In short: How much electricity a solar panel makes depends on how much sun it gets, which can vary greatly day-by-day. When measuring solar output, what’s most important is the total amount of solar energy available on an average day, expressed as kilowatt-hours per square meter, and also called peak sun hours.
We’ve talked about solar panel output, which means how much power a solar panel can produce under ideal conditions, but now let’s take that further and look at how much energy that panel can produce as it receives sun throughout the day.
In the above example, five solar panels each rated to produce 200 watts under full sun are shown on two different days. On the first day, the panels receive the equivalent of five “peak sun hours,” while on the second, more cloudy day, the panels receive two “peak sun hours.”
What’s a peak sun hour?
Put simply, a peak sun hour is an hour during which the sun shines at an intensity of 1,000 watts per square meter. Of course, the sun doesn’t shine that bright for that long, so peak sun hours are a measurement of the sun’s intensity over an average day, expressed as the equivalent of the sun shining at peak intensity for a certain number of hours.
During the day, as the sun moves across the sky, it shines with varying levels of intensity. That’s why stepping out into the midday sun feels a lot different than going out near sunset.
In the same way you can feel the difference in the sun’s intensity, special instruments can measure the amount of energy the sun is shining down, in watts per square meter. When the sun is highest in the sky, it shines down with an average intensity of 1,000 watts (or 1 kilowatt, or kW) per square meter.
Of course, the sun is at its peak in the sky for only a short time, so the graph of the intensity of the sun looks like a bell curve, with low intensity as the sun rises and sets, and high intensity during the middle of the day.
In order to create a useful number that can help determine the average amount of sunlight available in a day, the measurements of watts of energy available during all hours of daylight are added up and then divided by 1,000.
If that sounds complicated, it’s not really, it’s just getting all the area under the bell curve into a box of equal size, like this:
Putting it all together: A peak sun hour is the equivalent of the sun shining at an intensity for 1,000 kW per square meter for one hour (expressed as 1 kWh/m²). If you know the average daily peak sun hours for your location, you can calculate the kWh your solar panels will make on a daily, monthly, and yearly basis.
Annual solar output
Just as in the chart above, peak sun hours are generally calculated as a single number that takes into account the average amount of solar radiation a single location receives over a year, expressed as kWh/m²/day.
Here’s a great “insolation map” of the United States showing the average kWh/m²/day available to solar panels:
As an example, imagine a 1-kW solar installation in Dallas, TX. Dallas lies in the “5.0-5.5” color band on the map, so let’s just say there’s an average of 5.25 peak sun hours available in Dallas on an average day.
That means our 1-kW solar installation will generate about 5.5 kWh per day, 165 kWh per month, or just over 2,007 kWh per year. Of course, the sun is higher in the sky during the summer months and lower in the winter, so there is a monthly difference.
If you live in a state with good net metering rules, you can get credit for the extra solar energy your panels produce during sunny months. Net metering means you’ll be able to use that extra credit to erase your electricity bills during the cloudy months when you panels aren’t making as much electricity.
Solar panel output per month
The real reason anyone bothers to calculate the number of peak sun hours is to quickly estimate how much energy a solar panel installation will make over time. The specific reason you’d want a monthly total is tied to how electric utility companies bill you for usage every month.
If you live in a state without net metering protections, your utility company might offer you a minimal credit for those extra kilowatt-hours. They might not be required to give you any credit for your electricity at all!
In this case, it’s a good idea to select only as many solar panels as you need to offset usage during the sunniest month. That way you can be sure you’re not sending a lot of extra electricity back to your utility when they’re not giving you any credit for it.
Incidentally, not having good net metering rules can be a good reason to get a battery for your solar panel installation, and store your solar power for use when the sun isn’t shining.
Automate the peak sun hours calculation
Okay, that was a lot of math and explanation. Isn’t there just a tool that can help make this calculation simple? There is!
The National Renewable Energy Laboratory made a great tool to estimate solar production, and we can tell you how to use it! Check out our guide to using NREL’s PVWatts tool, and have a sunny day!
Last modified: November 6, 2019