How to test solar panel

[Jev88]

Hi.
So you inherit some solar panels on a MH. How can you simply test how well they are working after 10+ years?

Thanks all...... Multimeter at the ready.

 

[DBK]

Unless you can find a spec for what it was supposed to be when new I'm not sure what you can determine. But you could start by measuring the open circuit voltage then the short circuit current, both of course in sunshine. Compare these with what you might expect with a new panel of the same size.

 

[pappajohn]

As you don't have a new one to compare I doubt you can.
You could compare to manufactures data but that will be obtained under lab conditions....just like the data which says my car is capable of 70 MPG.... Laboratory lies...it does 47.3mpg

I believe a panel should be around 80% efficient after around 10 years.

 

[Richard n Ann]

If no battery monitor fitted I'd clip my clamp meter round one of the cables and see what current is flowing into the battery during daytime

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[autorouter]

If no battery monitor fitted I'd clip my clamp meter round one of the cables and see what current is flowing into the battery during daytime

If you measure the battery voltage at the same time, you can calculate the power, since you know the current in amps.
Watts = volts x amps.

To go one stage further, you could use a 'solar irradiance' meter, or even a solar irradiance app on your phone. Panels are rated at a standard irradiance of 1000 watts per square metre, at 90 degrees to the incoming light.

If the measured irradiance is 400 watts per square metre, then you can only expect 400/1000 = 0.4 or 40%. So a 100W panel should yield 40W, for example, when the panel is at 90 degrees to the incoming sunlight.

If the panel is not at 90 degrees to the incoming sunlight, you can calculate how much the irradiance reduces by. It's a bit fiddly but easy when you've done it a couple of times.

First measure the angle between the sunlight and the panel, in degrees

Find the sine of the angle. The sine is available on a 'scientific' calculator, ie anything better than a simple calculator. It is usually shown as 'sin'. If the calculator can work in 'degrees' then just enter them. The sine should be a number between 0 and 1, for example sine(60) is 0.86, sine(30) is 0.50.

Once you've found the sine of the angle, multiply the incoming irradiance by the sine of the angle to find the irradiance across the angled panel. For example an incoming irradiance of 400 W/m2 with the panel at 60 degrees results in a panel irradiance of 400 x sin(60) = 400 x 0.86 = 344W/m2. That's 344/1000 = 0.344 or 34.4% of the standard irradiance.

Then as before, a 100W panel should yield 34.4 watts if it is new and not degraded.

You can compare the expected power yield to the measured power, to see how much the panel has degraded.

This method is not perfectly accurate, as it doesn't account for temperature effects for example, but it will give you a good idea of whether your panel is OK or failing.

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[Speve]

As long as your batteries are being kept charged whilst not on hookup i wouldn’t worry about them

 

[autorouter]

If you're measuring the panel power output by looking at the solar controller volts and amps,(either input or output), remember the batteries must be actively under charge, ie nowhere near fully charged. As the battery gets full, the controller reduces the power to it. Unlike a house solar array that produces its maximum power all the time, and feeds any excess into the grid.