Solar sizing calculator
Work out the array, battery bank and charge controller you need to carry a chosen set of loads through a chosen number of sunless days.
1. What are you running?
2. Site and system
Your numbers
Daily load: 0 Wh
Select at least one load above to see results.
Assumes 75% round-trip efficiency, 80% array derate and 80% depth of discharge. Sizing guidance only — have any permanent install inspected.
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Questions people ask
- Why does the array come out larger than my daily load suggests?
- Because sun hours are not the same as daylight hours, and because energy is lost in the inverter, the charge controller, the wiring and the batteries themselves. This calculator assumes 75% round-trip efficiency and derates the array a further 20% for heat, soiling and tilt error. Sizing to the theoretical figure leaves you short in January.
- Should I use lead-acid or lithium?
- Lead-acid may only be drawn down 50% without wrecking cycle life, so a lead bank must be roughly 1.6x the nameplate capacity of a LiFePO4 bank doing the same work. Lead is cheaper per nameplate amp-hour and more tolerant of cold; lithium is lighter, lasts longer and gives you more usable energy per dollar over its life.
- What peak sun hours should I enter?
- Use your winter figure, not your annual average. The array has to carry the load in the worst month, and for most of the continental US that is December or January — often 2 to 3.5 peak sun hours even where the summer figure is above 6.