Solar calculators

Divide your monthly kWh use by 30.4 to get daily use, divide by your peak sun hours and by a derate of about 0.8 to get the system size in kW, then divide by the panel wattage in kW. For 855 kWh a month, 4.5 sun hours and 400 W panels, that is about 7.8 kW, or 20 panels.

For a fixed, year-round mount, tilt the panels at roughly your latitude and face them true south in the northern hemisphere. Tilting about 15° flatter in summer and 15° steeper in winter captures a little more energy if your rack is adjustable.

Peak sun hours are the number of hours per day the sunlight would deliver if it always arrived at 1,000 watts per square meter. Most of the US gets about 4 to 5 per day on average; the desert Southwest gets 5.5 to 6.5.

Multiply your daily kWh by the days of autonomy you want, then divide by the usable fraction of the battery (about 0.8 for lithium iron phosphate, 0.5 for lead-acid) and by inverter efficiency (about 0.9). Five kWh a day for two days needs roughly a 14 kWh lithium bank.

Use MPPT when the panel string voltage is well above the battery voltage, which is the case for most modern residential panels. PWM controllers are cheaper but only work efficiently when the panel voltage closely matches the battery voltage.