Solar Irradiance Calculator


Error: Please enter a valid location by selecting one from the autocomplete results.
Error: The National Renewable Energy Laboratory’s PVWatts Calculator does not have climate data for this location. Please try another location.
Optional: If left blank, we'll use a default value of 0° (horizontal). You can use our solar panel tilt angle calculator to find the best angle for your solar panels.
Error: The tilt angle must be a positive number between 0 and 90 degrees.
Optional: If left blank, we'll use a default value of 180° (south-facing) for locations in the northern hemisphere and 0° (north-facing) for locations in the southern hemisphere. You can use our solar panel azimuth angle calculator to find the best direction for your solar panels.
Error: The azimuth angle must be a positive number between 0 and 359 degrees.
Your annual average:
Your monthly averages:

Using this Solar Irradiance Calculator

  1. Enter your location in the corresponding input field. You can type in your state, city, or both of them. Then if you want to be more specific, enter your address alongside your city and state.
  2. Enter your solar panel tilt angle: This is optional, but providing this information will improve the accuracy of the irradiance calculation. Your solar panel tilt angle is the horizontal angle of your solar panel. For instance, if your solar panel is 15° from the horizontal plane, the tilt angle would be 15.
  3. Enter your solar panel azimuth angle: This is also optional. But if you provide it, you’ll get a more accurate result. The solar azimuth angle is measured clockwise north from your solar panels’ horizon to the sun’s position on the same horizon. Generally, for locations in the Northern Hemisphere, this angle is 180°. Then for places in the Southern Hemisphere, the angle is 0°. But you can enter a more precise value for your location.
  4. After filling in all the fields (or required fields), click the “Calculate” button. The solar irradiance calculator will display your annual average solar insolation and your monthly average solar insolations.

What Is Solar Irradiance?

Solar panels on grass in the farm

Solar irradiance measures the power density of solar radiation incident on a certain surface. It is the power per unit area a surface receives from the sun, measured in watts per square meter (W/m²).

Solar panels perform better with higher irradiance. But irradiance varies with factors like location, time, and season. This means that for some, low solar irradiation is almost unavoidable.

But even at that, you can get a solar array to perform optimally under such conditions.

For one, you can get an optimal amount of solar radiation if you adjust your solar panel to the optimal tilt and azimuth angle in a location with a low average irradiance.

Adjusting the solar elevation angle can also improve irradiance. You can readily estimate elevation angle using latitude, solar declination, and hour angle (the angle showing the sun’s position relative to solar noon).

Solar Irradiance vs. Peak Sun Hours

While irradiance is the basis for calculating peak sun hours, solar quantities differ. Peak sun hours are the hours in a day when irradiance averages 1000 W/m² or 1 kW/m².

Solar irradiance, however, is the number of watts per square meter at a given instance.

What Is Solar Insolation?

Solar insolation measures the energy density of solar radiation incident on a certain surface over a given period.

In other words, it is the energy production per unit surface area from solar power over a given period. It is measured in kWh per square meter (kWh/m²).

Why Solar Insolation and Solar Irradiance Are Different?

While solar insolation and solar irradiance are sometimes used interchangeably – even the result you get from our irradiance calculator is solar insolation – there’s a significant difference between them.

Insolation measures energy density or energy per unit surface area. Irradiance, on the other hand, measures power density or power per unit surface area.

To put this into context, using a battery as our analogy, solar insolation is like battery capacity. Solar irradiance, on the other hand, is battery wattage. Both are pretty similar, but they differ because one is summative while the other isn’t.

Insolation is summative because it adds up all the power generated by the sun on a surface over a given period (usually hours). Conversely, irradiance measures the sun’s power on a surface at a given instance.

Since they are used interchangeably, one can readily mix them up. But when you pay attention to the units, it becomes easier. The unit of insolation is kWh/m² while that of irradiance is W/m².

How to Calculate Solar Irradiance

Besides our solar irradiance calculator, you can calculate irradiance (and, by extension, insolation) using the following methods:

  • PVWatts® Calculator
  • National Solar Radiation Database (NSRDB) Viewer
  • Global Solar Atlas

PVWatts® Calculator

The National Renewable Energy Laboratory (NREL) is a very valuable tool for solar technology. Beyond helping calculate solar irradiance, you can also use it to figure out the azimuth, panel angle, and much more.

To determine irradiance using the NREL’s PVWatts® Calculator, follow these steps:

  1. Enter your location in the “Get Started” search field. Then click on the “Go” button.
    Enter your location in the "Get Started" search field
  2. You’ll be taken to a new page when you click the Go button. Confirm it shows your correct location on that page, then click the SYSTEM INFO tab.
    PVWatts SYSTEM INFO tab
  3. Adjust the values on the SYSTEM INFO tab according to your specifications. Then click on the RESULTS tab.
  4. After clicking on the RESULTS tab, the calculator will show you the estimated average daily insolation of your location for every month of the year. It will also show you the estimated yearly average.
    PVWatts RESULTS tab
  5. Besides insolation data, you’ll see average yearly and monthly power production, amongst others.

National Solar Radiation Database Data Viewer

The data used in the NREL’s PVWatts® Calculator is based on the National Solar Radiation Database (NSRDB). So, you’d get about the same result with either method.

To determine your optimal solar irradiance using the NSRDB viewer, do the following:

  1. On opening the NSRDB Viewer, enter your location into the “Search the NSRDB data catalog” search bar. In our case, we’ll enter New Haven, Connecticut.
    Enter your location into the "Search the NSRDB data catalog" search bar
  2. After choosing your location from the list of suggestions, a side menu will pop up on the left-hand side of the screen.
    NSRDB side menu on the left-hand side of the screen
  3. Choose a dataset that matches your preferred dataset from the side menu. We’ll select “USA & Americas (30, 60min / 4km / 2021)”. Depending on which of the two intervals we opt for, this will give us irradiance data for every 30 or 60 minutes of every day in 2021 for our selected location. After choosing a dataset, some options and input fields will appear beneath the dataset.
    NSRDB: Choose a dataset that matches your preferred dataset
  4. Scroll down in the side menu, and under the “Select Attributes” section, check “GHI.” If you want any other data besides GHI, you may also check them. You may also click on “SELECT ALL” to check every attribute.
  5. In the “Select Year” section, choose the year of the data you want. You may also click “SELECT ALL” to get yearly data available.
  6. In the “Select Interval” section, choose an interval. In our case, we have to choose between 30 minutes and 60 minutes.
  7. Finally, in the “Email” field, you’ll enter the email address where you want to receive the data. Then you’ll click on the DOWNLOAD button. Within a few seconds/minutes, you should receive the requested data as a ZIP file in the provided email.
  8. When you unzip the file, you’ll get a .csv document, which you can open with Excel. In that document, you’ll find robust GHI data: hourly GHI every day and every month in the selected year.
    NSRDB GHI data document

Global Solar Atlas

The Global Solar Atlas is a solar irradiance map of the world. But it offers information beyond solar irradiance. As the name says, it is an atlas, so it maps out solar data about various parts of the world.

To use the Global Solar Atlas to determine the solar irradiance of your location, follow these steps:

  1. Click on the “Search locations” search bar at the top left-hand side of the page. Then enter your city, state, or country. For higher precision, enter your city instead.
  2. While typing the name of your city, the search bar will show a list of suggestions; click on the option that matches your location best.
    Global Solar Atlas location suggestions
  3. After clicking on it, the atlas will return some data on the right-hand side of the page. You’ll find Global horizontal irradiation GHI, your location’s estimated solar irradiance.
    Global Solar Atlas GHI data
  4. By default, the Global Solar Atlas displays yearly data. So, the first GHI you see is your location’s yearly solar irradiance. To get daily irradiance data, click the “Per year” icon and choose “Per day.”
    Global Solar Atlas per day GHI data
  5. Now, you should have your daily Global horizontal irradiation on the screen.

What is GHI?

GHI is short for Global Horizontal Irradiance and is the amount of solar radiation that reaches a surface horizontal to the ground.

As highlighted in earlier sections, the value of this GHI affects the amount of solar power generated by a solar array.

The higher the GHI reaching solar panels, the higher the solar power generation. Then if GHI falls, solar power generation also drops.

GHI is made up of two other types of irradiances: direct and diffuse irradiance.

Direct irradiance (DNI) is the amount of solar radiation reaching a surface – in this case, a solar panel – directly from the sun.

Diffuse irradiance (DIF) is the amount of solar radiation scattered in the atmosphere that eventually reaches the surface of the target (solar panels).

How to Convert Solar Insolation to Solar Irradiance

To convert insolation to irradiance, divide the insolation value by the number of insolation hours. Then multiply the result by 1000.

\(Solar\ irradiance =
\displaystyle {\frac {insolation * 1000}{number\ of\ hours\ of\ insolation}}\)


If our insolation for the whole day is 8 kWh/m², our average irradiance for the day would be:

\(\displaystyle {\frac {8 * 1000}{24}} = \displaystyle {\frac {8000}{24}} = 333.3\ W/m²\)

In the example, we used 24 hours as the number of insolation hours because the insolation was for a whole day.

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