In some ways, installing solar panels is unlike other home improvement projects. Adding renewable energy sources to your house has an aspect of social good to it, because you’re using cleaner energy to power your lifestyle, reducing your energy and pollution footprint. And solar energy can also directly impact your budget by lowering the price you pay for electricity.
But in other ways, it’s just like any other project: You have to work out a budget for it. Installing solar panels can be an expensive proposition. The average cost is about $25,000, which is a significant price tag for most people. That’s why the first question you have to answer when you decide the time has come to go solar is: How many solar panels do I need?
This isn’t just about cost. The answer is crucial in terms of every aspect of the project, from how much space you’ll need to ensuring you get enough power from your panels.
Gather the data
You’re going to need all that grade-school math you thought you’d never need, because it's time to do some calculations:
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Your power needs. The first step is to know how much power you need to generate. The average home in the U.S. uses about 11,000 kilowatthours (kWh) each year, or about 900 kWh every month. But that’s just an average; your utility bill will give you a much more accurate sense of how many kWh of electricity you use every month. If you can see 12 months' usage, you can average that out to a number that takes into consideration seasonal variations in electricity use. Then divide that average by 30.42 (the average number of days in a month) to get your daily usage. For example, if you use 900 kWh per month, your daily usage is roughly 30 kWh.
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Sun exposure. The amount of sunlight your home gets every day is kind of a crucial aspect in solar energy. This varies greatly across the country, with areas in the Southwest getting an average of around six hours per day, and areas in the Northeast getting less than four hours (don’t forget, this isn’t just daylight hours—cloud cover and weather reduce the amount of sun exposure). Knowing how much sun you’re likely to get will tell you how much energy your panels will need to generate on a daily basis. For example, if you need 30 kWh of juice to power your home each month and you get five hours of sunlight every day on average, 30kWh / 5 hours = 6 kWh needed every day.
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Panel wattage. The amount of energy the solar panels you’re installing can produce is key. Most modern solar panels can output between 350 and 450 watts, or 0.35 to 0.45 kWh, of electricity from the sunlight they receive. That’s an average of about 0.40 kWh.
Fill in the formula
Now that you have all your data, it’s relatively simple to estimate how many panels you’ll need. The formula is
Daily kWh needed / per panel kWh
Using our numbers from above (we need 6 kWh each day using panels that produce 0.40 kWh), that translates to
6 kWh / 0.40 kWh = 15 panels
When doing these calculations, it’s a good idea to round everything up so you end up with slightly more than you need instead of slightly less. And keep in mind that a professional installer will make much finer calculations based on a lot more data (see below), so this number should be used as a rough estimate for determining costs and feasibility only.
Other factors
Your back-of-the-envelope calculation won’t be as accurate as a professional’s estimate; they’ll be taking something called Production Ratio into consideration. The Production Ratio applies to the whole system, and ranges between 1 and 1.8 or so. For example, a 10 kWh system that produces 16 kWh worth of electricity would have a PR of 1.6. A 10 kWh system that produces just 10 kWh would have a PR of 1.0.
Production Ratio is a more complex calculation that includes not just average sun exposure but also the shade around your house, the orientation of the panels (which may depend on where you can fit them), the geographical features on and around your property which may affect sun exposure, and other factors. If the PR is lower than expected, you might need to add some panels to get to the necessary energy production.
Finally, keep the physical size of solar panels and where you’ll be installing them in mind. Standard solar panels are about 66 x 40 inches give or take an inch or two, or about 17 and 7/8 square feet. If you need 15 of them, you’ll need approximately 268 square feet of space for them, so make sure that their intended location (your roof, or yard if it’s a ground-grid system) has enough space.
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