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I cannot understand why doubling the amount of solar panels gives only a few percent increase in total amount of power.

Here are the test results of a single 10-cell panel: Single solar panel efficiency tests http://imageshack.us/a/img43/6561/nlgd.png

Here are the test results of two panels connected in series: two solar panels connected in series

And here are the test results of two panels connected in parallel: two solar panels connected in parallel

My expectations were:

  • to see the voltage of 9.3V under load in serial connection, yet I've got 4.3
  • to see the current of 5.2A under load in parallel connection, yet I've got 2.6
  • to see the wattage doubled in both serial and parallel connection, yet I've got 10%,20%

Can someone explain the reasons behind such a weirdness?

The tests we repeated with the two different multimeters swapped in places to measure voltage and amperage.

The tests were conducted 1.5 - 1.0 hour before the noon, on a clear sky conditions, so there's a bias of few percent in measurements due to increase solar elevation angle.

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  • \$\begingroup\$ What is the "load" - is it just the shunt of the multimeter in 20A mode? Are you measuring the voltage and current at the same time (with two different meters)? \$\endgroup\$
    – pjc50
    Commented Jul 5, 2013 at 10:27
  • \$\begingroup\$ yes, the load is the shunt of the multimeter, I'm measuring the both - Amps & Volts simultaneously \$\endgroup\$
    – FlegmatoidZoid
    Commented Jul 5, 2013 at 10:36
  • \$\begingroup\$ electronics.stackexchange.com/questions/73226/… \$\endgroup\$
    – Phil Frost
    Commented Jul 5, 2013 at 11:41
  • \$\begingroup\$ I've constructed a dummy load with a replaceable 10W resistors. By controlling a current through MOSFET I was trying to find that particular threshold where current starts to drop and the voltage it at its maximum. Didn't work - peak wattage was 3W most from the two panels. Tried replacing resistors with 3W+3W LEDs, brushed motors etc. Then, I've plugged-in 7.4V LiIon battery. Inrush suddenly current peaked at [email protected] resulting in 24Watts total, or 12Watts per panel! \$\endgroup\$
    – FlegmatoidZoid
    Commented Jul 6, 2013 at 11:07

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http://en.wikipedia.org/wiki/Solar_cell_efficiency#Maximum_power_point

If you draw too much current from solar cells their overall efficiency falls. This is evidently affecting all three configurations to different extents. If you test with a set of resistors in the 1 - 20 ohm range you'll get different results. Given the power dissipation it may be easiest to do this with automotive lightbulbs of different wattage ratings (higher wattage rating for same rated voltage = smaller resistance). Ideally you'd want an adjustable 20W dummy load.

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