By now, photovoltaic (PV) panels are familiar to most Americans. You’ve seen them on your hand-held calculator, on top of illuminated highway signs, and maybe even on your neighbors’ roofs. With PV systems becoming more common, perhaps you’ve been dreaming of making some homemade electricity. The dream is achievable, as long as you own a sunny patch of lawn or an unshaded south-facing rooftop, and as long as you have a bank balance of several thousand dollars.
A PV array is made up of rectangular modules (or panels) that measure between 2 and 5 feet on a side. The most common type of PV module has an aluminum frame and a glass cover protecting a collection polycrystalline PV cells. When exposed to light, each PV cell produces 0.5 volt DC — so if you add up the number of cells and divide by 2, you know the voltage of the module. The best performing commercially available PV cells are roughly 20% efficient at converting solar energy into electricity.
Unlike polycrystalline PV cells, thin-film (amorphous) PV products are manufactured on a flexible sheet. These thin-film PV products have many applications; for example, they are used to make PV roof shingles and peel-and-stick membranes designed for use on metal roofing. Thin-film PV products have relatively low efficiencies — usually in the range of 10% to 12% — so they require almost twice the area required for a polycrystalline PV array with the same electrical output.
Both polycrystalline and thin-film PV arrays produce DC power. This DC electricity can be used directly to charge a battery; in most homes, however, the solar electricity is sent to an inverter that converts the DC power to AC. The inverter’s AC output can then be used directly by the homeowners or fed into the power grid.
Most grid-connected PV systems won’t provide any electricity during power outages, for two reasons: first, the type of inverter used for a grid-connected PV system won’t operate when the grid is down, and second, you can’t provide your home with electricity at night unless you have a big, expensive battery.
Of course, you can buy a battery and a special inverter if you want to, but that will add at least $6,000 to $15,000 to the cost of your PV system. Most homeowners who want backup power conclude that a gas-powered generator is cheaper than a big battery system and a special off-grid inverter.
Designing a PV system
The basics of PV system design can be quickly summarized with a few rules of thumb:
- Although some homeowners size a PV array to meet a specific electrical load, it is far more common to size a PV array to meet a specific budget. Residential PV systems now cost about $4.50 a watt, although prices can be higher or lower, depending on many factors.
- Polycrystalline PV arrays have a peak rating of 10 to 12 watts per square foot, while amorphous PV arrays have a peak rating of 5 to 6 watts per square foot.
- Solar electric potential varies by climate, from an average of 0.029 kWh per square foot per day in Seattle to an average of 0.049 kWh per square foot per day in Phoenix.
- A 1-kW PV system will generate an average of 970 kWh per year in Seattle and 1,617 kWh per year in Phoenix. (A useful free online tool for estimating the output of a PV array in different U.S. locations is PVWatts.)
- It’s almost always cheaper to buy very efficient appliances and a small PV array rather than ordinary appliances and a larger PV array sized to handle the increased load.
- Most buildings have a roof that is too small to accommodate a PV array sized to supply all of the building’s electricity.
Where should I mount the array?
A PV array can be roof-mounted, ground-mounted, or building-integrated. Most roof-mounted modules are installed on aluminum racks. These racks are best installed on an unshaded, south-facing roof parallel to the roofing, with an intervening air space of 3 to 4 inches. The air space under the array helps lower PV module temperatures; cooler modules produce more electricity than hotter modules. Maximum PV production usually occurs on clear winter days; ideal conditions require snow on the ground (but not on the modules) and a few fluffy cumulus clouds to reflect additional sunlight on the solar array.
Roof-mounted arrays dominate the PV retrofit market, but they aren’t the only option. Installing a ground-mounted array avoids one of the major drawbacks of a roof-mounted array — the need to disassemble the array when the roofing needs to be replaced. It’s also usually easier to remove snow from a ground-mounted array than a roof-mounted array.
Ground-mounted arrays require a site without any nearby trees or buildings to the east, south, or west. Such an array can be installed at a fixed angle or on a pole-mounted tracker that automatically adjusts the array’s angle as the sun moves across the sky.
Although trackers can increase the output of a PV array by 15% to 30%, they add complexity, cost, and potential maintenance headaches. Many PV installers advise homeowners to use the money that they would have spent on a tracker to simply buy more PV modules; in many cases, the end result is a simpler system with about the same electrical output.
PV roofing
PV arrays can be integrated into a variety of building components, including roofing, vertical façade components, translucent glazing, and awnings. Of these, roof-integrated PV arrays are by far the most common.
Manufacturers sell PV roofing products designed for integration with concrete tile roofs, asphalt shingle roofs, metal roofing, and low-slope membrane roofs. Since these products don’t stand proud of the roof, they make residential PV arrays less conspicuous; however, all of these roof-integrated products are more expensive than a conventional polycrystalline PV array.
Net metering
Many states have net-metering regulations that compel utilities to offer two-way electricity meters to customers with renewable energy systems; these meters credit customers for any on-site electricity production. While there is currently no federal mandate requiring utilities to offer net metering, 42 U.S. states have established net-metering mandates or guidelines. The details of these diverse net-metering programs vary widely.
The Network for New Energy Choices, a New York nonprofit group, issues an annual report, “Freeing the Grid,” that rates states on the friendliness of their net-metering laws and interconnection standards. According to the most recent report, the states with the best net-metering and interconnection regulations are Delaware, Massachusetts, and Utah, while the worst states are Georgia, Minnesota, Oklahoma, and South Carolina.
Here’s an example of how net metering usually works: If you have a PV array that generates 200 kWh during a month when your home uses 500 kWh, you’ll receive a bill for only 300 kWh. Some utilities roll over credits indefinitely, but most allow credits to roll over for only 12 months.
In most cases, homeowners can’t get a check from the utility for excess power production. That’s why grid-connected PV systems are rarely sized to generate more electricity on an annual basis that a home is expected to use.
PV payback
With Chinese factories now churning out PV modules at a furious pace, the cost of PV modules has been dropping fast. As Jesse Thompson, an architect in Portland, Maine, pointed out in his recent GBA blog, PV Systems Have Gotten Dirt Cheap, the installed cost of a grid-connected PV system now ranges from $4.10 and $4.50 a watt. At that price, PV-generated electricity is now cheaper than grid power in many areas of the country.
The financial case for PV systems is boosted by a variety of federal, state, and local incentives. Every U.S. taxpayer is eligible for a tax credit equal to 30% of the cost (including labor) of installing a residential PV system. New homes as well as existing homes are eligible for the credit, and there is no upper limit to the size of the credit.
Some state and local governments (and some local utilities) also offer further PV incentives. To learn more about the incentives available in your area, check out the Database of State Incentives for Renewable Energy (commonly known as the DSIRE website).
Although Jesse Thompson calls PV systems “dirt cheap,” most Americans swallow hard when they learn how much they cost. The typical zero-energy home needs a PV system rated at 5 to 10 kW; such a system costs between $21,000 to $42,000. Of course, tax rebates and utility incentives can significantly reduce that cost.
Let’s say you can afford a 6-kW PV system; how much electricity will that system produce? The answer depends on your location. In Chicago, Illinois, such a system would generate an average of 7,056 kwh per year — or $1,086 worth of electricity at the local rate of 15.4¢ per kWh. So if the system costs $17,500 after your federal tax rebate, you’ll break even if the system lasts about 16 years — assuming, of course, that you don’t have any maintenance costs.
Rising electricity costs would shorten the payback period, and if the system lasts longer than 16 years, you end up with cheap electricity. (For more information on payback, see Payback Calculations for Energy-Efficiency Improvements.)
Homeowners don’t have to make their own power
Before you invest thousands of dollars in a PV system, you need to ask yourself whether you really want the responsibility of maintaining power-generating equipment. Many energy experts argue that electricity generation is best done on the scale of a neighborhood or town rather than a single building.
Most homeowners have no interest in troubleshooting inverter problems or figuring out how to dismantle their PV array when it’s time for a new roof, so they’re happy to leave the job of power generation to their local electric utility. That’s just as well, since utility-scale wind or solar projects are almost always more cost-effective than residential-scale PV systems.
For some building owners — especially those willing to make a significant investment to reduce their carbon footprint or those living in states with generous PV subsidies — investing in a PV system makes sense. Many hobbyists get a kick out of watching the meters on their PV system spike on a sunny day, and some homeowners appreciate the security that comes from paying up front for 30 years’ worth of electricity.
Of course, an on-site renewable energy system is not a prerequisite for green construction. “I always tell clients that solar is the last thing I want you to do,” said Steven Strong, the president of Solar Design Associates in Harvard, Massachusetts. “Build the envelope with the best materials you can. Buy the best windows — don’t even tell me what they cost. I don’t care.”
Remember: many energy-efficiency measures — including air sealing work and investments in energy-efficient appliances and lighting — have a much faster payback than a PV system. Such measures, often referred to as “the low-hanging fruit,” should always be implemented before you make an investment in PV.
What about an off-grid system?
If you’ve ever dreamed of building or owning a zero-energy house, perhaps you’ve also dreamed of living off the grid. Here’s the fantasy: you build a cabin in the woods with its own well and septic system, and you obtain all of your electricity from a PV array on the roof. What could be better? You’re self-sufficient!
Well, not quite. In most climates, it’s actually quite difficult (and expensive) to be electrically self-sufficient. First of all, you’ll need a bank of lead-acid batteries to run your appliances at night and on cloudy days. Most off-grid homeowners pay between $1,200 and $8,000 for a set of batteries. The batteries usually have enough capacity to run the house for only two or three days. And they don’t last very long; every 8 or 9 years, you’ll have to invest in a new set.
If you anticipate three weeks of cloudy weather in November — a typical occurrence where I live in northern Vermont — you could theoretically purchase a battery bank large enough to get your house through three cloudy weeks. However, the batteries would probably cost $40,000 or more — far more than a gas-powered generator and a lifetime supply of gasoline. That’s why most off-grid homeowners don’t size their battery bank to get them through the winter.
Another problem with an off-grid PV system is that much of the electricity produced during the summer is wasted. How is that? Since most off-grid homes don’t have an air-conditioner — air conditioning uses far more electricity than the typical off-grid array can supply — these homes don’t require much electricity during the summer. Days are long, and the need for lighting is greatly reduced. So on a sunny June day, the battery systems of many off-grid homes are completely full by 10:00 a.m. At that point, the system’s charge controller disconnects the PV array from the battery, and all of the electricity produced for the rest of the day is wasted.
Grid-connected systems solve the summer problem as well as the winter problem. During the winter, when sunlight is rare, grid-connected homeowners can buy electricity from the local power company. During the summer, when their PV array produces a surplus, they can sell the power to the grid. It’s a win-win situation.
The last $10,000
When energy nerds gather for conversation, they often discuss how to spend “the last $10,000” when designing a new home. As PV systems continue to drop in price, designers need to keep their pencils sharp and rethink their assumptions about the last $10,000.
The “last $10,000” question assumes that you’ve already designed a “pretty good house,” meaning that you’ve paid attention to air sealing, have installed insulation in a conscientious manner, and have installed windows that at least meet Energy Star requirements.
Ten years ago, the answer to the “last $10,000” question almost always involved more insulation or better windows. These days, however, it often makes sense to spend the last $10,000 on a PV array.
So, the next time a client asks you a question about “last $10,000,” you should be ready with an educated answer that applies to your climate and your local utility costs. Before you recommend installing R-60 attic insulation, triple-glazed windows, or a condensing gas water heater, do the math. The answer you get may surprise you.