Handy solar info for lead acid batteries

pianotuna wrote:
"All solar panels lose power at higher cell temperatures. This is why most panels are designed for 16.5 to 17.5 volts output at room temperature - when the panel cell temperatures get up to 150 degrees F or so, voltage output can drop as much as 20%"

150 f = 65 C.

If the panel is putting out 17.5 volts @ 25 C then at 65 C the voltage would still be 14.

Voltage drop at 14 for a 180 watt panel which would service 300 amp-hours of battery bank with a 15 foot run would amount to a further 0.31 volts drop with #8 wire. That still leaves the charging voltage at a comfortable 13.69 volts input to the controller.

Therefore for a 180 watt system #8 wire would work well.

Going down in size to #10 wire "costs" an additional .25 volts (.56) leaving 13.44 volts to the controller.

Going down to #12 "costs" a total of .84 leaving 13.16 to the controller. Still adequate but not "great".

Going to #14 costs 1.4 volts leaving 12.6 volts to the controller. That's not good.

This is pretty much based on a 'worst case' scenario where the panels hit 65 C (150 f). I think that is unlikely, at least where I live.

Another solution would be to use a higher voltage panel.

If I remember correctly handy suggests wire sizes much larger than those I've listed.

In any event, if you want low line losses go MPPT and get the voltage up to say 48, or even 140. You will pay for a more expensive controller to do so--and probably it is not worth it until about a 600 watt threshold.

When your small percentages apply to the OVERALL voltages, then yes they are small. HOWEVER, when you apply these differences to the VOLTAGE THAT MATTERS (ie the voltage difference between charging voltage and battery voltage the difference that will actually push AMPS into the batteries) then these "small" losses become MUCH more important.