backchannel: assessing solar panels with a multimeter

This comment on a YT video is correct, although perhaps overstated.

The best, and simple way to check the best Solar Panels is to measure the Open Circuit Voltage and the Short Circuit Current.

Well, the best and simplest way would be to read the specs on the panel’s label.  :-)

In this case, though, I think OP is talking about a hands-on assessment of a panel that is being considered for purchase.

... take 80% of the Open Circuit Voltage

This is a reasonable heuristic for estimating voltage at which the panel makes max power (Vmp) from open circuit voltage (Voc).  Vmp for a panel is typically around 80% of Voc.  I have suggested elsewhere that a slightly smarter PWM could guess (not track) Vmp based on 80% of Voc and run the panels at that voltage.  No tracking necessary, no fancy hardware or software required.  Already-present PWM switching would handle power reduction when a setpoint is achieved.

This would eliminate the main issue that hamstrings PWM;  low output when the batteries are deeply discharged.  This occurs because PWM drives the panels at battery voltage which in the morning is usually a long way from Vmp.

multiplied by 90% of the Short Circuit Current,

I’ve never heard this stat but it seems right.  In more formal terms “Imp is typically ~90% of Isc”.

to give you the POWER The Open circuit Voltage should be fairly consistent regardless of how much sun or cloud there is.

I understand Voc is subject to temperature-related depression the same way Vmp is.  If it didn’t then the 80% heuristic would only work at one temperature.

Excluding temp derating, panel voltage is indeed reasonably steady when insolation is >= 20% (200w/square meter).  Cavet:  MPPT tweaks that voltage for its own purposes.

It is the Short Circuit Current that is the all important one that varies with how bright the Sun is. **Good panels will give you more Short Circuit Current under identical conditions. **{emphasis added}

I’m not sure that last claim is supported. Ideas to consider:

  • Isc is a known value, and is printed on the label.

  • for any panel with the same rating (300w, say), the panel with the lower Vmp will have a higher Isc.  This does not make it a better panel.

Divide your power by the surface area of your solar panel in square meters, to get the Power per sq.meter. These should be around 16% to 18% efficiency from the actual Sun's energy on your panel.

Yes, that’s how efficiency is measured.  The 16%-18% numbers there are sane, but there is variance on both sides.

Full Sun on a cloud free dark blue sky around midday should deliver about 1,000W.

Depends on latitude.   Solar noon is a relative term;  it’s the highest point the sun will attain on any given day.  Ideal conditions are 1000w/meter square but this is not a promise that clear blue skies will provide that at a local solar noon. 

So at 17% efficient, you could get 170W.

Yes, you’d get 170w out of a panel rated for 170w.  In other words, capturing 17% of the 1000w/sq meter that we stipulate as lab conditions.

Quality may then simply come down to how long they will last. Solar Panels can last 50 years or more, if looked after and protected from damage.

That’s a useful concept.