Solar Panels

Choose the string first, then the panel. A panel's open-circuit voltage (Voc) climbs as the temperature drops, so the string you wire has to stay under your charge controller's or inverter's maximum input on the coldest morning of the year, not the average one. Work it out from the panel's Voc and its temperature coefficient, both on the datasheet. After that, compare on cost per watt. Efficiency earns its price where space is the limit: a van roof, a small roof or one with shade.