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Peak sun hours: solar panel output per day, month and year

Avatar for Ben Zientara
Published on 11/06/2019 in
Updated 11/06/2019
Peak sun hours can be calculated by looking at all energy produced by the sun on its path through the sky

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.

Image source: Geloven thuis

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.

A slide from Solar Energy International's PV training class describing the same solar panels during days with a different number of peak sun hours.

An example used by Solar Energy International in their online solar energy job training class.

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.

The sun’s position in the sky changes based on time of day.

The sun’s position in the sky changes based on time of day.
Source: University of Duisburg-Essen

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:

How the bell curve of solar production can be expressed as a box of peak sun hours

The average daily solar irradiance bell curve and equivalent peak sun hours. Source: National Technical University of Athens

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:

Insolation map from NREL for photovoltaic panels tilted at latitude

Source: NREL

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

7 thoughts on “Peak sun hours: solar panel output per day, month and year

  1. Avatar for Luc Luc says:

    What of the optimal space between rows for ground mounted panels? How many hours per day (ignoring clouds or rain) should one consider when specifying row spacing to avoid shadow from the row in-front?

    1. Avatar for Ben Zientara Ben Zientara says:

      Hey Luc-

      The optimal space between ground-mount rows (or even rows on a flat rooftop), has to do with three things: panel dimensions, tilt of the panels, and latitude of your location on earth. What you’re trying to do here is determine the distance the shadow of one row will fall onto the ground on the day the sun is lowest in the sky, so you can prevent any shadow from falling on the row behind on any day. Here’s a decent guide on how to do the calculations.

  2. Avatar for Steve Steve says:

    But STC is a lab number in my experience. A 100 watt panel will never produce 100 watts in the field. Maybe 80 at peak? Perhaps in high altitude sun with a cold breeze. I’m here, looking for typical numbers out of curiosity. My flexible panels are such under performers that right now, in the heat of the afternoon at sea level San Diego, two 120 watt panels are yielding 140 watts total. I’m using an MPPT with the panels in series on a 10′ 12 ga cable. But flexible panels are know to give very poor yields to ratings and mine are now aged.

  3. Avatar for Jasper Jasper says:

    Thanks for this informative article! I am trying to fully understand it and have a question about the “insulation map”. The paragraph immediately above the map states the map shows units “available” to solar panels. Does that imply the map shows the theoretical levels available if the solar technology worked at 100% efficiency? If so, what is a reasonable percentage multiplier to determine the actual amount captured by the solar panels?

    1. Avatar for Ben Zientara Ben Zientara says:

      Hi, Jasper-

      That map is unfortunately kind of vague, and I apologize that the explanation of how it works didn’t capture all the nuances. The map does indeed show average available solar energy per square meter per day for solar panels tilted at latitude (e.g., if you live in St. Louis, MO, you’re at 38.627 degrees north, so the map assumes the panels are tilted up 38.627 degrees from horizontal). And yes, if solar panels were 100% efficient, they would generate this much energy per square meter.

      But solar panels aren’t 100% efficient. The best ones on the market are around 22% efficient, and an average is more like 18.7%. Here’s the thing: that 18.7% efficient solar panel takes up about 1.63 m². In order to produce an amount equal to 1 m², you’d need about 3.3 solar panels (3.3*1.63*0.187=1.005873)

      Here’s an example: St. Louis, MO is firmly in the “4.5 to 5.0” section of the map. Let’s call it 4.75 kWh/m²/day. At 18.7% efficiency, that’s 0.898 kWh/m²/day. At 1.63 m², each solar panel can make around 1.45 kWh per day, and 3.3 of them can make 4.79 kWh/day—pretty dang close to the 4.75 kWh(again, on average. Sunny summer days lead to higher production than snowy winter ones, but it averages out over the course of a year).

      But wait! There’s an easier way!! Manufacturers of solar panels report the performance specifications of their solar panels, usually right in the model number. They take the module’s efficiency and size into account and produces a solar production number called watts-DC under “Standard Test Conditions,” or STC for short. Watts-DC STC just means the number of watts a solar panel can produce under perfectly full illumination.

      That 18.7% efficient, 1.63 m² solar panel we mentioned above? It can produce right around 300 watts under full sun. and 3.3 of them can put out about 1 kilowatt (kW) under full sun. That’s why we just used shorthand in the article when we explained the map. To put it all together: to produce as many kWh of electricity as that map shows in each area, it takes 1 kW of solar panels installed at that location.

      Solar system size is measured in kW, and given the numbers above, you can get a pretty good estimate for how many kWh each kW of your system will produce on an average day. Multiply by 365 to get an annual production estimate. This makes it a LOT easier that doing all the conversions by hand.

  4. But the production depends not only on the “peak sun hours” but also on the angle of incidence of the light on the panels, which differs not only during a day but also during the year with the sun being at the lowest angle in the winter during the entire day. Optimum angles of the panels varies, at my location, from 12° summer to 65° winter.

    1. Avatar for Ben Zientara Ben Zientara says:

      Absolutely right, Phillip! The map above from NREL estimates insolation when the panels are tilted at latitude all year round. Of course it depends on your location in the country, but that’s about equal to the average between 12 and 65! You can eke out a bit extra by re-titling your panels a couple times a year, but this is a pretty decent estimate of what the average fixed-tilt system enjoys.

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