How to Use the PVWatts Calculator to See Your Monthly Savings From Solar Power

Find Out How Much You’ll Save with Solar Energy

Take me straight to how to use the calculator!

Give me some numbers!

That’s what everyone really wants, right? You don’t want some salesman making vague promises or giving you data from a house in Arizona when you live in Alabama.

The NREL (National Renewable Energy Laboratory) created the PVWatts Calculator to help you find the actual savings you can reasonably expect when you install new solar panels –at your location.

The PVWatts calculator will tell you three useful amounts – all for each month of the year:

  • How much solar radiation hits your location
  • How much AC energy, in kWh, you can expect for the system size you want
  • How much money you’ll save in “energy value”

Of course – these are projections. Perfect predictions are impossible. But their calculator is about as specific as you can get, pre-purchase. It takes into account the most important factors, including:

  • Your location, and the weather that comes with it
  • The size of the system you want to install
  • The tilt of your roof
  • Which way your house faces
  • The type of solar panels you’re considering installing
  • And much more

As specific as that list is, it also might make you a little nervous. It sounds complicated, and may make you wonder:

Is the PVWatts Calculator Easy to Use?

Yes and no. It’s easy to use in that the interface is really simple, and they give additional information to help you understand it. But it does require you to know a few things.

When you go to the calculator at, you’ll see this screen:

PVWatts calculator main screen – enter your address

To access the PVWatts calculator, you have to enter the address where you intend to install your solar panels. Type it in the space where you see the arrow pointing to the red circle.

They want this for very good reasons, and it’s not about sending you advertising. They need it because the location and orientation of your home or business will affect the amount of sunlight (solar radiation) that hits your panels.

Click here for 9 questions you need to ask before buying solar panels.

They also take into account the weather history at your geographic location. Amounts of cloud cover, how often, and in which months, affects the amount of solar radiation you receive.

So type in the address where you’ll be installing your solar panels. The next screen shows you the location you entered. Make sure it’s correct on the map below. Then, click the arrow on the right, which will take you to the actual calculator. (see red circle and arrow below)

location and weather data will give accurate values for your home, farm, or business

How to Use the PVWatts Calculator

Now you’re ready to use the actual calculator and get your numbers for monthly solar radiation, AC energy in kWh your solar panels will produce, and energy cost savings at your location.

You should see the following screen, which is the only page you have to complete:

PVWatts calculator with all the parameters you need to fill in

On this screen, the NREL has entered default values for all these fields. We’re going to take you through each one to help you understand what they mean, and how to fill it in.

DC System Size

This will play the greatest role in the numbers you get from the calculator. Most residential systems will be less than 10kW. Commercial systems and large farms can be quite a bit larger, even into the megaWatts range (1 MW = 1000 kW).

How do you know how big your solar power system should be? Eventually you’ll need to talk with an expert. But if you look at your power bill, you can see how many kilowatt-hours you’re using each month.

A 1 kW solar energy system means it produces 1 kilowatt of energy per hour – of direct sunlight. So in southeastern states like Georgia, South Carolina, Florida, and Mississippi, you get a lot of sunlight. If you average 8 hours of direct sunlight per day, then a 1 KW solar system will produce 8 kWh of energy per day. Multiply that by 30 and you get your kWh per month.

Here’s a simple formula you can use:

x KW system * hours of sunlight per day * 30 = approximate kWh per month

See how much sunlight Georgia averages per day

If you look at your current bills, you might want to do this in reverse to find out how many total kWh you need to produce. If you want to produce 300 kWh of solar energy per month, then divide that by 30 to get your kWh per day. Then divide that by the average hours of sunlight per day. That tells you about how large a solar power system you’ll need to get free electricity.

Module Type

Here, you have just thr