Chapter 5

Properly Selecting a Site for a PV System

IN THIS CHAPTER

Bullet Preparing yourself to conduct a site survey

Bullet Identifying the basic pieces of information you need to gather about a site

Bullet Recording data about magnetic declination, tilt angle, azimuth, and shading

Bullet Evaluating the information obtained during your site survey

One of, if not the, most exciting portions of the overall system installation process is the site survey. Why? Because you get to work with a blank slate (after all, the client shouldn’t have another PV system anywhere on his building or property) and create a PV system from the ground (or roof) up. As the person who performs the site survey, you need to be able to identify any potential trouble spots and the best ways to address the issues they pose from the beginning of the project.

The site survey is generally your one chance to obtain all the required information about the site to create a proposal that works for both you and the client. It’s also the only time you can really work with your client to establish his goals and expectations for his PV system before you’re too far along in the process. I explain the steps and methods you need for a site survey in this chapter. I also give you an idea of what to do with all the data that you gather.

Setting the Stage for a Site Survey

When you’re at a client’s house or business to perform a site survey, you must be diligent about collecting the information you need. You don’t want to leave out any information that may prove critical for you to provide an estimate and a quality design for your installation crew. Return trips to gather information that should’ve been collected the first time around do nothing but waste your time and risk making you look less than professional. In this section, I provide some pointers on how to prepare for a site survey.

Putting aside enough time

Remember You can’t conduct a solid site survey without allowing yourself ample time and staying focused (a site-survey form can help focus your itinerary; see the next section). I suggest allocating at least an hour of on-site time. If through preliminary phone or in-person conversations with the client you get a feeling that he’ll require extra time, then schedule yourself enough time to answer his questions while leaving enough time to collect the information you need.

Speaking of questions, the one you’ll hear most often (aside from “What’s the average cost?”) is “How much money will this save me each year?” Although this can be a difficult question to answer without knowing a number of specifics, you should have an answer for it.

Use the PV Watts tool to become familiar with what a 1 kW PV array can produce in the client’s area. (Figuring out what a 1 kW system can produce means you can easily do the math to adjust the values for your client’s site.) This free tool, found at www.nrel.gov/rredc/pvwatts, takes information from you and applies weather information from collected data and returns estimated energy values for a PV system. One of the best features of this tool is the ability to vary input factors such as the direction you want your array to point or what tilt angle you want it to have. With just a few mouse clicks, you can run some scenarios and be able to determine the best solution for your site. (Note: If you're new to PV Watts, I suggest using Version 1 first and then moving on to Version 2 as you become more proficient with the tool.)

By taking the time before the site survey to acquire this information, you can give your client an estimate for the array size and the amount of money the system can save him each year after you complete the site survey. Just be sure to give the disclaimer that you won’t know the exact size of the array or how much energy it’ll produce until the design is done.

Creating a standard site-survey form

So that you don’t forget any of the information you need to acquire while conducting a site survey, seriously consider carrying a standard site-survey form. Using a standardized site-survey form keeps you on track by reminding you what to collect and giving you a place to write all of that down. It also improves the quality of information you walk away with and can save you time when a client whom you gave a bid to a year ago suddenly calls and says that he’s ready to buy. With all the necessary information recorded on your site-survey form, you can save yourself another trip out to the site.

Remember A good site-survey form can be easily replicated by others within your organization, meaning no matter who’s conducting the site survey, the required information will be obtained and recorded. In a program such as Microsoft Word, you can quickly create a site-survey form that contains places for the following information:

  • The site address and client’s contact information (e-mail and phone number)
  • The type of system desired (grid-direct; utility-interactive, battery-based; or stand-alone, battery-based)
  • The utility company (along with contact information if you’re not familiar with the local utility)
  • The information from the client’s main distribution panel (see the later “Collecting Basic Information during a Site Survey” section for the how-to on obtaining this data)
  • Space for notes on the proposed inverter location
  • A note section for options for wiring the array to the inverter
  • The roof tilt and azimuth if the array will be roof mounted or the estimated array tilt and azimuth if the array will be ground mounted
  • Information on the type of roofing material and its condition
  • A note section for any issues with the structure
  • A list of pictures to take
  • Space for notes about the customer’s requests or special site considerations

Toting a site-survey bag

When you go to do site surveys, you’ll be climbing on roofs, traipsing through fields, and walking through all sorts of buildings — all while carrying the tools you need to complete the survey. For these reasons, I can’t stress how essential a dedicated site-survey bag is.

Tip I prefer a tool bag with multiple pockets and a heavy-duty strap so you can carry the bag over your shoulder. The strap is key because you’ll be carrying the bag up and down ladders and in and out of access hatches.

Remember The following tools should always be in your site-survey bag:

  • A good-quality digital camera (see the next section for tips on using this tool during a site survey)
  • Measuring tapes, including a
    • 25- or 30-foot tape
    • 100-foot tape
    • Wheel tape (this one’s critical for commercial applications)
  • A compass
  • An angle finder (see the later “The tilt angle” section for the scoop on this tool)
  • A calculator
  • A clipboard and notebook with pens and pencils
  • A flashlight and extra batteries (note that a headlight works great in attics while allowing you to keep your hands free)
  • A shading-analysis tool (I describe several of these tools later in this chapter)
  • Screwdrivers (you can get away with just one if you have a 10-in-1 screwdriver)
  • A digital multimeter (see Chapter 3 for an example)
  • Tinted safety glasses

Remember Although a ladder won’t live in your bag, make sure you have it on hand. Very few site surveys can be completed without accessing a roof or attic.

Picture This: Documenting Your Entire Site Survey with Digital Photos

One of the best pieces of advice I ever received regarding site surveys was to take enough of the right photos so I could re-create the site when I got back to the office. A good-quality digital camera is therefore an essential tool for any site-survey bag.

If you’re in the market to buy a new digital camera, do a little research first. I suggest looking for a camera that can sustain some abuse (like being dropped and/or being exposed to water). Simple point-and-shoot cameras are sturdier (for your purposes, anyway) than the fancy digital SLRs out there, plus you can get high-enough-quality pictures from them. Compare different models within your price range using the solid reviews found at reviews.cnet.com/digital-cameras.

Tip Chances are you’ll find yourself doing multiple site visits in a day, so before you do anything else during a site visit, take a photograph of the front of the building. Doing so helps you establish the location of all the pictures that follow. Without establishing your location in the beginning, you can easily jumble all of your pictures together.

Remember To help yourself make sense later on of the photos you take, be consistent with your procedures. By keeping a consistent order to your site survey, you’ll be more likely to remember the information needed at each step. I personally like to start at the PV array location and follow my way through the rest of the system, but I also know some installers who like to work in the opposite direction. Find the method that makes sense to you and stick with it.

Here are some tips for taking photos during your site survey:

  • When you’re looking at mounting the PV array on a roof or on the ground, take multiple pictures from each corner of the array location so you can see potential issues from each vantage point during the design process.
  • Use reminders as to where you’re taking the pictures from. For example, if you’re planning to use a satellite image or a roof plan for recording measurements, then take a picture of the image/plan you’re using while pointing at the location you’re at. So if you’re on the southwest corner of the roof, take a picture of the roof plan while you point at the southwest corner of the roof. Then take the pictures from that vantage point. You can do the same thing if the array will be mounted on the ground; just follow the same process to fully document the site.
  • Repeat these steps for every location where any PV equipment will be installed. I recommend identifying alternate equipment locations too. After all, you never know whether the location you covet for the inverter is really reserved for some prize-winning rose bushes.

Tip Another trick is to use the video feature of your digital camera and record the same vantage point you just photographed. Doing so gives you the ability to talk to the camera and remind yourself of certain points or give co-workers some insight into factors or considerations worth noting.

Collecting Basic Information during a Site Survey

You need to gather certain pieces of information during each and every site visit; you also need to look out for oddball things that may cause problems later. In the sections that follow, I look at the most basic (yet critical) pieces of information you need to acquire.

Warning When performing a site survey, gathering the information is vital, but so is maintaining your safety. Any time you’re climbing on roofs, walking through attics, or opening electrical boxes, you’re doing real work. All of these activities have inherent dangers that you need to think through. If during your site survey you find yourself in a situation that feels unsafe or makes you uncomfortable, immediately stop what you’re doing and find a different way to perform the task. Chapter 15 covers typical safety issues and how to handle them; the guidelines I present there apply to site surveys as well.

General site information

When you show up to perform a site survey, it’s easy to get so focused on all the little details you need to document that you forget to look at the big picture. Yet taking an objective look around and asking a few questions can save you from wasting your time. Following are four tidbits of general site information you should find out before you start delving into minutiae:

  • What’s the shading like at the proposed location? I explain how to perform a basic shading analysis later in this chapter, but you also need to account for the other shading-related site conditions that may cause problems. The one that catches people the most is future shading — objects that aren’t there now but may shade the array in the future. For instance, those small trees on the south side of the building won’t always be small, and the empty land to the south of the property may one day be the home of several tall buildings.

    Tip If the view from your client’s site is clear now, you may be able to protect it from any potential shading issues. Many areas throughout the United States have solar-access laws that allow a person to protect his solar resource from shading sources on adjacent properties. These laws are typically enforced at the municipal level, so check with the local building department to see whether solar-access laws apply in your client’s region.

  • Are there any restrictions for the site? Such restrictions can include but aren’t limited to homeowners’ associations, city covenants, and historic districts.

    Remember At the very least, your client should know whether he belongs to a homeowners’ association; you can then ask the association’s contact person about any association restrictions. The local building department can provide you with any other special requirements that are in place based on the building’s location.

  • What agencies need to give approval for the installation? You absolutely need to know what (if any) restrictions are in place due to local specialty codes and firefighter access. The best place to find out what’s required for agency approval is the local building department (the same office that issues permits, as I explain in Chapter 14). You also need to check in with the local utility to make sure you meet its requirements (Chapter 17 has more on what you need from the utility).
  • What’s your client’s target budget? You can easily establish a rough estimate of the PV system’s cost based on your equipment choices and the installation specifics, but you need to know what your client’s financial thresholds are. For example, if there are substantial unforeseen issues, how much money is the client willing to spend to fix an underlying problem before moving on?

Structural and mechanical information

How much physical space is available for the installation? Typically, PV systems are installed on the roofs of buildings or on free land space. Your task during the site survey is to make sure the space available will suffice for the client’s desired PV system. Your client may have an idea of where he wants the array to go, but it’s your job to make sure a better alternative doesn’t exist.

Remember The area you have your eyes on may be the same as someone else. Always verify that other plans don’t exist for the space you want to use, such as plans for solar thermal collectors or skylights.

Here are some additional structural and mechanical questions that you should ask if the array will likely be mounted to a roof:

  • What are the dimensions and shape of the roof area available? Taking the dimensions of the roof area you plan to install on will help you sketch out the roof later when you’re ready to plan how the array will be arranged on the roof (later in this chapter, I give you tips on how to use satellite photos and three-dimensional drawing programs to help keep this sketch accurate). During the site-survey process, you also need to identify obstructions (such as plumbing vents, chimneys, and attic vents) on the roof as well as their locations.

  • What condition is the roof material in, and how old is the roof covering? Placing an array on a roof that will need to be replaced in a few years doesn’t make a lot of sense. If a reroof is in order, suggest it be done now and be sure to work closely with the client and the roofer to coordinate phases of the project so you can continue with the PV system design and installation in a timely fashion.

    Tip As I state in Chapter 16, a possibility for dealing with a reroof and keeping your project moving is to do some of the rack installation before the roofers come in and allow them to seal the roof attachments.

  • What’s the roof framing like? The roof’s framing plays an important role. Most modern homes and commercial buildings (those built since the mid-1970s) tend to have roof framing that’s adequate for a PV array mounted parallel to the roof so long as a single layer of lightweight roofing material (such as composition asphalt shingles or wood shake) is used as the roof covering. Why? Because the roofs of modern homes are designed to handle multiple lightweight roof layers. As long as only one layer is present, adding the weight of a PV array will be less than the structure’s limitations.

    If the building was constructed before the mid-1970s, plan to spend a few hundred dollars to have a structural engineer evaluate the roof for you and outline any changes you need to make to safely support the array. Make sure this consultation happens as early as possible in the system design process. Note: Many commercial buildings are built with the minimum requirements; therefore, they may not be able to support a roof-mounted PV array of any size.

  • What are the dimensions and spacing of the roof framing? Most residential roofs have rafters or trusses that are made of dimensional lumber that’s either 2-x-4 or 2-x-6 and are spaced 2 feet apart, which gives you a relatively narrow space to hit for your array’s attachment points. For commercial roofs, the structures vary greatly. Some buildings have lumber, similar to residential roofs; others use very large wood support members; and some use steel supports. Consequently, you should take the time to verify the roof structure in order to properly attach the array in any system that’s being installed on a commercial roof.

    Remember Always do your best to verify the roof framing composition and orientation when conducting your site survey. (See Chapter 16 for more information about mounting an array.)

    Warning Be sure to carefully evaluate rafters that are overspanned — a situation where the rafter has too much space between vertical support members. Different spans are allowed based on lumber type and roof-loading restrictions, but as a general rule, if the rafters have a span of more than 7 feet between supports, you should investigate the need for adding support by consulting a structural engineer.

After roofs, ground mounts are the most popular type of racking system. Unless your client’s site has unusually loose soil (like sand), you can work with a racking company (and maybe an engineer, if necessary) to determine the best possible mounting solution for the array. Of course, before you start talking to a racking company, you need to make sure the location is suitable for mounting an array. I cover the issues you need to consider for ground-mounted PV arrays in Chapter 16.

Electrical information

If you’re installing a stand-alone, battery-based system, you don’t really need to worry about examining the existing electrical service and equipment because the stand-alone system will be providing all the energy for the client moving forward. (You will, however, have a lot of components to size and to specify, as I explain in Chapters 12 and 13.) For utility-interactive systems (whether grid-direct or battery-based), you have a number of items to review while you’re on-site because you’ll eventually connect the PV system to the utility:

  • What are the specifications for the main distribution panel (MDP) and the main circuit breaker protecting the panel? The ratings on the MDP and the main circuit breaker play a major role in determining a PV system’s maximum size. When looking at the existing electrical service, you need to document the specifics on the MDP and any subpanels you want or need to use (I cover these panels in Chapter 2), including their physical locations. The voltage for most electrical services in residential applications is 240 VAC at various current levels; for commercial applications, the voltage is usually 208 VAC or 480 VAC.

    Busbars are the pieces of metal in the back of the MDP that connect the circuit breakers in the panel to the wires coming from the utility (you can’t see them when the cover of the MDP is on). Every MDP has a rating for its busbars on the label attached to the inside of its cover. This rating is a value for the amount of current that can flow on the busbars inside the panel without causing any problems. Standard residential MDPs have either a 200 A or 225 A busbar rating and may simply be labeled 200 A Max (or 225 A Max); commercial busbars typically start at 200 A for small facilities and can exceed 1,200 A in large facilities, with a number of options in between.

    The other specification for the MDP (and any subpanel used) is the rating of the main circuit breaker protecting the panel. For the MDP, this is often the same size as the busbar rating. The ratings for circuit breakers in subpanels vary based on the loads located in the subpanels.

  • Are there any open breaker spaces on the main electrical panel? Look in the MDP or subpanels to check for available space to put a breaker. Then connect the inverter’s output wires to one of these panels by placing a breaker in the panel and wiring the inverter to it. (This process is similar to putting a breaker in the panel for a new set of outlets or a new load except that the electrons are running in the opposite direction.) If the panel is full, you need to either make room or replace the existing panel with a larger panel.

  • Where will the inverter and required disconnects be located? Determining a location for the inverter and required disconnects means you have to know the specifics of the equipment you’ll be using as well as what the local jurisdiction requires. Typically, the inverter needs to have disconnecting means (both AC and DC) within sight of the inverter.

    Remember The majority of the inverters on the market today have integrated disconnects, but they aren’t all considered AC and DC disconnects, which means you need to know for sure what type of product you’re using. If the inverter doesn’t have integrated disconnects, make sure it has disconnects next to the inverter and in locations that satisfy the National Electrical Code® (NEC®). Refer to Chapter 17 for exact location requirements.

    In addition, some utilities require visible, lockable disconnects at the utility’s meter location. If this is a requirement at your client’s site, become familiar with the utility requirements before conducting a site survey so you aren’t surprised later. Ask the utility for its net metering or PV interconnection rules. If the utility does have a requirement, make sure you reserve the proper space for the required disconnect.

Remember The electrical connections are a critical part of the whole installation (as you find out in Chapter 17). Before you get too far into the system design, however, you need to know what the licensing requirements are in the jurisdiction in which you’re working. Some jurisdictions require that the people doing certain portions of the work hold specific licenses. (The most common example is the requirement that a licensed electrician make the final wire connections from the inverter to the utility.) Consequently, you may need to hire certain tradespeople to do certain portions of the work.

Tip To find out the requirements in the state where you’re working, visit the state’s construction contractors board or licensing department Web sites. These sites will list the requirements for working in the construction trades.

Measuring Information in Degrees

Some factors that are relatively constant on sites within a geographic region are the magnetic declination (the direction a compass points toward north versus true north), insolation data (the number of peak sun hours), and local climate conditions. Factors that differ on nearly every installation, no matter the location of the site, include the tilt angle of the array (the number of degrees it is off of the horizon), the azimuth the array faces (the number of degrees from true north), and the shading considerations.

In the following sections, I explain how to measure the three pieces of vital information that are measured in degrees: magnetic declination, the array’s tilt angle, and the array’s azimuth. Flip to Chapter 4 to find out how to measure peak sun hours and consider seasonal effects; head to the later “Exploring Shading-Analysis Tools” section in this chapter for pointers on analyzing shading concerns.

Understanding magnetic declination

The term magnetic declination refers to the number of degrees that a compass needle differs from true north. (If you do a fair amount of navigation — flying, boating, mountaineering, and the like — you may refer to this as magnetic variation; that’s just another term for essentially the same thing.) For locations in the Northern Hemisphere, you want a PV array to be facing true south as much as possible — something you can’t make happen if you follow exactly what a compass tells you. If you were to use your compass and point the array the same direction as the “south” needle on the compass, you’d be as much as 20 degrees off from true south, depending on your location. To make sure an array points toward true south, you need to know what your magnetic declination is.

Depending on where you are in the United States, a compass’s north needle points either to the east of north (the western half of the country) or to the west of north (the eastern half of the country); the Mississippi River is roughly the dividing point. The designations are referred to as eastern (positive) declination for locations in the western part of the country and western (negative) declination for locations in the eastern region of the country. These designations refer to the direction that the north needle is pointing in reference to true north.

Remember Ultimately, you need to concern yourself with true south more than true north (which is contrary to what most people are taught). Why, you ask? Well, for the Northern Hemisphere, the sun is located in the southern half of the sky, which is where you want to point your PV modules. You need to turn yourself 180 degrees and think about how the declination value affects the magnetic south and true south locations. For example:

  • When you’re on the west coast and using a compass to find south, the south needle is actually pointing southwest of true south (because the north needle is pointing northeast of true north). To account for this, you need to turn yourself to the east to find where true south is.
  • When you’re on the east coast, the opposite occurs. The south needle points southeast of true south (because the north needle points northwest of north). Turn to the west to find true south.

Figure 5-1 gives you the visual of these two examples. (Don’t worry if the concepts still don’t make sense; they can be difficult to grasp. I still have to stop and think the process through to make sure I get it right on a roof.)

Images of two compasses depicting (left) eastern (positive) declination and (right) western (negative) declination.

FIGURE 5-1: Compass views showing positive and negative magnetic declination.

To add to it all, magnetic declination is a dynamic value, meaning the number of degrees your compass lies to you is constantly changing. It doesn’t change rapidly, but it does change. True south, however, never changes, so after you find it, you don’t have to worry about going out to the client’s PV array every five years and adjusting it a few degrees.

Tip The National Oceanic and Atmospheric Administration (NOAA) has a great declination calculator on its Web site that can help you find the exact value for your client’s location; you can find it at www.ngdc.noaa.gov/geomagmodels/Declination.jsp. To find the point on your compass that relates to true south, you need to know the magnetic declination for your client's area and subtract that from 180 degrees (north is 0 degrees, so south is 180 degrees on the compass):

  • For locations in the western part of the United States, true south is designated on the compass as 180 degrees – declination.
  • For locations in the eastern part of the United States, you still subtract the declination value, but because the declination is a negative number to begin with, the equation becomes 180 degrees + declination.

Remember Technology can help you evaluate the declination at a site during a site survey, but even the greatest technology requires you to input the starting information correctly. If you can at least keep the general rules about magnetic declination in mind, you should see consistently accurate data — provided, of course, that you’re not trying to use your compass in a location that’s close to steel objects, which cause a compass needle to point in any number of directions and prevent it from finding magnetic south. If you can’t find magnetic south, you’ll have a tough time finding true south by subtracting the magnetic declination value.

Calculating the array’s tilt angle and azimuth

In a typical PV installation, the array is located on top of a building. In this scenario, the most cost-effective installation method places the array parallel to the roof and pointing in the same direction as the roof because the additional requirements for securing an array that isn’t parallel to the roof surface can become overwhelming. Aesthetics are another consideration. I know, I know. Beauty is in the eye of the beholder, but an array that looks awkward on a roof doesn’t make passersby say, “Gosh, I sure would like that on my roof!”

Knowing an array’s tilt angle and azimuth can help you properly place one on a roof. I explain what you need to know to calculate these values during a site survey in the following sections. (If these concepts don’t sound familiar to you, check out Chapter 4 for a quick introduction.)

Note: The approaches in the following sections refer to roofs and the methods used to calculate an array’s tilt angle and azimuth in relation to a roof. If you’re installing an array on the ground, the same approaches apply; you just don’t have a roof to reference and need to use landmarks instead.

The tilt angle

You have a couple options for figuring out the tilt angle of an array that will be both functional and eye pleasing:

  • Use an angle finder. To use this tool, which is also called an inclinometer, simply place it on the face of the surface to be measured and wait until the rotating dial comes to a stop. The face of the tool has degree measurements on it in relation to a flat surface so you can record the angle off of the horizon by noting the degree location where the dial stops spinning. Because most PV arrays are mounted parallel to a roof, this angle is the same for both the roof and the array.

  • Use a little math. If you can measure the amount of height the array changes (rise) over a certain horizontal distance (run), you can calculate the corresponding angle. After all, who doesn’t love some good old-fashioned trigonometry? A common approach is to give the rise of a roof over a distance of 12 inches.

    Tip You may hear a roof slope (angle) referred to as X:12. What this is saying is that the roof rises X inches for a run of 12 inches. The smaller the X number is, the lower the slope is for that roof. If you’re given the roof slope this way, you can quickly convert the variable to a number of degrees by using a calculator with basic trigonometry functions. The calculation is: arctan (rise ÷ run). The arctan function is represented as tan–1 on many calculators. Therefore, if I tell you that a roof has a slope of 6:12, you can calculate the number of degrees with this operation: arctan (rise ÷ run) = arctan (6 ÷ 12) = arctan 0.5 = 26.6 degrees.

    Tip If you aren’t given the roof slope, the easiest way to figure it out is by using an angle finder. However, you can also use a bubble level and a tape measure. If you choose to go this route, hold the level with one end touching the roof and the bubble centered in the viewing glass (in other words, hold it so that the level is level). Use the tape measure to see how many inches the roof surface is from the end of the level that’s above the roof. You now know that the roof rises that many inches (from the tape measure) over a run that’s equal to the length of the level.

The azimuth

Satellite images are an immense help when conducting a site survey, especially when you need to calculate the azimuth. With the help of satellite images, you can know the azimuth of the roof with magnetic declination accounted for before you even set foot on it.

These images show the site in relation to true north and true south. So if you find your array location (typically a building) on the satellite image, you can determine the roof’s azimuth without having to use your compass. Why? Because the satellite images have a built-in compass that’s pointing true north. If you can find your client’s building on the satellite image, you can use this built-in compass to estimate the true azimuth of the building (with magnetic declination accounted for).

Tip Another handy online tool is available at www1.solmetric.com/tools/RoofAzimTool.htm#. It lets you find a satellite image of a building and use your mouse to determine the building’s azimuth.

Warning The downsides to satellite images are their resolution and the time when they were taken. A remote location may have poor image resolution. Other times, the image is a few years old, and the building you’re looking for isn’t in the image yet. In these situations, you have to obtain the information you need on-site. The address look-up tools on these products aren’t perfect either, so you may need to confirm with your client which building is the right one before you head to the site.

Depending on the services you need, one of these options may work for you:

  • Google Earth (earth.google.com) and Bing Maps (www.bing.com/maps) let you see the building you're looking for with a decent amount of detail after you type in the address. Some of these images can also be used in conjunction with free three-dimensional drawing programs (such as Google SketchUp, found at sketchup.google.com) to really help you evaluate and analyze your site.
  • Subscription-based programs such as Pictometry (www.pictometry.com) can give you even more detail.
  • Smartphones offer applications that can come in handy during a site survey. You can download apps to your phone that perform shading analysis, calculate tilt angles, and even help with some of the NEC® calculations I present in Chapter 13. These phone apps are currently available solely for the iPhone, but with the speed at which the wireless world moves, I'm sure that will all change very quickly.

Tip If you print a couple copies of the satellite images for your client’s site beforehand, you can take notes and record measurements directly on the printouts without having to re-create the building or array location with hand drawings. I recommend printing at least two or three clean copies so you have plenty of space to make notes for yourself.

Exploring Shading-Analysis Tools

When you perform a shading analysis during your site survey, you look at the area surrounding the proposed PV array location and estimate the amount of sunlight that’s blocked from obstacles like trees and buildings. This analysis is what allows you to give your client a realistic expectation of the energy that can be delivered by the PV system over the course of a year.

Remember Shade on a PV array can drastically reduce the array’s power output, which in turn reduces its energy production. Although a perfectly sunny spot is preferable, the reality is that shading is a simple fact of life in many locations. Some situations can be altered (a tree can be trimmed or removed), but others can’t (the neighbor’s house isn’t going anywhere). The goal for any PV site is a location that’s shade free three hours before and three hours after solar noon (I define solar noon in Chapter 4). This time frame allows the PV array access to full sun during the portion of the day when the irradiance values are at their highest. (If this time window isn’t an option, like in the winter, look for a window of two hours before and two hours after solar noon as a minimum.)

By using a shading-analysis tool, which recognizes the shading objects in the PV array’s solar window (all the points between the sun’s lowest and highest points in the sky) and calculates the effect that shading has on the array, you can effectively determine the shading effects from objects located in the array’s vicinity. Figure 5-2 illustrates how a shading-analysis tool transfers the obstructions in the site’s solar window to a sun chart that you can analyze (I explain how to perform this analysis later in this chapter). You can also use shading-analysis tools to propose “what-if” scenarios, like what’s the overall impact if we remove that tree in the distance?

Illustration depicting how a shading-analysis tool transfers the obstructions in a site’s solar window to a sun chart.

FIGURE 5-2: Obstructions as viewed on a sun chart.

The shading-analysis tools I outline in this section work the same way. I’ve used all three tools and feel that they’re quality products that deliver good information when used correctly. (Because they all work off of the same principle, you may want to check out the sun-path information in Chapter 4.) Of all the tools I suggest you keep in your site-survey bag (see the earlier “Toting a site-survey bag” section), a good shading-analysis tool will be your single biggest expense (unless of course you buy the best-of-the-best digital camera). It’s a tool you can’t do without though, so research your options and think about how (and how often) you plan to use it before buying one.

Remember Always use a shading-analysis tool at the proposed array location. If you can’t get to that, use the manufacturer’s instructions to adjust the information you collect so that it describes shading at the actual array location.

Tip One of the greatest features of the following three shading-analysis tools is their ability to evaluate a site for an entire year at once. They all use site-specific solar charts and local weather data for the site analysis, which means you can go out to the site at any time (as long as enough light is available so you can see the area surrounding you) and collect all the information you need.

  • ASSET: The Acme Solar Site Evaluation Tool (abbreviated as ASSET and found at www.we-llc.com/ASSET.html) is a shading-analysis tool that uses a digital camera and software to evaluate a site. All you have to do is set the camera up in a special base that incorporates a level and compass. ASSET then takes seven pictures, starting with the camera pointing due east and rotating the camera to the west. The result is a panoramic picture of the site.

    After you have your panoramic view, you download the pictures into the ASSET software so the program can return a single picture (made of the seven individual ones) with the solar path on top of the site. ASSET also recognizes shading objects and reports the overall loss of the solar resource due to shading.

  • Solar Pathfinder: The Solar Pathfinder (found at www.solarpathfinder.com) has a long history within the solar industry. It consists of a plastic base that integrates a level and compass and holds a paper sun chart. (The company makes sun charts for various latitudes, so the Solar Pathfinder can be used virtually anywhere.) The final piece is a transparent dome that sits directly over the sun chart in the base. This dome reflects the surroundings and allows you to project those objects down to the sun chart below. You can then either trace those objects directly onto the sun chart or take a digital picture of the tool and download that picture into Solar Pathfinder's optional software for further analysis.

    Tip The software option is a powerful tool that can help you better estimate the shading effects on that site. In fact, if you’re purchasing a Solar Pathfinder for the first time, don’t even consider the software optional; just buy it. The additional information you get from the software is worth the investment.

  • Solmetric SunEye: One of the newest entrants in the shading-analysis-tool market is the Solmetric SunEye (found at www.solmetric.com). It’s a fully digital tool that incorporates a fish-eye camera, compass, and level into a base with a screen showing the site complete with sun chart and obstructions. The machine evaluates the site by taking a photo at the array location and using some site-specific data, such as magnetic declination and local climate readings (note that you have to input some basic site parameters in order for the SunEye to be able to retrieve the proper information). It then returns detailed information about the shading effects.

Interpreting the Data and Bringing It All Together

During your site survey, you go to the site and gather all the data you need (described earlier in this chapter). That’s great, but what do you do with all that data afterward? As you find out in the following sections, you compile it and analyze it to determine the best PV system solution for your client. Specifically, you must evaluate the following:

  • The area available for the array
  • The options for mounting the array
  • The electrical considerations
  • The array orientation
  • The shading effects

Analyzing reports from your shading-analysis tool

Shading-analysis tools generate a report that’s associated with the site. This report helps you during the design and installation portions of a job because having this information allows you to avoid shading issues that may reduce the overall energy output. Here are some important pieces of information revealed in a typical report from a shading-analysis tool (this is the information your client is most likely to care about):

  • Remember The effect of shading on an annual basis: This is the most basic level of information you absolutely have to walk away with. It may be reported as a percentage of the ideal insolation or as a new amount of insolation. Either way, you can have your shading-analysis tool break down this information month by month so you can see where the big hits are. Ideally, you’ll have very few losses due to shading. In the next section, I show you how to incorporate the shading losses and the array’s orientation losses to account for all the site-specific losses (note that the combination of shade and orientation losses shouldn’t reduce the site’s potential by more than 25 percent).

  • Estimations of the energy production based on different scenarios: These estimates allow you to vary the tilt and azimuth to see how such adjustments may benefit the installation.
  • Predictions of how the PV array would perform if the obstructions were removed: You can estimate the array’s production if, say, a tree weren’t in the way and allow the client to choose between keeping the tree or obtaining additional energy from the array by removing the tree.

Considering the total solar resource factor

Remember When shading-analysis tools evaluate a site, they typically look at the shading effects as well as the effects of the tilt and azimuth. The combination of these factors is defined as the total solar resource factor (TSRF). The individual components of the TSRF are often referred to as the shading factor (SF) and the tilt and orientation factor (TOF). (Orientation is another word for azimuth.) Both are reported as a percentage of the ideal situation. For shading, the ideal situation is an absence of shading; for tilt and orientation, the ideal situation is pointing a stationary array at a specific tilt and azimuth to have the array produce the maximum amount of energy annually. You need to figure out what the TSRF is so you can accurately predict the amount of energy that the PV array will produce annually.

The SF value is, obviously, totally dependent on shading. If a site were to have no shading, the SF would equal 100 percent. That’s pretty difficult to achieve, but an excellent site has shading factors greater than 95 percent (meaning less than 5 percent of the solar resource over an entire year is lost due to shading).

The TOF value, given in terms of a percentage, is estimated by evaluating the PV array’s tilt and azimuth compared to what the absolute ideal tilt and azimuth for a PV array in that same location would be. These values are typically represented as a graph with concentric circles (or ellipses) that represent different levels of the resource. Figure 5-3 shows one such graph. The vertical axis represents the tilt angle of the PV array, and the horizontal axis represents the array’s azimuth. The innermost circle represents the ideal tilt and orientation for this location. As the array moves away from that circle, the amount of solar resource is reduced. To estimate the TOF with such a graph, simply find the point on the graph where the proposed array lands and apply that percentage for the TOF.

Illustration of a typical tilt and orientation graph to determine the shading effects from objects located in an array’s vicinity.

FIGURE 5-3: A typical tilt and orientation graph.

Remember To calculate the TSRF, you multiply the SF by the TOF. The result is the total resource available after these inefficiencies are considered. For example, if you perform a shading analysis and find the annual reduction of the resource due to shading is 9 percent, that would equate to an SF of 91 percent. If you located the array in a location where Figure 5-3 applied and the array was tilted at a 20-degree angle and a 165-degree azimuth, the TOF value would be 97 percent. You can determine this percentage by placing an X at the intersection of the 20-degree tilt and the 165-degree azimuth on the tilt and orientation graph. The X falls between the 95-percent ring and the 100-percent ring, so you can estimate the value to be 97 percent. The resulting TSRF value would be 91 percent × 97 percent, or 88.3 percent. This TSRF value tells you that the site’s output will be reduced by nearly 12 percent when compared to a PV array oriented toward the ideal location with absolutely no shading.

Remember The TSRF should be greater than 75 percent for your PV arrays. Anything less than that is marginal at best. Many rebate programs require that the TSRF exceed a minimum value (typically 75 to 80 percent), so be sure to find out the restrictions tied to any rebate money your client may be expecting.

You can then use the TSRF to estimate the amount of energy the array will produce by multiplying it by the peak sun hours data available from sources such as the NREL Redbook (see Chapter 4). Another source for this information (as well as handy tools to estimate the TOF values) is Solmetric, the manufacturer of one of the shading-analysis tools I recommend earlier in this chapter; you don’t have to use Solmetric’s tool in order to use some of the features of its Web site (www.solmetric.com).

Using other collected information to plan out the design and installation

The other information you collect during your site survey (such as general site information, structural and mechanical information, and electrical information) comes in handy during the design and installation process. Many municipalities and rebate programs want to see some basic drawings of the site and associated hardware used. You’ll need to reference the information you collected to establish a bill of materials and the estimated cost of installing the system. Using the information gathered during your site survey, you should be able to

  • Determine the size of the PV array that can go in the proposed area. The square footage of the area available and the module of choice for the array (see Chapter 6) dictate the size of the array. You may also need to account for walking paths around and between rows of modules, so the total available area may actually be less than what it appears at first glance. A good plan for a roof-mounted array is to allow for access around the array. If you pack a roof with modules, not only will the installation process be difficult and more dangerous, but you’ll also make it nearly impossible for anyone to get on that roof in the future.

    Tip A good number to keep in your head is 10 W per square foot of open roof area. So if you have 200 square feet of open roof area, an off-the-cuff estimation for the PV array is 2,000 W, or 2 kW. This value accounts for space that can’t be fully utilized by the array. It’s slightly on the conservative side, meaning you can probably fit more wattage in that area, but it’s a great starting point and generally within reason.

  • Establish any potential upgrades to the structure for either the array or the other components. If you spot a potential problem area in the roof framing information that you collected during the site survey (see the earlier “Structural and mechanical information” section), you can use this information, along with an engineer’s report, to justify the addition of more bracing or supports to the existing roof.
  • Identify the racking structure used. You can narrow down your options based on the defined array location and the array’s potential size.
  • Decide on the best method to run all the conductors to and from the inverter and tie into the electrical system. As part of the site survey, you examine the potential wire runs and the best methods for getting from the array to the inverter and then to the MDP (or subpanel).
  • Identify all the major components for installation per local requirements. By becoming familiar with the local jurisdiction’s requirements as well as the local utility’s requirements, you can establish all the major components needed and their approximate locations to save yourself some time and hassle during the installation process.