Use our free solar system size calculator to estimate how much solar you need for your house.
Solar System Size Calculator
- Data source: National Renewable Energy Laboratory PVWatts Calculator
- This calculator does not take into account shading.
- This calculator assumes the solar system will cover 100% of your energy usage and will be roof-mounted. (If you’d like to be able to choose the coverage percentage or mounting location, message me here. I’ll add these options if enough people are interested.)
How to Use This Solar Sizing Calculator
1. Enter your address, city, or zip code and then select your location from the search results. For this example, I’ll use the address of Los Angeles City Hall.
2. Enter your average energy usage in kilowatt hours (kWh) and then select your timeframe. You can find this number in your power bill. For instance, if you look at your last 3 power bills and see that you use, on average, 600 kWh per month, you’d enter “600” and then select “kWh per month” as your timeframe.
3. Optional: Select your roof pitch from the list. If you have a rough idea of the angle of your roof, you can always select the option closest to that. For instance, if I estimate that my roof angle is around 30°, I could select the “7:12 (30.3°)” option from the list.
4. Optional: Select your roof orientation from the list. If you have a sense for which side of your roof is best suited for solar panels, select the direction it faces from the list. If my sunniest roof faces southeast, I’d just select that option.
5. Optional: Enter the size of solar panels you want in watts (W). If I know I want 350-watt solar panels, I’d simply enter the number 350.
6. Click “Calculate Solar System Size” to get your results. In this example, the calculator estimates that I need a 4.7 kW solar system — which works out to 14 350-watt solar panels — to cover 100% of my annual electricity usage with solar.
7. Click “Get a Free Solar Quote” to get a more accurate estimate. The results from this calculator are rough estimates because they don’t take into account factors like roof size and shading. I’ve partnered with a solar advisory service called EnergyPal to give free custom quotes to readers who want more accuracy.
How Much Solar Do I Need?
Nowadays, online solar calculators tend to be more accurate than manual calculations because they can pull from historical solar radiation data. Some even use satellite imagery to calculate how much sun your roof gets.
But, for the curious, here’s a basic way to estimate your solar needs yourself. (I made a free spreadsheet calculator that you can copy and use to help with these calculations.)
1. Look through your 3 to 12 most recent power bills and get an average of your monthly energy usage in kilowatt hours. The more months you include, the better your average will take into account seasonal variations.
2. Divide your average monthly energy usage by 30 to estimate your daily energy usage. Let’s say your average monthly energy usage is 600 kilowatt hours.
600 kWh per month ÷ 30 days = 20 kWh per day
3. Multiply your daily energy usage by the percentage of your power bill you want to cover with solar. If you want to cover half of your power bill, for instance, you’d multiply your daily energy usage by 50%. This gives you an estimate of how much energy your solar system needs to produce on an average day.
20 kWh per day × 50% = 10 kWh per day
4. Find your location’s average peak sun hours. To do so, you can use our peak sun hours calculator or the following solar irradiance maps provided by the National Renewable Energy Laboratory and Global Solar Atlas. 1 peak sun hour is equal to 1 kWh/m2, so if your location averages 5 kWh/m2/day, that would be equal to 5 peak sun hours per day.
5. Divide your solar system’s daily energy production by your location’s average daily peak sun hours. This estimates your solar system size in kilowatts (kW). Let’s use a value of 4 peak sun hours in this example.
10 kWh per day ÷ 4 peak sun hours per day = 2.5 kW
6. Multiply your solar system size by 1.2 to cover system inefficiencies. There are inefficiencies in any solar system due to factors like shading and soiling. So this step is a simple way to try to account for system losses.
2.5 kW × 1.2 = 3 kW
So, in this example, you’d need a 3 kW solar system to meet half of your daily energy needs.
Note: The above steps have been modified from the US Department of Energy factsheet titled How to Size a Grid-Connected Solar Electric System.
How Many Solar Panels Do I Need?
Once you’ve sized your solar system using the steps outlined in the previous section, there are only a few more to determine how many solar panels you need.
(Another plug: make a copy of my free spreadsheet calculator to help with these calculations.)
1. Decide what solar panel wattage you want in your system. You could base this off of the available options from your brand of choice. Or you could consider your roof’s dimensions and look at panels that would fit the area. Or you could just assume a common solar panel wattage, such as 300 watts.
2. Convert your solar system’s size to watts. To convert kilowatts to watts, simply multiply kilowatts by 1,000. (I’ll use the solar system size we calculated in the previous section.)
3 kW × 1,000 = 3,000 W
3. Divide your solar system size (in W) by your desired panel wattage. For this example, I’ll use a solar panel wattage of 350 watts.
3,000 W ÷ 350 W = 8.57 panels
4. Round up to the nearest whole number.
8.57 rounded up = 9 panels
So, in this example, you’d need 9 350-watt solar panels for a 3 kW solar system on your roof.
3 More Ways to Calculate Solar System Size
Besides our solar sizing calculator at the top of this page, here are 3 more free tools you can use to calculate solar system size:
- Google Project Sunroof
- PVWatts Calculator
- Global Solar Atlas
Let’s run through how to use each tool. Again, I’ll use the address of Los Angeles City Hall as an example.
1. Google Project Sunroof
Project Sunroof is another one of those excellent free Google products. It uses satellite imagery to estimate how much sun your roof gets and calculate solar system size and savings.
1. Go to Google Project Sunroof.
2. Enter your address and select it from the search results. Project Sunroof only works with US and Puerto Rico addresses at the time of publishing. (If you live elsewhere, keep reading for alternative tools.)
3. Input your average monthly power bill to fine-tune your estimates.
4. Get your estimated solar system size. Project Sunroof also estimates costs, savings, and environmental impact.
5. Learn more about how to go solar or click “Search for Solar Providers”. Dive deeper by reading Project Sunroof’s guide on the solar buying process, or look at local installers. Clicking the “Search for Solar Providers” button just searches Google for “solar installers near me”, which isn’t a half bad way to get started.
2. PVWatts Calculator
PVWatts is a more technical solar calculator from the National Renewable Energy Laboratory. It has data for plenty of locations around the globe, so it’s a good tool to try if you don’t live in the US or Puerto Rico.
1. Go to PVWatts.
2. Enter your city or address in the search bar and click “Go”.
3. On the Solar Resource Data page, scroll down to the map and confirm that the calculator selected the right location. If it did, click “Go to system info.” If it didn’t, click “Change Location” at the top of the page and try again.
4. Select your array type, input your array’s tilt and azimuth angles, then click “Go to PVWatts results”. If you’re putting your panels on your roof, your array type would be “Fixed (roof mount)”, your tilt would be your roof pitch (in degrees), and your azimuth angle would be your roof orientation (in degrees clockwise from north). Leave the “DC System Size” field as the default value for now.
5. Record the estimated system output. This is an estimate of how much a solar system of PVWatts’ default 4kW size would output at your location.
6. Go back to the System Info page and experiment with different values in the “DC System Size (kW)” field until you find the system size that generates your desired amount of electricity per year. For instance, let’s say I want a system that produces around 5,000 kWh per year. In that case, I’d experiment with different values until I settle on 3.2 kW, which would generate an estimated 5,050 kWh per year.
There you go!
3. Global Solar Atlas
The Global Solar Atlas is another free tool that can also help you estimate your system size. It isn’t as robust as PVWatts, but it is a little easier to use.
1. Go to the Global Solar Atlas.
2. Enter your location in the search bar and select it from the search results. Or just click on your location on the map. I’ll use Sydney, Australia as my location for this example.
3. Select what kind of PV system (i.e. solar system) you want. I selected the “Small residential” option.
4. Click “Change PV system”, input your azimuth and tilt of PV panels, and click “Apply”. Again, your azimuth would be your roof orientation (in degrees clockwise from north) and your tilt would be your roof pitch (in degrees). Since Sydney is in the southern hemisphere my azimuth is closer to north in this example.
5. Record the annual average output of this system size. It will be in either kilowatt hours (kWh) per year or megawatt hours (MWh) per year. 1 megawatt hour is equal to 1000 kilowatt hours.
6. Click “Change PV system” again and experiment with different values in the “System size” field until you find the size that generates your desired amount of electricity per year. I tested out a few different sizes trying to find which one output around 6 MWh (6,000 kWh) per year. I eventually found that a 4.1 kW system would do the job.
You’ll Probably Need to Talk to Solar Installers Eventually
Online solar calculators can only get you so far. Unless you plan on installing your system yourself, you’ll eventually need to have a local solar installer assess your property’s solar potential and put together a quote.
There are many ways to go about getting quotes from local installers, but I’ll walk through my preferred way.
1. Go to EnergyPal. They’re a solar advisory service that walks people through the solar buying process and helps get free quotes from local installers.
2. Go through their form’s steps and click “Get a Free Quote” at the end.
3. Wait for your quotes to arrive. Someone from EnergyPal may reach out to see if your house is a good fit for solar and find out what sort of solar system you’re looking for. If it’s a match, they’ll mock up a system design and start work on getting you some quotes.