The effect of temperature on the efficiency of photovoltaic panels

A few years ago while arguing about the economic viability of photovoltaic (PV) electricity in Ireland I learned that the ambient temperature can have a significant impact on the performance of the PV panels and it is an important variable in return-on-investment calculations. The lower the temperature the more efficient the panels. [1]

Today we have a rare heat wave in Ireland, so I thought it was a good time to see this effect for myself. I recorded the experiment on video. What I've got here are 2 x 15 watt peak (rated) panels charging a 40Ah sealed lead acid (SLA) battery and a Raspberry Pi [2]

Conclusion

At the start the panels were reading 55°C (read from a non-contact thermometer) and were delivering 710mA of current to the battery/Pi system. After pouring water to cool the panels the temperature dropped to 42°C and the current rose to 820mA. I'm going to use current as a measure of efficiency here [3]. So that's about 14% efficiency increase for a 13°C drop in temperature.

Footnotes

[1] http://pveducation.org/pvcdrom/solar-cell-operation/effect-of-temperature

[2] It's a Raspberry Pi Model A (the one without ethernet or inbuilt USB hub). I've got an external unpowered hub connected to the model A's sole USB socket and to that a WiPi WiFi dongle and a Arduino Leonardo.  A 12/5V DC/DC car converter down converts the 12V to the 5V required by the Pi. The current draw from the Pi at the 12V end of the converter is 210mA.

[3] Measuring and extracting power from photovoltaics is not a trivial topic.