How to use this battery backup calculator
- Total load, the combined wattage of everything you want to power.
- Backup time, how many hours you want that load to keep running.
- Battery voltage, pick 12, 24 or 48 V to match your bank.
- Capacity per battery, the rated amp-hours (Ah) of one battery.
- Battery type, lead-acid or lithium, which sets the depth of discharge.
- Inverter efficiency, leave it near 90% unless you know your inverter's figure.
Results update as you type. Not sure of your total wattage? Open the appliance checklist above the results. It holds 28 common household loads with typical running watts, from a 15 W Wi-Fi router up to a 3,500 W central AC compressor, and it writes the total straight into the load field.
How inverter battery backup is calculated
The energy your load needs, in watt-hours, is watts times hours divided by inverter efficiency. Efficiency belongs in there because an inverter loses part of the energy as heat while it turns stored DC into mains AC. A 90% unit makes the bank hold roughly 10% more than the bare load suggests.
Usable energy in one battery is volts times amp-hours times depth of discharge. Rated capacity is not usable capacity. Lead-acid is normally held to about 50% depth of discharge to protect its cycle life. LiFePO4 lithium runs happily to about 80%.
Divide the energy you need by the usable energy per battery and round up. That is your battery count. Run it the other way and you get the backup time from one battery.
Take the defaults the tool loads with. A 300 W load for 4 hours at 90% efficiency needs 1,333 Wh. A 12 V 100 Ah lead-acid battery holds 600 Wh of that usable, so you need three batteries, and one on its own lasts 1.8 hours. Now switch the type to lithium. Nothing about the battery changes except the depth of discharge, which lifts usable energy to 960 Wh, so two batteries cover it and a single one runs 2.9 hours. That one dropdown is worth a third of your bank.
Why a 100 Ah battery rarely gives you 100 Ah
Here is the part most backup calculators leave out. It matters more than the depth of discharge point above.
Lead-acid capacity is quoted at the 20-hour rate, written C20. A 100 Ah battery earned that badge by giving 5 A for 20 hours in a lab. Pull harder and you get less. Wilhelm Peukert described the effect in 1897, and the modern form is t = H(C/IH)^k. H is the 20-hour rating period, C the rated capacity, I your actual current, and k the Peukert exponent,typically 1.1 to 1.3 for lead-acid. Victron ships its battery monitors witha default exponent of 1.25as a fair average.
Put that same 100 Ah battery on a 300 W load at 12 V and it is supplying about 25 A. That is five times its rating current. Peukert at k=1.25 puts real runtime near 2.7 hours, so the battery gives up roughly 67 Ah, not 100.
Lithium barely suffers this way. Its exponent sits close to 1. That is a second reason a lithium pack beats a lead-acid one of the same sticker capacity.
This calculator, like every other one, uses the plain rated Ah. So on heavy short bursts, read its lead-acid numbers as an optimistic ceiling and add headroom.
Reading the per-appliance breakdown
The breakdown table answers a different question. It always assumes one battery of the voltage, capacity and type you set at the top, never the whole bank, so if the main result told you that you need three batteries then every hour figure in this table is for a third of that system.
Add appliances from the dropdown, or type your own name and wattage. Each row then shows how long that one battery would last if the appliance were the only thing drawing from it, and the footer row shows the same battery running the whole list at once. Remove a row and the numbers redraw.
Try it. The default 12 V 100 Ah lead-acid battery holds 600 Wh usable. A 60 W CPAP machine runs about 9 hours on it. A 1,000 W window air conditioner lasts half an hour. That gap is the whole argument for choosing what stays on during a cut.
How large a home bank is allowed to get
If your answer comes back as a wall of batteries, US fire code stops you before your budget does.NFPA 855 Chapter 15 limits one residential energy storage unit to 20 kWh of stored energy. Totals are then capped per location: 40 kWh in a dedicated utility or storage room, and 80 kWh in an attached garage, on an exterior wall, or ground-mounted outdoors. Your local building authority has the final say, so check before you buy.
Treat these numbers as a plan, not a promise
Real backup time moves with battery age, temperature, cable losses, and the surge when a motor or compressor kicks in. A five-year-old bank does not hold what it did when new. Size with headroom rather than sitting on the calculated limit, and check the maker's recommended depth of discharge for your exact cells.
The maths runs in the page as you type. Nothing you enter is saved or sent anywhere, and there is no sign-up.
Frequently asked questions
How do I calculate inverter battery backup time?
Work out the energy you need, which is load in watts times hours, divided by inverter efficiency. Then divide that by the usable energy in your bank, which is volts times Ah times depth of discharge. That gives the number of batteries. Reversing it gives how long one battery lasts.
How many batteries do I need to run my home during a power cut?
Add up the watts of what you want running, pick your hours, and the tool returns a count. The answer swings wildly with what you include. A fridge, fans and lights sit around 400 W, while one window AC unit alone is 1,000 W and a well pump is 750 W.
Why does a 100 Ah lead-acid battery not deliver 100 Ah?
Because that rating is measured over a slow 20-hour discharge. Draw the battery faster and Peukert's law says the available capacity drops. At a 25 A draw a typical 100 Ah lead-acid battery gives closer to 67 Ah.
Why does switching to lithium change the battery count?
Depth of discharge. Lead-acid is limited to about 50% of rated capacity, lithium to about 80%. Same 12 V 100 Ah battery, but 600 Wh usable against 960 Wh, so lithium often cuts the count by a third.
Does the appliance breakdown use one battery or the whole bank?
One battery, at the voltage, capacity and type you set at the top of the page. Multiply the hours shown by your battery count for a rough bank figure, assuming the batteries share the load evenly.
How accurate is this calculator?
It is a sizing estimate. It uses rated capacity and a fixed depth of discharge, so it ignores Peukert losses, temperature, ageing and surge current. Expect real runtime to land below the figure shown, especially on lead-acid with a heavy load.