How many solar panels does your home need? If you’re thinking about going solar, that question just might have crossed your mind. But how do you calculate that number? And what factors go into the calculation? Get ready to explore the ins and outs of how many panels would be best for your rooftop!
How many solar panels do I need?
Start with these factors to think through how many solar panels would work for your roof:
- Sunlight in your location
- Solar panel output
- Solar panel size
- Your electricity usage
How much sunlight does your home receive?
With just over 200 days of sunlight a year, the DC area is excellent for solar. And it’s important to remember that solar panels can still generate power on cloudy days too, just not quite as much. Besides days of sun a year, the solar industry typically considers peak sun hours. What are those? They aren’t every single hour in a day when the sun in shining, but when the sun shines bright enough to produce 1,000 watts per square meter. The DMV region receives an average of 4.0 to 4.9 peak sun hours per day, depending on the time of year. That’s good, since estimates say a location should receive at least four peak sun hours to work well for solar.
What’s the output of your solar panels?
How much power your particular panels produce matters because it’ll give you an idea of how much power your array will generate, and how much of your current electricity use (more on that in a minute) your panels will offset.
The average solar panel wattage is 350 watts, and solar panels range from around 250 to 400 watts or slightly above. Uprise typically installs 400-watt panels, so we’ll go ahead and use 400 watts in our calculation below, but feel free to ask about the wattage of the panels included in your design.
What are the size of your solar panels?
Solar panel size will help determine how many panels can physically fit on your roof. Solar panels are typically around five feet long and three feet wide.
This factor doesn’t matter quite as much, unless your roof is oddly shaped or doesn’t have much room available. We’d be more than happy to take a look at your roof if you’re not sure, and we also wrote a blog post on which roofs work best for solar if you’d like to dig more into the topic.
How much electricity do I use?
You can figure out your current electricity usage in kilowatt-hours (kWh) by logging into your utility account. For example, if you have Pepco, log into your account on the Pepco website. On the banner along the top, hover over “My Account” and select “My Usage.” Then click “View My Usage.” A graph will pop up, likely showing your energy usage over one billing cycle. If you want to get an idea of how much electricity you consume in a year, right above the graph, change “Bill View” to “Year View.” Then you can see how much power you used each month in kWh. If you add up those 12 numbers, you’ll get your current electricity use.
According to the U.S. Energy Information Administration, American residential customers used an average of around 889 kWh a month, and 10,791 kWh a year.
Putting it all together: how many solar panels do I need?
It’s the moment of truth! We’ll bring all the information above together through this calculation from MarketWatch:
- Take your annual electricity use and multiply that by 1,000
- Divide that number by your peak sun hours times 365
- Divide that number by one of your solar panels’ output
- That number is how many panels you need
Let’s work out this calculation with some average numbers. If the average American family uses 10,791 kWh, multiplying that by 1,000 gets us 10,791,000. We’ll be conservative with our peak sun hours, using 4. So dividing 10,791,000 by 4 times 365 gets us 7,391.1 (rounding up that decimal place). Let’s say our solar panel output is 400 watts, so we’ll divide 7,391.1 by 400 to get 18.48. So we’re looking at 18 to 19 panels.
Again, how many we can physically install depends on the size of your roof, and which parts of it receive sunlight. According to EnergySage, the average home in America could offset their power bills with 17 to 25 panels.
I still have solar questions!
Curious about how many panels would work best for your home, or have other solar questions? We’d be more than happy to chat, and create a personalized design for you. Reach out at (202) 280-2285 or schedule a consultation here.