Do solar panels require direct sunlight?

Solar panels can work with almost any amount of sunlight, but that doesn't mean you should install solar panels on your house unless less than ideal conditions. Here's why.

Photo of partly sunny sky
Credit: Bojan Bogdanovski (Unsplash)

Solar panels work by turning photons (ie. light) into moving electrons (ie. electricity). Known as the photovoltaic effect, this continues to work under even very low light conditions.

Because of that, solar panels don’t need direct sunlight to generate electricity. They’ll continue to generate power even if its cloudy. If your panels are being used for non-critical applications, such as keeping the house battery of your RV topped up, indirect sunlight and cloudy conditions might be just fine.

However, if you’re making a major investment by installing solar panels on your home, you want as much direct sunlight as possible so that you maximize your electricity generation and recoup your investment quickly. While solar panels do work under indirect sunlight, a seemingly small reduction in incoming light can have a big effect on your electricty production.

How does light intensity affect the power output of a solar panel?

The power output from solar panels increases with more light, and decreases as the light intensity drops. This relationship is roughly linear: if you cut the amount of light in half, the power output will drop by about half as well.

This linear relationship starts to break down under very low light, but holds true under typical daytime conditions.

How much solar power generation can you expect under indirect light?

Even though a cloudy day might still seem pretty bright, that’s partly just perception: your eyes are adjusting to the change in light by changing your pupil size.

In fact, even thin clouds will cut the amount of sunlight by roughly half. Here’s a chart that describes the relationship between weather conditions and the amount of light that roughly corresponds to those conditions:

ConditionsLight level
full sun100%
slightly overcast50%
heavy overcast12.5%
open shade, sunset, or sunrise6.25%

(Tip: old school photographers might recognize this as the Sunny 16 Rule.)

Since electricity generation is in direct proportion to the light level, you can see that even light clouds will have a big negative impact on solar panels. Heavy clouds or full shade? Forget about it.

Cloudy days are normal and okay for solar panels

This doesn’t mean that you can’t ever have a cloudy day if you own solar panels. When you get an estimate for the annual power output for home solar panels, whether that’s from a solar installer or the Solar Nerd calculator, the local climate is factored into the calculation.

Whether you live in a famously cloudy city like Seattle, Washington or a really sunny place like Mesa, Arizona, the climate models that solar installers use to estimate your power generation will ensure that the average number of cloudy days is factored in. This means that if it’s cloudy and rainy all the time where you live, your solar quote and production estimate will account for that.

Should you install solar panels under indirect sun?

If you’re talking about using solar panels for a small application, like maybe keeping a battery charged so that you can have lights on in a backyard shed, then yes, you can go ahead and use solar panels. If it’s a shady spot in your yard, you can always add more panels if the battery isn’t staying charged.

It’s a different matter for a home solar array. If you have shade trees, it’s generally a bad idea to install solar panels if they don’t have a clear view of the sky. As described in the table above, full or even partial shade from trees will really impact your electricity production.

If you don’t have a roof that faces south, that’s actually a pretty common situation. Many homeowners have their panels facing east or west, and that can work especially well if you have a time-of-use plan where electricity is more expensive in the evening.

Even north-facing solar panels can work if you live in a more southern latitude or use reverse-tilt frames.

Bottom line: when it comes to a major investment in home solar panels, you’ll want to make sure they have as much direct sunlight as possible with minimal shading from trees and nearby buildings (including your own). Otherwise, you’ll see a big loss in power output.

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