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Sum all essential loads you want to power from batteries — refrigeration, lights, communications, well pump, and heating controls. Use actual wattage and runtime hours.
Daily load (kWh) = Σ (Watts × Hours) / 1000Multiply daily load by the number of days you want to run without sun or grid power. Off-grid systems typically target 2–3 autonomy days; backup systems target 0.5–1 day.
Raw capacity = Daily load × Autonomy daysLithium batteries safely discharge to 80–90% DoD; lead-acid should stay above 50% DoD for cycle life. Divide raw capacity by usable DoD fraction.
Required kWh = Raw capacity / DoD fraction / Inverter efficiencyUpdated: July 2026
5 kWh/day × 2 days = 10 kWh raw. At 85% DoD lithium and 95% inverter efficiency, need ~12.4 kWh bank. Two 6 kWh lithium modules or one 13.5 kWh Powerwall equivalent.
Refrigerator, lights, router, and phone charging total ~3 kWh/day. One day backup at 90% DoD needs ~3.5 kWh — a single 5 kWh battery covers overnight outages.
Full home backup for one day at 25 kWh with 80% DoD lithium requires ~32 kWh bank. Three stacked 10 kWh modules or two Tesla Powerwall 3 units at 13.5 kWh each.
Whole-home backup requires enormous battery banks. For grid backup, size for critical circuits only — refrigeration, lights, and communications — to keep costs manageable.
Discharging lithium to 0% or lead-acid below 50% dramatically reduces cycle life. Size the bank so normal operation stays within manufacturer DoD recommendations.
Battery storage sizing determines how long your home runs off-grid or during outages. This calculator estimates required battery bank capacity in kWh from daily energy consumption, desired backup days, depth of discharge limits, and inverter round-trip efficiency.