Birds eye view of residential street:
You're standing in the driveway looking up at your house. You've decided to go solar but have no clue on how large a system your roof can support. Questions are myriad: Where should I put the panels? How many kilowatt-hours will the system generate? How many solar panels do I need? Why type of panels should I buy?
Before we answer these queries, let's start with the basics: Solar, or photovoltaic (PV) cells, are electrical devices (usually 6 inches by 6 inches) that convert the sun's energy into electricity. In order to do this in any meaningful way, 60 to 72 cells have to be connected to one another in a solar panel. These panels come in different lengths, widths, and weights. For residential use, the most common panel is 65 inches by 39 inches, while depth can fluctuate between 1.4 inches to 1.8 inches. You can see the specifications for all residential panels available in the US in 2018 on SolarReviews and you can even compare the specifications and efficiency of solar panels for homes side by side.
In addition, not all solar panels are created equal, so they won't all generate the same amount of power. Solar panels are rated based on the watts they generate. The higher the wattage rating, the more power your solar module will produce. Most residential solar panels have power output ratings from 250 to 400 watts depending on the panel size and how well they convert sunlight into energy. A typical solar panel produces 290 watts. One manufacturer sells a panel that will generate 445 watts.
Because there is such a wide range of quality and efficiency, it is difficult to generalize about which panels might be right for you. For example, in New England, where sunlight is often a commodity especially in the winter, homeowners will often purchase higher-rated panels that generate more electricity than those living, say, in sunny Arizona, Nevada, or California.
The upshot of all this is that no matter where you live, more efficient panels generate more wattage, which means the fewer panels you have to install on your roof.
No One-Size Fits All: How many square feet of solar panels do I need for a solar roof?
Residential PV systems can be as small or as large as you want or need. There's no one-size fits all, however, so each has to be custom designed and built. Large PV systems, of course, produce the most electricity and allow you to save the most money. Large systems also reduce your carbon footprint more than a smaller system. Yet, even if you can afford to install the largest PV system, you might be limited by the size of your roof.
Before you climb a ladder and start measuring, it's important to figure out how much solar power you actually need. First, pour through a year of electric bills to determine how much electricity in kilowatt-hours (kWh) you use. That will give you a good indication of which sized system you might want to install. The average U.S. household uses about 11,000 kWh of electricity a year.
While you may want your PV system to offset your entire energy usage, it might not be wise or practical. First, large systems are expensive to install even with incentives. Second, limited roof space can be a problem. If that turns out to be the case, you have to maximize the yield from your solar panels by deciding which part of your roof receives the most sunlight. In nearly all cases that's the southern part of the house.
However, not every region, or home for that matter, lends itself to such optimal conditions (you might not have a south-facing roof, for example, or the south side of the house is shaded with trees). While south-facing roofs are best, anything in a 180-degree arc between east and west is okay, too.
Rule of Thumb: How many solar panels will I need?
The next thing to determine is how many panels you will need. One rule of thumb is every square foot of roof space generates 15 watts of solar energy, but that's if the sun and the panels are in the right place and the panels are a standard rating. Most residential solar electric systems require between 100 square feet for small "starter" systems and 1,000 square feet for larger systems.
On average, the typical homeowner in the United States will need to install 28 to 34 solar panels, but again, this depends on the rating of the panels. Remember, it is best to optimize the sun's light. Although it's okay to install solar panels in an area on the roof that doesn't receive direct sun, you will need to figure out how much it will reduce the total power output, or have your solar installer do that for you.
Roof Size: How big of a solar panel system do I need?
Finally, check to see if your roof can accommodate the system you want. The average residential PV system installed in the United States in 2018 has a peak DC capacity of 6 kW. Since the most common solar panel size is 65 inches by 39 inches (5.41 feet by 3.25 feet), or 17.5 square feet, if your panel is rated at 265 watts, you would need to install about 23 solar panels for that sized system, which will take up a total of 400 square feet. You could also install "high-efficiency panels," which cost a bit more but generate more energy per square foot. Today, the most efficient residential solar panel on the market is 400 watts. If you used 400-w panels, you would only need 15 panels to get your 6-kW output but each panel would contain 72 solar cells and so would be physically bigger and so would still take up around 360 square feet.
You also need to be aware that states often mandate a "solar panel setback" for safety. The setback is the amount of clear space around the edges of the roof. In California, for example, the setback is 3 feet, which can eat up a lot of valuable roof space. Setbacks can eliminate about 25 percent of total roof space, leaving only 75 percent of your south-facing roof available for the PV system.
If you want to calculate how many panels you will need and how much solar energy production your roof can support, this formula will give you a good indication.
Multiply the square footage of your roof by .75 to account for the setback. (Remember, only a portion of your roof can accommodate a PV system.)
Next, divide the answer by 17.5, the average size in square feet of one solar panel. That will give you the number of panels that will fit on your roof.
To figure out the size of your system, complete these two additional steps.
Since PV systems are measured in kilowatts, you need to divide the number of watts produced by each panel by 1,000. If each panel generates 290W, for example, each panel produces .290 kW.
Next, multiply the kilowatt hours by the number of panels that will fit on your roof. The answer will be the maximum size of the system you can install.
If you don't want to do the math, use this calculator. Here are some examples that we plugged if for you: If you wanted to install an 11kW system in western Connecticut, but only have a 600 square foot roof, you won't be able to accommodate that size. In fact, the largest system you can install on that sized roof is 9kW. However, if you live in Phoenix, you can install a 12kW system on an 800 square foot roof.
If you have a small roof or a roof that is irregular in shape or partially shaded by trees or other buildings, you might want to consider smaller, more efficient panels. They cost more, but you should be able to achieve your desired energy goals.
How much will a solar power system that fits on your roof and produces the right amount of electricity for your usage cost?
We have gone to lengths above to point out above every house will have a different optimal solar power roof solution based on your available roof space and power use.
You can work out this solar system size for yourself using the information above. Once you have a size in mind then you can look at websites that disclose average residential solar panels cost and are up to date for 2018 prices. Once you have an idea of the average cost per watt of solar in your city you can then choose from the top brands of residential solar panels in 2018 and the best manufacturers of inverters to come up with the system you want to buy. The equipment you choose for your solar roof can vary the cost by 10-30% and so it is a significant part of the decision making process.