Off-Grid Solar Calculator — System Size & Battery Bank
Calculate the complete off-grid solar system size you need — panels, battery bank, inverter and generator backup. Based on your daily power usage.
· Free · No signup required
How to use this calculator
- Enter your total daily electricity consumption in watt-hours.
- Set your peak sun hours for your location.
- Set how many days of battery backup you want (autonomy days).
- Choose your battery type — LFP batteries allow deeper discharge.
- See the complete system specification: panel kW, battery kWh and estimated cost.
Understanding your results
Off-grid system sizing logic: The calculator uses standard engineering ratios: solar array is sized to produce 125% of daily needs during peak sun hours (the 25% buffer accounts for cloudy days and system losses). Battery bank is sized to provide your chosen autonomy days at the battery’s usable depth of discharge. Inverter is sized at 2× daily kWh load to handle surge currents from motors and appliances.
Why battery chemistry matters so much off-grid: LFP (lithium iron phosphate) batteries tolerate 80–95% depth of discharge versus 50% for AGM lead-acid. This means an LFP bank needs 40–50% less capacity for the same usable storage. LFP also lasts 3,000–6,000 cycles (8–15 years of daily use) versus 300–700 cycles for AGM (1–3 years with daily cycling). Despite 2–3× higher upfront cost, LFP is almost always cheaper over the system lifetime for permanent installations.
Generator sizing and fuel planning: For reliable year-round off-grid power, a 2,000–4,000W generator is recommended for winter backup. Size it to recharge your battery bank from 20% to 80% in 4–6 hours. A 3,000Ah 48V LFP bank (144 kWh) draws 30 kW to charge at 1C rate — a 4,000W generator will charge it in 36 hours, which is why generator backup complements solar rather than replacing it.
ITC for off-grid solar: Fully off-grid solar systems qualify for the 30% federal Investment Tax Credit if the property is your primary or secondary residence and the system is used for residential electricity. Off-grid systems for vacation cabins used personally (not rented) typically qualify. Commercial or rental property off-grid systems qualify under the commercial ITC (Section 48E). Consult a tax professional for your specific situation.