In short: PVWatts is a highly useful tool that can show you a close estimate of annual electricity production from a solar array of a given size. But by using a reverse-engineering trick and dividing your annual energy usage by the amount of solar energy 1 kilowatt of panels can make, you get your ideal system size. Start at the top to learn more about how PVWatts works, or jump ahead to use the tool to size your system.

The National Renewable Energy Laboratory’s (NREL) PVWatts tool is the gold standard for estimating solar panel production without actually scanning a home’s roof with LIDAR or some other technology. Also, it’s FREE to use, unlike many of those other solutions.

By inputting a few variables, including your location, roof pitch, panel type, and system size, you can get a pretty accurate prediction of solar energy output for your solar system.

But wait, how do you come up with the solar system size that fits your needs? For the answer to that question, combine the PVWatts engine with our favorite reverse-math method to determine the ideal size for your solar array.

How PVWatts works

NREL designed the PVWatts solar calculator to take a few simple inputs and produce estimates of the performance of a solar array for each month in a year. You can use it to check whether a solar installer’s quotes are accurate by adding information about the system you were quoted.

PVWatts is so trusted, some installers use data directly from the tool in their quotes to you. The data is output for each month in an average year, and includes estimated savings from solar panels based on the current electricity prices in your area.

Here’s how the results look for a pretty standard grid-connected 4-kW system:

Where does NREL’s data come from?

Believe it or not, between the National Weather Service and NREL, the US government has been collecting solar radiation and supplementary meteorological data since 1961. Now in its second iteration, using data from 1998 to 2014, the National Solar Radiation Data Base (NSRDB) contains insolation data for everywhere on the North, Central, and South American continents between about 21 degrees South latitude and 60 degrees North.

The data is stored for about 2 million 4km² “pixels” on the surface of the continents, meaning each data area is about the size of 50 city blocks. We’ll show one of those “pixels” a bit further below when we look at the PVWatts interface. NREL keeps track of the average number of kilowatt-hours (kWh) each square meter of the earth receives in a day. Written out as “kWh/m²/day,” this number is also called “peak sun hours.”

How to use PVWatts to figure out system size

Here’s the main idea of this article: how to find your ideal solar system size based on your electricity needs.

Just follow these steps:

• Gather the data: your annual electricity usage, roof slope (angle above the ground), and azimuth (direction your roof faces in compass degrees)
• Enter variables into the PVWatts calculator
• Divide your usage by kilowatt-hours per kilowatt of solar

Gather the data for energy usage, azimuth, and roof slope

This step might sound complicated, but it’s actually pretty easy to get close to the right numbers quickly.

Finding annual electricity usage

These days, most folks have access to an online portal for their electric utility. If that describes you, you’re golden. If not, you need to look at your electricity bills. Some utility companies now print annual usage numbers on every bill, but if yours doesn’t, get ready to find your bills for the past year or two or let your fingers do the walking and call your provider.

What you’re looking for is total annual usage, measured in kWh, ideally for more than 1 year. Using your company’s online portal, here’s how the last 13 months of your electricity usage might look:

You can hover your mouse over the blue bars to see electricity usage, and record those numbers to see your total annual need. But we can get even better data by clicking the green button in the lower right there.

Using the button, you can download electricity usage numbers going back to the beginning of your account. Taking an average of all monthly usage, the example home we’re using needed 361.7 kWh per month. Multiplied by 12 months, that’s 4,340 kWh per year. We’ll use that number to find my ideal solar system size.

Roof azimuth and pitch

PVWatts also needs some information about your roof. Think about it from two perspectives: overhead and horizontal.

“Azimuth” is just a fancy way to say “direction.” It has to do with where your best roof face is pointing, and it’s measured in degrees of a circle. North is 0° azimuth, and the numbers rise as you go clockwise. Here’s a guide image with a few numbers for convenience:

North-facing roofs in the northern hemisphere are not ideal for solar panels. East (90°) is okay, west (270°) is better, but south (180°) is best. Select the number that corresponds with your roof, and remember it for the next step.

As for roof pitch, that’s the angle at which your roof rises from the edge to the top. Most roofs in the USA are pitched between 25° and 35°, which is actually good, because the optimum tilt of solar panels in the USA is about in the same range.

For the purposes of this estimate, it’s okay to select your angle based on an eyeball appraisal. If you choose a solar installer, they’ll do a much more thorough and accurate job planning your installation.

Entering variables into PVWatts

Here’s where the rubber meets the road. We’re almost done! Navigate to the PVWatts home page, and enter your address in the top bar. It’s okay to enter just your zip code if you want, because chances are it’ll be in the same 4-km² “map pixel” we talked about above. Here’s an example:

You can zoom in further and move the little dot to exactly your roof, if you’d like, but it’s not necessary. Click the “go to system info” arrow and enter the numbers we gathered above:

Enter the data like this:

• Important: enter “1” for System Size—this sets up the calculation based on production per 1,000 watts of solar panels
• Module type: Premium (please don’t use junk solar panels)
• Array type: Fixed (roof mount)
• System losses: Default is 14.08%, but we use 15% to be safe
• Tilt: Your roof pitch. 20° is default, but 30° is more likely in the USA
• Azimuth: The degrees of your roof’s direction from the last step—180° is south

Then click “Go to PVWatts Results!” The calculator gives you an estimate of total production, a likely range based on climatic factors for any given year, and monthly estimated totals:

Converting kilowatt-hours of usage into kilowatts of solar panels

The final step is simple enough to do with a pocket calculator (aka, your phone). Remember the average annual usage from our example from above? It was 4,340 kWh per year. Divide that number by the PVWatts production estimate for 1 kW of solar panels in your area. In this case, that was 1,128, and the result of the calculation is about 3.8.

So 3.8 kilowatts is the ideal size of the solar system needed for this example home.

Things you can do with the PVWatts data

Okay, time for next steps! Now that you know your ideal solar system size, you can figure out how many solar panels you need.

Also, you’ll be more informed when you get quotes from solar installers. If you’ve done the math right and ended up with a 4-kW system but someone’s trying to sell you a 10-kW system, you need to question why they’ve estimated that.

If you’re ready to get quotes, we can help you with that!

Other considerations

The calculations above assume you’ve got a house that’s perfect roof for solar power. If that’s the case, great! But if you have any shade, or your roof is made from cedar shake shingles, or it’s pitched very high or low, there may be additional considerations.

Shade can be overcome (as long as you don’t have too much) by using microinverters or panel optimizers that allow you to maximize output and keep your  solar panels working in cloudy weather and shaded conditions.

Over- or under-sizing your system, and why you would

As a final word, there are some reasons you may want to under- or over-size your solar system.

You may want to under-size if you live in a state with terrible net metering rules, because excess solar energy in those states won’t earn you much of a credit on your bill (if any). Of course, you can ensure you use all your solar power if you add a home backup battery to your solar array, but that tends to be more expensive in the long run.

You should also under-size if your usage is artificially high, because of past wasteful usage. If you’ve done your homework and recently gotten a home energy audit, your needs could be significantly smaller in the future.

On the other hand, you may want to over-size your system if you plan to increase your usage in the near future by adding central air conditioning or getting an electric vehicle. The average electric car requires 15 kWh per day to drive 50 miles, which means your electricity needs could increase by nearly 5,500 kWh per year if you want to power your electric car with solar panels.

In our example above, the homeowner only required 4,340 kWh per year before and could get away with a 3.8-kW system. By adding the usage from their EV to the mix, the usage would increase to 9,800 kWh per year, for which they’d need an 8.7-kW system.

The verdict on PVWatts

In the end, PVWatts is a great entry-level solar system calculator that can show you how to turn solar watts to kWh produced. It’s an indispensable tool for homeowners to estimate the product from potential PV installations.

Unfortunately, though, the PWatts calculator has some drawbacks to its design. Our solar calculator uses the same PV watts solar irradiation and production data - however, it is linked to the pricing tables of each utility as well as live pricing data provided by 200 solar installers throughout the US. This means that our solar calculator only requires you to know the name of your utility and the amount of your average power bill. With that information, our calculator can tell you what size system you need and how much it will cost. With the PV Watts calculator, you need to input all of this information yourself.