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About this tool
Size your home inverter VA and battery Ah from your real appliance load, with backup and charging time.
The Inverter & Battery Backup Calculator sizes a home inverter and battery bank from a real appliance list, dividing total connected watts by a 0.8 power factor to get the VA rating and dividing watt-hours by (system voltage × inverter efficiency × depth of discharge) to get the required amp-hours. It is built for homeowners and electricians choosing between a 600, 900, 1100, 1500 or 2500 VA unit and a 100–220 Ah battery. You get the recommended standard size, the surge headroom needed when a motor starts, real backup hours and an estimated recharge time.
Open Inverter & Battery Backup Calculator on AltFTool — it loads instantly in your browser.
Pick an appliance from the grouped dropdown - each option shows its watts and any starting surge - press Add, then set Qty and Hours on the row, or enter 'Desired backup (hours)' and press 'Apply to every appliance'.
Choose the System voltage (12, 24 or 48 V) and adjust Inverter efficiency, Depth of discharge and 'Charging current (A)'.
Read the Recommended rating (total W divided by the 0.8 power factor, rounded up to 600, 900, 1100, 1500 or 2500 VA), the required Ah snapped to a standard 100, 150, 180 or 220 Ah size, 'Real backup at' your load and 'Time to charge from flat'; 'Copy sizing report' copies the lot.
Adds the worst starting spike in your list — a fridge compressor at about 4× running watts, a pump at about 3× — and flags when your chosen VA size would trip.
Snaps the computed VA and Ah up to real market sizes (600/900/1100/1500/2500 VA, 100/150/180/220 Ah) instead of leaving you with an unbuyable number.
Works out series count for 12 V, 24 V and 48 V systems, how many parallel strings you need, and recharge hours at your charger current with a 20% charging-loss allowance.
Divide total watts by the power factor — this calculator uses 0.8, the usual figure for a home inverter, so 800 W of load needs 1000 VA. Then pick the next standard size up and aim to run at roughly 80% of the rating so the unit stays cool and has headroom for motor starts.
Backup hours = (Ah × system volts × inverter efficiency × depth of discharge) ÷ load watts. A 150 Ah 12 V battery at 80% efficiency and 80% DoD gives about 1,150 usable Wh, which is roughly 5 hours on a 230 W load of fans, lights and a router — and just over an hour (about 1 h 9 min) on a 1,000 W electric iron.
No. A 1 ton non-inverter AC draws about 1,200 W running and roughly 3,600 W at startup, and a 15 L geyser pulls about 2,000 W continuously — a standard 12 V home inverter will trip or flatten the bank in minutes. Running an AC needs a high-capacity pure sine wave inverter on a 48 V or larger bank.
Tubular is the default for frequent or long cuts: roughly 1,200–1,500 cycles and 4–6 years, with 70–80% usable depth of discharge. Flat plate is cheapest but lasts about 2–3 years at 50–60% DoD, while LiFePO4 gives 3,000–5,000 cycles and 80–90% usable capacity at several times the price and needs a compatible inverter and charger.
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