Solar Irradiance Calculator (with Map)

Use our solar irradiance calculator — or jump to our solar irradiance map — to easily find out how much solar radiation your location gets.

Solar Irradiance Calculator

Calculator Notes

• Data source: National Renewable Energy Laboratory PVWatts Calculator
• The calculator does not take into account shading.
• The calculator assumes you will be using a solar array with a fixed tilt and azimuth angle, rather than one with 1-axis or 2-axis solar tracking.

How to Use This Calculator

1. Enter a location such as your address, city, or zip code. For instance, if you live in San Francisco, CA, you can simply type in “San Francisco.”

2. Select your location from the autocomplete results. In this example, you’d select “San Francisco, CA, USA” from the results.

3. Optional: Enter the angle at which your solar panel(s) will be tilted. For instance, if your solar panels will be tilted at 30° from horizontal, you’d enter the number 30.

Note: If you don’t know which angle to tilt your panels to, you can use our solar panel angle calculator to find the best angle for your location.

4. Optional: Enter the azimuth angle (direction) your solar panels will be facing. For instance, if your solar panels will be facing southwest (i.e. 225° clockwise from north), you’d enter the number 225.

Note: You can use our solar panel azimuth calculator to find the best direction to face your panels.

5. Click “Calculate” to get your results. In this example, your solar array would receive on average 5.5 kWh/m²/day of solar energy.

Solar Irradiance Map

Here is a solar irradiance map of the United States provided by the National Renewable Energy Laboratory:

And here is a global solar irradiance map provided by the Global Solar Atlas:

Solar irradiance is useful when determining how many solar panels you need. Knowing how much sun your location gets is a key step in estimating solar system size.

There are multiple ways to measure solar irradiance. The main two you’ll see are Global Horizontal Irradiation (GHI) and Direct Normal Irradiation (DNI).

GHI is the most relevant for solar panels because it includes sunlight that directly hits a surface (direct irradiation) and sunlight that is scattered by the atmosphere (diffuse irradiation). Also, GHI is measured at a surface horizontal to the ground — hence the “Horizontal” in “Global Horizontal Irradiation.”

DNI, on the other hand, only measures sunlight that directly hits a surface. And it is measured at a surface perpendicular to the sun, which means it must be measured by tracking the sun, something which many solar installations don’t do.

The Global Solar Atlas also provides a measurement called Global Tilted Irradiance at optimum angle (GTI_opta, or just GTI). This is a measurement of the solar irradiation that would reach a solar system whose angle is fixed and set to the optimum tilt angle for its location.

3 More Ways to Calculate Solar Irradiance

I’ll run through 3 more free tools for calculating solar irradiance for your location:

1. Global Solar Atlas
2. PVWatts Calculator
3. National Solar Radiation Database Viewer

Let’s look at how to use each one.

1. Global Solar Atlas

The Global Solar Atlas is the best solar map I know of. It’s easy to use and has scores of solar data for nearly every spot on the globe. Here’s how to use it to calculate solar insolation at your location:

1. Type your location in the search bar and select it from the autocomplete results.

2. Locate Global Horizontal Irradiation (GHI) in the Site Info section. This is the estimated solar irradiance your location receives per year.

3. Change the results from “Per year” to “Per day” to get your average daily solar irradiance.

Simple!

2. PVWatts Calculator

The PVWatts Calculator is a free solar calculator provided by the National Renewable Energy Laboratory. It’s a great tool for estimating energy production of a solar power system. It can also be used to calculate solar irradiance for your location. Here’s how:

1. Enter your city or address in the search bar and click “Go.” For this example, let’s say you live in Denver, CO.

2. 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.

3. On the System Info page, enter your array type, tilt and azimuth then click “Go to PVWatts results.” Those are the three values that affect your solar irradiance results. You can edit the other values if you want. Doing so will improve the accuracy of your system’s energy production estimate, but it’s not necessary if you just want to calculate solar radiation.

4. On the Results page, find your location’s solar irradiance estimates in the Solar Radiation column. Daily estimates of solar insolation are given for each month and for the entire year, in kWh/m²/day.

3. National Solar Radiation Database Viewer

PVWatts uses data from the National Solar Radiation Database (NSRDB). You can visualize and explore the data with the NSRDB Viewer. It’s a bit clunky to use, but here’s how to find your location’s solar radiation data with it.

1. Click the map pin icon in the bottom right of the map.

2. Click on your location on the map. Zoom in until you find your location and then click it to drop a pin there.

3. Select the data layer that includes your location. For instance, if you’re looking up a location in the United States, you’d select the “USA & Americas: GHI” data layer.

4. Click “Request Query Data” to get solar data for your location.

6. Scroll down to the Point Data section to find the average daily GHI (solar irradiance) for your location. The units are kWh/m²/day.

Solar Irradiance vs Solar Insolation

Solar irradiance is an instantaneous measurement of solar power over a given area. Its units are watts per square meter (W/m²).

Solar insolation is a cumulative measurement of solar energy over a given area for a certain period of time, such as a day or year. Its units are kilowatt hours per square meter (kWh/m²).

As an analogy, irradiance is like speed, how fast you’re moving at a particular instant, while insolation is like distance, how far you’ve travelled over a certain period of time.

In practice, you’ll see solar irradiance and solar insolation used interchangeably throughout the solar industry. (In fact, I’ve used them interchangeably in this article.) Just look at the units being used to determine whether you’re actually being given insolation or irradiance values.

I often see people use the term “irradiance” with the units kWh/m²/day or kWh/m²/year. Technically, this means they’re providing insolation values but calling it irradiance. It’s a bit confusing.

Solar Irradiance vs Peak Sun Hours

Sometimes, you’ll see solar radiation data expressed in peak sun hours. Solar insolation and peak sun hours both express how much solar energy a location receives over a period of time.

One peak sun hour is defined as 1 kWh/m² of solar energy. So, if a location receives 6 kWh/m²/day of sunlight, you could say that location gets 6 peak sun hours per day.

Using peak sun hours makes it a bit easier to communicate how much sun a location gets. To the solar novice, kWh/m²/day makes little sense.

But if you instead say that London gets on average 5 peak sun hours per day in July, it’s a little easier to grasp. The total sunlight London receives per day in July is equivalent to 5 hours of full sun.

If you’d prefer your solar radiation data in peak sun hours, check out our peak sun hours calculator.

References

1. Sengupta, M., Y. Xie, A. Lopez, A. Habte, G. Maclaurin, and J. Shelby. 2018. “The National Solar Radiation Data Base (NSRDB).” Renewable and Sustainable Energy Reviews 89 (June): 51-60.
2. Global Horizontal Irradiation Map by the Global Solar Atlas is licensed under CC BY 4.0.