Energy
Estimate usable and nominal battery capacity for essential-load backup.
Enter your numbers to see the answer.
Home battery size calculator guide
A home battery is sold by nominal kilowatt-hours, but what you actually get to use depends on the chemistry’s safe depth of discharge and on the energy lost charging and discharging. This calculator starts from your essential load in kWh per day and how many hours of backup you want, works out the usable energy required, then grosses it up for round-trip efficiency and depth of discharge to give the nominal capacity you should shop for. It is written for anyone comparing a LiFePO4 wall battery with a bank of tubular lead-acid cells for an IPS, or checking whether a quoted 5 kWh or 10 kWh system really covers a night of load-shedding. Set depth of discharge to about 50 percent for lead-acid and 80 to 90 percent for LiFePO4 or NMC lithium. Round-trip efficiency is typically 85 to 92 percent for lithium systems with a good inverter, and 70 to 80 percent for lead-acid. Pair the result with the inverter-size calculator, since a battery’s kW output limit matters as much as its kWh.
Usable energy needed (kWh) = daily essential load (kWh/day) × (backup hours ÷ 24) ÷ (round-trip efficiency ÷ 100). Nominal capacity (kWh) = usable energy needed ÷ (depth of discharge ÷ 100).
Worked example with the defaults: an essential load of 8 kWh per day, 12 hours of backup, 90% depth of discharge, and 90% round-trip efficiency. Energy over the backup window = 8 × (12 ÷ 24) = 4 kWh. Gross up for efficiency: 4 ÷ 0.90 = 4.44, displayed as 4.4 kWh usable energy needed. Gross up for depth of discharge: 4.44 ÷ 0.90 = 4.94, displayed as 4.9 kWh nominal, the figure the calculator shows. Change depth of discharge to 50% for lead-acid and the nominal requirement becomes 4.44 ÷ 0.50 = 8.9 kWh, almost double.
The daily load is in kilowatt-hours per day; take it from your bill (monthly kWh ÷ 30) or from the essential-loads calculator. Backup hours are hours within one day; the tool scales the daily load by hours ÷ 24, so 12 hours means half the daily energy. Depth of discharge and efficiency are percentages. The result is nominal kWh; to convert to amp-hours for a lead-acid bank, divide by battery voltage: 8.9 kWh at 12 V is 8.9 × 1,000 ÷ 12 ≈ 740 Ah, for example five 150 Ah batteries in parallel.
This is a modelled estimate, not a meter reading. Real consumption moves with weather, occupancy, appliance age, standing charges, and tariff structure, and equipment rarely runs at its nameplate rating. Use the figure to compare options against each other, then confirm against your own bill and the manufacturer's specification before committing to a purchase. On this page the figure rests entirely on essential load, backup duration, usable depth of discharge, round-trip efficiency, market / jurisdiction, effective date, data source, update owner and review date, so start there if the home battery size calculator returns something you did not expect.
Good to know: the two percentages compound, so entering 100 percent for both because “the label says 10 kWh” hides a third of the shortfall on lithium and half of it on lead-acid. Do not also subtract the inverter’s reserve setting twice — either lower the depth of discharge or reduce the usable figure, not both. Backup hours longer than 24 need a multi-day load, so enter the total kWh for the outage as the daily load and 24 hours. And a battery with enough kWh but a 3 kW continuous limit will still trip on a 3.5 kW load; check the power rating separately.
Energy results depend on tariffs, equipment behavior, region, weather, and installation assumptions. Verify rates and electrical decisions with the utility, manufacturer, or qualified professional.
Sources
How it works
Estimate usable and nominal battery capacity for essential-load backup. This tool explains the calculation so you can adjust the assumptions to match your situation.
Worked example
With Essential load = 8 kWh/day · Backup duration = 12 hours · Usable depth of discharge = 90 % · Round-trip efficiency = 90 % → 4.9 kWh nominal (estimated battery capacity). Change an input above and this example updates with your numbers.
Common questions
With essential loads totalling 8 kWh per day you need about 4 kWh of usable energy for 12 hours, which is roughly 4.9 kWh nominal for a LiFePO4 battery at 90% depth of discharge and about 8.9 kWh nominal for lead-acid at 50%. Whole-home loads of 20 to 30 kWh per day need 12 to 18 kWh of lithium storage.
A 12 V 150 Ah lead-acid battery stores 1.8 kWh nominal, or about 0.9 kWh usable at 50% depth of discharge. Divide your usable kWh requirement by 0.9: a 4-hour evening load of 400 W (1.6 kWh) needs about two batteries, while 3 kWh of backup needs three to four.
A 5 kWh LiFePO4 battery gives about 4.5 kWh usable, enough for a night of essentials (fridge, lights, fans, router, TV) at 300 to 500 W, or roughly 3 to 4 hours if a 1 ton AC is added. It is not enough for electric cooking or water heating.
Depth of discharge is the share of nominal capacity you draw before recharging. Lead-acid batteries lose cycle life quickly below 50%, so plans use 50%; LiFePO4 handles 80 to 90% routinely and some makers rate 100%. A 10 kWh battery at 80% DoD gives 8 kWh usable.
LiFePO4 costs more per nominal kWh but delivers almost twice the usable energy, three to five times the cycle life (3,000 to 6,000 cycles versus 500 to 1,500), and 90 percent or better efficiency. Lead-acid wins only on upfront price and easy local replacement; over five years of daily load-shedding cycles lithium is usually cheaper.