When choosing a solar energy system for your home, there are three major components that you’ll need to be concerned about: solar modules, solar racking or support frames, and inverters. The components you choose will determine the reliability and output of your solar array for the duration of the system’s time on your home.
In a grid-tied system, electricity is initially generated by several to many PV solar modules. Chapter 2 provided a detailed description and analysis of the various types of solar modules available. Once the type and size of the system are chosen, a simple rooftop mounting frame, to which the modules can be attached, will be installed. See example in the photo on the following page.
There are some good options for solar mounting equipment. Solar tracking devices are now more economical and easier to install than earlier systems, with new products coming to market all the time.
Of course, the mounting frame is installed on the side of the roof that provides maximum sunlight exposure. In the northern hemisphere, this would logically be on the south-facing part of the roof, which is slanted toward the equator. The more perpendicular to the equator the mounting frame supports are, the better, but this line does not need to be exactly 90 degrees for the PV solar system to function well.
A top view of three rows of support rails mounted on the roof with micro-inverters already attached and waiting to be connected to the PV modules.
The type of mounting system you’ll need depends on where you plan to install the modules of your PV solar energy system. With crystalline panels, you have three options for location: mounting the panels on your roof, on the ground, or on a pivoting stand.
Most people think a roof mount is the most convenient and aesthetically pleasing, but there are many reasons that people choose other options. For example, if your roof is small, unstable, in the shade, or if you aren’t able to face the panel towards the equator (facing south in the northern hemisphere or facing north in the southern hemisphere), you may consider mounting your system elsewhere. You may find that you like the simplicity of a ground mount if you have extra land available.
Pivoting stands are an attractive alternative because they’re able to follow the sun throughout the day, so they can be far more efficient. But they’re also more expensive. If you have enough open space and your roof has major disadvantages, then a ground mount may be the best choice. With any of these mounting options, you should make sure there are no local ordinances or homeowners association rules against them.
There are viable solar panel racks and mounting hardware for every solar panel installation style; your solar array depends on your property. If you’re not sure about the different choices, any qualified solar contractor can help you make the right decision.
Two rows of support rails installed and waiting for installation of the PV panels and, if applicable, the micro-inverters first go on the rails before being connected to the PV panels.
Flush support frames are the most popular choice. They provide an inexpensive and simple option that is suitable for most roof-mounted solar panel installations. They’re generally not adjustable and are usually designed to position the solar panels at a consistent height above the surface of the roof on which they are mounted. There are suitable leak-proof anchor bolts or fasteners for any kind of roof. Tilted mounts, which improve the angle of the PV panels for roof installations, are also available. These are definitely recommended for flat roofs.
Ground-mounted support frames can provide an excellent alternative—space permitting, and assuming there are no serious shading problems—that will allow you to install your solar panels at ground level anywhere on your property. Ground mounts are usually designed with a fixed-tilt angle. But they’re often adjustable, allowing you to tilt your solar modules to the appropriate level for optimal solar collection, depending on your latitude.
There are also fully automated, adjustable-tilt tracking mechanisms that can substantially increase the solar efficiency of your system. Generally speaking, the lower the latitude, the less tilt your mount will require. Four-way tilt-control tracking systems can maintain the panels in a perpendicular position to the sun’s rays from east to west, following the arc of the sun.
For those requiring a non-roof installation, pole mounts are another solution. These mounts come in one of three alternative types that are distinguished from one another by how they’re positioned on the pole: top of pole, side of pole, and pole with tracking device.
Regardless of the type of mounting you and your solar contractor decide on, your support frame or pole-mount system will provide safety and security for your solar investment.
While most people might want to focus on panels and inverters, it’s important to remember that the solar panel mounting frames can also be critical to the success of your overall system. Besides the orientation and shading issues we discussed above, you also need to find out the wind category for your area, and also check the conditions of the ground if you plan to install a good ground-mount system.
Even with off-the-shelf parts, many permit offices will not give you a permit if the proposed ground-mount system doesn’t have a civil engineer’s stamp of approval. You should be able to hire an engineer for $500 or even less, depending on your location.
Now let’s look at another component of a PV system: disconnect switches. A shutoff switch, more commonly known as a “disconnect switch,” separates the panels from the rest of the system so that you’ll be safe if you ever need to make any repairs or changes to the system, such as adding additional panels. The first disconnect switch is the DC disconnect switch.
The DC disconnect switch is relatively small and can be easily installed anywhere convenient along the chosen routing for the PV cables that connect the rooftop modules to the inverter. Usually, you would choose your routing for the PV cables in the least conspicuous layout, taking advantage of any chimney, rooftop sun windows, attic bedroom roof protrusions, or eave troughs. You will want to pick a color for your PV cables that will tend to blend in with the existing rooftop colors, most often dark brown. Here are a few photographs of standard disconnect switches.
A DC disconnect switch, central inverter, AC disconnect switch, and metering device.
An off-grid disconnect switch.
A solar PV system typically has two safety disconnects. The first is the PV disconnect (or Array DC Disconnect). The PV disconnect allows the direct current coming from the modules to be interrupted before reaching the inverter for service and safety reasons.
The second disconnect is the AC disconnect. The AC disconnect is used to separate the inverter from the house’s main distribution panel and also from the electrical grid. In a solar PV system, the AC disconnect is usually mounted to the wall between the inverter and utility meter. The AC disconnect may be a breaker on a service panel, or it may be a stand-alone switch. This disconnect is sized based on the output current of the inverter or micro-inverters.
A schematic of a simple, small PV system installed on a garage.
Having chosen your solar modules, you’ll need to decide on a solar inverter. Solar inverters are a critical component to your PV solar energy system. Inverters change the direct current coming from the panels into the alternating current that your appliances and light fixtures use. There are several considerations here. First, you need an inverter that can handle what your panels generate, so make sure the wattage of the inverter is at least as strong as the wattage of your array. Second, you can consider solar micro-inverters, which are smaller inverters that connect to each panel instead of a central inverter that converts the DC of the entire system as a whole.
Below is a photograph of a typical inverter unit. There are several well-known manufacturers of inverters, including SMA and InfluxGreen. They all make quality equipment that has been tested and approved by national and international electric standards authorities.
A typical central inverter.
Grid-connected inverters must supply AC electricity in a “sinusoidal” form that is synchronized to the grid frequency. These inverters limit feed-in voltage to no higher than the grid voltage and disconnect from the grid if the grid voltage is turned off. Islanding inverters need only produce regulated voltages and frequencies in a sinusoidal wave shape, as no synchronization or coordination with grid supplies is required.
A solar inverter is usually connected to an array of solar panels. For safety reasons, a circuit breaker or disconnect switch is provided both on the DC and the AC side of the central inverter to enable maintenance. The AC output is connected through an electricity meter into the public grid and into the home’s main panel. In some installations, a solar micro-inverter is connected to each solar panel. We’ll discuss the subject of micro-inverters in more detail a little later.
Single phase off-grid PV inverters such as those made by InfluxGreen (Models IGSCI-0.5kVA to 7.0kVA) are an integrated system comprising a solar charger, AC charger, inverter, AC bypass switch, and transformer—all with battery control options. This inverter is versatile and can be used with a grid-tied system or with an off-grid system where the grid is not readily available. The IGSCI solution provides a cost-effective, reliable, and efficient stand-alone system with battery backup to meet specific user needs.
Solar installations typically have a central inverter that takes the direct current generated by a group of PV panels and converts it into AC for the home and/or for the electrical grid. Micro-inverters function essentially the same way but are installed on the back of each solar panel. They have some distinct advantages over central inverters. Micro-inverters make a rooftop PV system or large commercial PV installation more efficient. They also make it easier to monitor power generation of each panel and quickly pinpoint a failure in the system. This can save a lot of time and money. The idea is basically that the added expense of using micro-inverters is more than offset by the savings created by increased efficiency and lower maintenance costs. Hanwha SolarOne is one of several manufacturers that incorporate a micro-inverter into every solar module.
A standard Enphase M215 micro-inverter—industry leader for many years.
A side view of the same three rows of support rails mounted on the roof with micro-inverters already attached.
When using a central inverter, economies of scale can certainly decrease the cost-per-kilowatt-hour of your solar PV system as you increase the size of the array. Many DC/AC central inverters are sized for systems up to five kilowatts. However, if your PV array is smaller (three or four kilowatts, for example), you might still be wise to buy the same 5 kW inverter. This is because the difference in the cost of the inverters is minimal. But this will facilitate adding more PV panels later on and will end up saving you money.
Until recently, central inverters dominated the solar industry. The introduction of micro-inverters marks one of the biggest technology shifts in the PV industry to date. Manufacturers of micro-inverters claim at least a 15 percent increase in power output over central inverters, which in the long run can result in substantial savings for the homeowners who choose micro-inverters for their PV rooftop system.
Micro-inverters have been available since 1993. In 2007, Enphase Energy was the first company to build a commercially successful micro-inverter. More than one million units of the Enphase Micro-Inverter Model M175 have been sold since its release in 2008. Several other solar companies have since followed suit and launched their own micro-inverters, validating their potential and reliability.
Nine poly PV modules mounted after having the micro-inverters connected from the support rails and then to the modules.
The conduit wiring from the micro-inverters across the roof and down to the electrical control equipment.
Micro-inverters are quite small and are installed on the back of each solar panel in the upper right-hand corner. Alternatively, the micro-inverters may come separately and installed directly onto the support rails before being connected to the PV modules. Each micro-inverter is protected by the aluminum edge of the PV modules. In this way, the inverter doesn’t interfere with stacking the panels when they’re laid flat on top of one another for shipping and storage.
There has been a lot of debate on whether micro-inverters are better than central (string) inverters, but for most PV rooftop systems micro-inverters have distinct advantages.
Additionally, homeowners who are subject to shading issues should definitely consider micro-inverters for their system, as they’ll likely perform better compared to installing a central inverter. An analysis of the comparative advantages and disadvantages of the two systems will usually come out in favor of micro-inverters. For example, one central inverter would normally cover the requirements of an entire residential PV rooftop solar system (assuming that the central inverter has enough capacity for your entire array). Micro-inverters, on the other hand, sit on the back of every solar panel and offer several important benefits over central inverters.
For each individual PV system, the homeowner will want to determine whether the benefits of using micro-inverters outweigh the extra costs. Earlier it was stated that for those installations where shading is an issue, micro-inverters are preferred over a central inverter. This is because with even a small shading problem, a central inverter will negatively affect all of the PV modules in the array, whereas with micro-inverters only the shaded modules are affected.
Ultimately, micro-inverters make the PV system more modular and easier to expand. To subsequently increase the size of your solar electric system, you can simply add one or more panels with a micro-inverter for each new panel. Also, these new additional panels can be of different wattages and even from different manufacturers, two more significant advantages.
With the latest technological advancements, the new micro-inverters being developed may be able to harvest up to 20 percent more energy than central inverters over their lifetime. That’s a lot of energy! Therefore, even though they cost more initially, it appears likely that they’ll recoup the difference in a relatively short time. This also means more income from feed-in tariffs for the PV system owners who have a grid-tied or hybrid system.
Micro-inverters make a rooftop PV system or a large commercial installation more efficient. They also make it easier to monitor the power generation of each panel and enable a service technician to quickly pinpoint a failure in the system.
Micro-inverters are gaining acceptance in the solar energy market, particularly in residential applications. The market leader for many years has been Enphase, but there are a number of companies who claim to be at various stages of developing better and cheaper micro-inverters. Two relatively new companies building micro-inverters are Enecsys and SolarBridge. These two companies appear determined to be competitive; they both initially offered longer warranties than Enphase. In response, Enphase has extended their warranty period.
GreenRay, another new micro-inverter manufacturer, recently developed an interesting innovation. They have fully integrated a micro-inverter into the solar panel. Another new competitor is InfluxGreen. Their IGSM series of grid-tied PV micro-inverters (single phase 200W to 270W) appear to be cost-competitive, and the manufacturer states they can achieve a peak efficiency of up to 95 percent.
An Influx inverter—view from the front.
Although the market is rapidly expanding, it’s not likely to be large enough to support all of the existing and proposed micro-inverter competitors. The market is becoming overly competitive, and some of the smaller companies will likely drop out of the race at some point. However, this need not worry the homeowner, because different makes of micro-inverters can operate together in the same array of PV panels without any compatibility problems.
An Influx inverter—view from the back.
Micro-inverters optimize output for each solar panel—not for the entire system, as a central inverter would. This enables every solar panel to perform at its maximum potential. In other words, one solar panel alone cannot drag down the performance of the entire solar array, whereas a central inverter can only optimize output according to the weakest link. For example, shading of as little as 9 percent of a solar system connected to a central inverter can lead to a system-wide decline in power output by as much as 50 percent. If one solar panel in a string had abnormally high resistance due to a manufacturing defect, the performance of every solar panel connected to that same central inverter would suffer.
One of the tricky things about solar cells is that the voltage needs to be adjusted to the right level for maximum output of power. In other words, the performance of a solar panel is dependent on the voltage load that’s applied to the inverter. Maximum power point tracking, or MPPT, is a technique used to find the right voltage, or the “maximum power point.” With micro-inverters, MPPT is applied to each individual panel as opposed to the solar system as a whole; performance will naturally increase.
Unlike central inverters, micro-inverters are not exposed to high power voltages or to high heat loads. Therefore, they tend to last significantly longer. Nowadays, micro-inverters typically come with a warranty of 25 years, averaging ten years longer than central inverters. The 25-year warranty for micro-inverters conveniently matches the warranty period of the solar modules, particularly if the micro-inverters are installed into the PV solar modules at the panel manufacturer’s plant.
Central inverters come in limited sizes, and you might end up having to pay for an inverter bigger than what you actually need.
I recommend that you purchase or ask your solar contractor to install a monitoring system to follow the energy output of your entire PV array. But remember, with a central inverter you cannot see the output at the individual panel level. With micro-inverters, you can connect each panel to a computer monitoring system so you can easily pinpoint which panel is experiencing lower efficiency rates. This is helpful in discovering faulty panels or equipment. With micro-inverters, web-based monitoring on a panel-by-panel basis is usually available for both the homeowner and the installer. The DIY installer should invest the small extra amount required for a monitoring system. Continually analyzing the health of your PV solar rooftop system can pave the way for additional performance improvements. There are even mobile applications that enable you to monitor your PV system when you’re travelling.
The main electrical panel for the house. In this case, there is a grid-connection via a back-fed breaker in the panel. While this does serve as a disconnecting means for the panel, utilities generally require a separate, accessible, dedicated AC disconnect switch such as those illustrated earlier in this chapter under the heading “DC and AC Disconnect Switches.”
Solar panels are connected in a series before they’re fed into a central inverter, typically with an effective nominal rating of 300-600 VDC (volts of direct current); the larger central inverters can reach up to 1,000 volts DC. This current is potentially life threatening if safety precautions are ignored—for instance, if the DC disconnect switch is left in the “ON” position while maintenance is being done on the system. You should always refer to the safety data sheets for the module and the inverter and follow all safety instructions. However, micro-inverters eliminate the need for high-voltage DC wiring, which improves safety for solar installers and system owners—an important consideration, especially for DIY installers. Once again, we recommend reading the safety data sheet of whatever type of inverter you plan to install.
Micro-inverters also generate significantly less heat than central inverters do, and as a result there’s no need for active cooling. This enables micro-inverters to operate without any appreciable noise, another advantage over central inverters.
Regarding the higher initial cost, it must be said that for any given PV system, micro-inverters are a more expensive option than using a central inverter. But micro-inverters are definitely worthy of consideration due to the superior long-term benefits. In recent years, central inverters had an average cost of about $0.40/Wp (watts–peak), while the average cost of micro-inverters has been about 30 percent higher: $0.52/Wp. If the micro-inverters are already incorporated into the PV modules, then you’ll simply compare the combined cost of the modules and micro-inverters versus the cost of the central inverter plus the cost of the PV modules.
Finally, installing solar panels with micro-inverters is simpler and less time consuming, which typically cuts about 10 percent off the installation costs.
A few years ago, dual micro-inverters were introduced to the market. They essentially do the same thing as regular micro-inverters, but they convert the DC of two solar panels instead of one. This lowers the initial system cost slightly, but at the price of performance, so there may be very little net benefit. A homeowner will often ask the contractor, “Are micro-inverters, or dual micro-inverters, or a central string inverter the best choice in my situation?” It depends on the site conditions, the homeowner’s budget, and other factors. However in most situations, regular micro-inverters should be given serious consideration.
However, it’s only fair that we touch on the characteristics of central inverters before you make your final decision. First, central (or string) inverters are less expensive initially and have fewer moving parts when comparing the two systems overall. But in the long run, micro-inverters are usually more economical.
Still, the original single central inverter is very popular among homeowners and investors alike. The main reasons are familiarity and trust: Central inverters have simply been on the market longer, and they’re believed to be efficient since they have a history of proven results. A typical central inverter has a maximum efficiency rate of 95 percent, and if there are no shading issues, they perform well. Because of this, they hold great promise for large industrial-and utility-sized projects, because solar systems designed for those projects usually don’t confront shading challenges. For these larger installations, central inverters are significantly less expensive than micro-inverters.
Central inverters have only one point of failure. An analogy might be a ceiling with ten track lights versus a ceiling with a single lamp. Over the past year or so, there have been many claims about the reliability or unreliability of one technology over the other, but more time is needed to determine conclusively whether central inverters or micro-inverters are more reliable. You can easily guess which special interest group is making which claim. In any case, each installation should be analyzed separately. Both inverter systems have a valid role to play, depending on the site conditions, the size of the PV system, financial considerations, and desired system flexibility.
Though both central inverters and micro-inverters have a place in the market, micro-inverters are gaining ground and have become the inverter of choice for many residential PV solar installation companies throughout the US and overseas. We can evaluate the benefits of micro-inverters versus central inverters by looking at two numbers:
• Lifetime costs ($)
• Lifetime energy production (kWh)
These two figures, calculated for both types of inverters, are essential numbers. Divide costs by energy production and you can determine how much money you’ll pay for every kWh your solar system will produce. Every situation is different. There are several variables to take into account in order to find these two numbers.
Enecsys, one of the leading micro-inverter manufacturers, sums it up like this: “A total cost-of-ownership analysis of a PV solar system can only be carried out after detailed examination of capital and maintenance costs, and an understanding of how much energy will be harvested over the life of the system.”
However, if your solar contractor recommends using a standard central inverter for your installation, and some good reasons are provided, this is fine. The central inverter is a readily available and well-proven system.
Your central or string inverter unit is not much bigger than a typical disconnect switch, and they can be installed side-by-side for convenience and an aesthetically pleasing appearance. Both the disconnect switch and the inverter can be installed close to your main breaker panel or breaker box. Between the local grid line transformer and the main breaker panel, your electrician will install a “smart meter,” which will measure in real time the electric power provided by the PV solar system to your residence. If you have selected an in-grid system, the smart meter will also be connected to the external power grid, probably near the existing conventional electric meter.
Below are three schematic diagrams of the different available metering systems, “FiT,” “PPA,” and “Net Metering”:
The smart meter will measure and record the power in kilowatt hours consumed by your home, as well as the excess amount of power fed back into the external grid of your utility company. Thus, the term “net metering” is used to describe the net amount of power consumed. As mentioned before, most utility companies will pay you in the form of credit for any excess daily power generated from your PV system. For domestic consumers, there’s usually an upper limit to these credits that is equal to the charges for power consumed. So, theoretically, you could have some months with a net metering month-end bill of zero. To qualify for net metering, your PV system must be less than a specified maximum generating capacity, which is regulated by your electric utility. In the US, the most common maximum size cap is 10 kW for residential systems.
A schematic of a FiT meter connection..
The location of your existing breaker panel box may be where you install the PV solar DC disconnect switch, inverter, and smart meter. The breaker panel is usually installed on a wall in a garage, or in a hallway near the back door, or on a wall outside of the house.
Power moves from the DC/AC inverter to your home breaker panel box and is distributed to the rest of the household. A power meter with net metering capability is a little different from the standard meter you have now. It’s capable of measuring power going into the grid or being pulled from the grid at the end of the line, and it will measure the amount of electricity that is either consumed or being sold back to the utility company.
A schematic of a PPA meter connection.
A schematic of a net metering connection.
With net metering, safety is an issue as well. The utility has to make sure that if there’s a power outage in your immediate area or neighborhood, your PV system won’t continue to feed electricity into power lines that a lineman might think are dead. This is a dangerous situation called “islanding,” but it can be avoided with an anti-islanding inverter. Most inverters incorporate the anti-islanding protection feature.
The thought of living at the whim of the weatherman probably doesn’t thrill most people, but three main options can ensure you still have power even if the sun isn’t cooperating. If you want to live completely off the grid but don’t trust your PV panels to supply all the electricity you’ll need in a pinch, you can use a backup portable diesel generator when sunlight is low or if you wish to have power after dark or during a blackout in your area.
The second stand-alone system involves energy storage in the form of batteries. Several batteries connected together form what is commonly called a “battery bank.” Unfortunately, batteries can add a lot of cost and maintenance to a PV solar rooftop system. But installing a battery bank is a necessity if you want to be completely independent of the electric utility company grid. However, if you find that the cost of a battery bank and backup generator large enough for your requirements is excessive, then the alternative is to connect your PV solar system to the utility grid, assuming it’s available, buying power when you need it, and selling it back to the utility when your PV system produces more power than you use. A more detailed description and several images of battery banks are provided in Chapter 2.
If you decide to use batteries instead, keep in mind that they’ll have to be maintained, and they must be replaced after a certain number of years. Most solar panels tend to last at least 30 years—and improved longevity continues as a main research goal—but batteries just don’t have that kind of useful life. Depending on various factors, solar batteries might last up to 10 or 11 years before they must be replaced. Also, be aware that PV battery bank systems can be potentially dangerous because of the energy they store and the acidic electrolytes they contain, so you’ll need a well-ventilated space and a nonmetallic enclosure or rack where they can operate safely.
Although several different kinds of batteries are commonly used for PV systems, make sure they’re deep-cycle batteries. Unlike your car battery, which is a shallow-cycle battery, deep-cycle batteries can discharge more of their stored energy while still maintaining long life. Car batteries discharge a large current for a very short time to start the car engine, and then the alternator immediately recharges them as you drive. PV solar batteries generally have to discharge a smaller current for a much longer period of time, such as at night or during a power outage, while being charged during the sunny parts of the day. The most commonly used deep-cycle batteries are lead-acid batteries (both sealed and vented) and nickel-cadmium batteries, both of which have various pros and cons. Refer to Chapter 2 for more information, and, as recommended above for inverters, be sure to read the safety data sheet, or SDS, that comes with every important piece of electrical equipment. Any distributor or contractor can provide you with the SDS for any piece of equipment they sell or install.
The use of batteries requires the installation of another component called a “charge controller.” Batteries last a lot longer if they aren’t overcharged or drained too much. That is one of the functions of a charge controller. Once the batteries are fully charged, the charge controller doesn’t let current from the PV modules continue to flow into them. Similarly, once the batteries have been drained to a certain predetermined level of voltage, the charge controller won’t allow more current to be drained from the batteries until they have been recharged. The use of a charge controller is essential for long battery life.
The other challenge, besides your energy storage system, is that the electric current generated by your solar panels—or extracted from your batteries, if you choose to use them—is not in the form that is supplied by the utility, namely AC, which is also the form of current required by the electrical appliances in your home. A solar system generates DC, so you need an inverter to convert it into alternating current. As discussed earlier, apart from switching DC to AC, most inverters are also designed to protect against islanding if your system is hooked up to the power grid.
Most large inverters will allow you to automatically control how your system works. Some PV modules, called AC modules, actually have a small inverter already built into each module, eliminating the need for a central inverter and simplifying wiring issues; refer to the comparison of inverters and micro-inverters above.
In addition to your PV array of panels and mounting support hardware, the wiring, conduits, junction boxes, grounding equipment, over-current protection, DC and AC disconnect switches, net metering equipment, and other accessories identified above will round out the essential components of a PV solar system. Of course, both DIY installers and solar contractors must follow regulations governed by electrical codes; there is a section in the National Electrical Code just for PV solar systems. There may also be state regulations or municipal regulations that apply to the installation of your PV solar rooftop system.
I recommend that you contact a licensed electrician who has experience with PV solar systems to connect the electric control equipment and measuring devices during your installation. In Chapter 9, I provide links to websites that supply contact information for experienced solar contractors and PV electricians, information that will help you contact experienced solar contractors in your hometown or nearby. Generally speaking, solar contractors are willing to let DIY enthusiasts handle much of the PV solar rooftop installation process, including mounting the support system, the PV modules, and the conduits.
Once installed, a PV solar rooftop system requires very little maintenance, especially if no batteries are used, and will provide electricity cleanly and quietly for 25 to 30 years.
