I have a battery, light bulbs, a solar charge controller, and a solar panel. The actual issue has come with me testing the solar panel. I have both a 30W and a 50W solar panel, so I am assuming the issue is not with both of these solar panels. I have been using a multi meter to test them. When it is sunny out, it usually gets over 20V. This part is fine. The issue is what happens when I measure the current of the solar panels. On the 10 Amp setting, I am measuring 0.2 Amps. This would mean that the solar panel is only producing 4 Watts. I have tried moving the solar panel and adjusting its tilt angle as well.

Additionally, when I connect a circuit with light bulbs, the light bulbs light up. These are 5W. Yet somehow the multi meter says that there is less current moving through the circuit. So this is where my questions come in:

Because it is a 10 A scale, does that mean that the .2 is actually 2 A or not?

Is there another reason that both solar panels would have this discrepancy?

Am I measuring something wrong or should I be doing it a different way?

What would you consider the best way to measure the efficiency of the system?

This isn't working at all well even in good sunlight and at the right tilt.


1 Answer 1


Maximum power is not the product of open circuit voltage and short-circuit current but when the impedance is matched and this is generally around 75% of that so instead you should use a three watt load but not a tungsten bulb as this starts as a 30W load.

You need optimum solar input to get max power and the MPT voltage starts at ~ 82% of Voc then drops with solar input to ~72%.

Depending on Panel characteristics, your results will vary.

  • \$\begingroup\$ So would I get proper data if I were to just buy a 3W lightbulb and hook that up to the system? Would I just connect that to the solar panel and the multi meter alone? \$\endgroup\$
    – Maxonton
    Mar 19, 2018 at 23:52
  • \$\begingroup\$ No it has to be a linear R load or equivalent. Bulbs are not linear. But an LED string can be made sort of linear using 200mA LEDs with 20V*75% =15V/3V= 5 LEDs rated for 200mA with heatsink. \$\endgroup\$ Mar 19, 2018 at 23:52
  • \$\begingroup\$ try V/I= 20V/0.2A= 100 Ohms @ 4W rating and read about MPT tracking also try 150 Ohms or 12V stripleds cut to length that only draws 150mA \$\endgroup\$ Mar 19, 2018 at 23:56
  • \$\begingroup\$ Thank you so much for all of this by the way! So do I just want to find 5 LEDs each at 200 mA? Should those be connected in a series then? Or is there a better alternative to lightbulbs? I really just need any way to measure this maximum output. \$\endgroup\$
    – Maxonton
    Mar 20, 2018 at 0:00
  • \$\begingroup\$ White LEDs vary from 2.85 to 3.2 typ. so five is.... But 12V stripleds use 3 LEDs in series + R so they can run from 10 to 15V so if each string takes 50mA (depends on type) then 3 ~4 strings from strip LED \$\endgroup\$ Mar 20, 2018 at 0:01

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