How battery size is calculated
Nightly use ÷ depth of discharge ÷ inverter efficiency. The calculator takes your daily kWh, multiplies by the share you use at night (50% is typical), then divides by the battery’s usable share — 0.9 for lithium — and by inverter efficiency (0.95). A home using 30 kWh a day with half its use at night needs 15 ÷ 0.9 ÷ 0.95 = 17.5 kWh of usable storage, rounded to a 20 kWh battery (for example, 2 × 10 kWh).
Lithium is now standard because it safely discharges to about 90%; older lead-acid batteries only reach about 50%, so they need twice the rated size for the same usable energy. Battery cost is estimated at about $600 per kWh installed — a 20 kWh battery is about $12,000.
A 10 kWh battery delivers about 9 kWh of usable energy: enough for an evening of lights, TV, fridge and charging, but not a full night of air conditioning or electric heating. Batteries add cost and lengthen payback, so many homes start with panels only.
Size the panels first with the solar panel calculator, or work out your savings with and without a battery.