This past summer I installed a couple of 12 volt 100 watt solar cells on the roof of my caravan.
I was never naive enough to actually expect to get 100 watts out of them, but I've still been very disappointed with the actual power produced.
Here are my sums, have I missed something?
The Solar cells says 100W – that means it should produce 100 watts in the noonday sun on the equator at 25 C with an assumed solar radiation of 1000 watts per square meter.
There are maps published which suggest that in the UK were I live the peak solar radiation should be around 650 watts per square meter.
This brings the power I think I can expect down to 65 watts.
Then I need to adjust for the fact that performance goes down as temperature goes up. (click here for details) .The Temperature Coefficient of Pmax for my panels is -0.40% per degree. That means for every degree above 25C it loses 0.40% of its power. I’m guessing that the temperature on the roof of my caravan is quite warm – lets say 50C, that’s 25 degrees above test conditions, or a total power reduction of 12.5% due to the temperature. So deduct 12.5% off my UK power of 65 watts and I’m down to about 57 watts.
I'm calculating my actual power output simply by measuring the voltage across the solar panels and multiplying by the current through them (P=IV). The largest value I've seen is about 18-22 watts - less than half what my most pessimistic theoretical calculations have suggested.
I've been doing my test in the middle of the day, with the sun vertically overhead in the middle of the best summer the UK has experienced for 40 years.
Either I've missed something in my calculations, or I've made an error in my testing, or my panels are just cheap cr4p.
Further info.....
- I used to be an electronics engineer so I hope I’ve avoided any really stupid mistakes, however this is my first time working with solar panels so their could be something I’ve missed.
- Current and voltage has been measured repeatedly with a number of quality fluke multi-meters, similar values from all of the them.
- Current and voltage are being measured on the input side of the solar regulator so my measurements should be giving me power before taking into account any regulator inefficiencies.
- I've installed two identical solar cells and have had very similar results with both, so it's unlikely to be just a faulty unit.
The specification for the panels (according to the sticker on the back) is...
Guangzhou Jinsheng New Energy Technology Co Ltd
Model No: JS100W-12MP
Wattage: 100W
Dimensions(mm):1100*540*2
Cell Type: Monocrystaline
Peak Power (Pmax) : 100W
Maximum Power Voltage (Vmp) : DC18.0V
Maximum Power Current (Imp) : 5.6A
Open circuit Voltage (Voc) : DC22.5V
Short-circuit Current (Isc) :5.8A
Maximum System Voltage (V) : DC 1000V
Test Conditions : AM1.5, Ec=1000W/m2, TC=25C
Made in China
In summary, the most power I've ever managed to get out of a 100w panel is about 20w. Why?
UPDATE - I'm not sure on the spec for the solar regulator, I don't have it in front of me now, it was a closing-down-sale item from Maplin, so I'm guess it was pretty low spec. I can remember that when wired up as-per the spec the battery charging voltage would be about 14.4 volts, and the solar-cell voltage would drop to about 16 volts (2 volts below the max power voltage quoted)
I don't currently have access to my caravan (it's in storage for winter) and when I do it's not currently sunny enough to produce much power, so I'd like to re-phrase my question. Can anyone point me in the direction of some worked-examples of how to calculate the practical power output of solar cells and what I'd need to do to achieve maximum power? Comments so far seem to be suggesting that my solar regulator may not be optimum. But maybe there's something else that I've missed?
