Free tool
Battery Backup Runtime Calculator
Pick the appliances you want to keep running and see how long your batteries will carry them, or work backwards to how many batteries you need.
Two questions people ask constantly, answered both ways round. Pick the appliances and see how long the batteries will carry them, or set the hours you want and see how many batteries that takes. The appliance figures are realistic running loads rather than nameplate maximums.
What is drawing the power
| Appliance | Quantity | Watts each | Total watts | Share |
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Why the label on the battery is not the energy you get
A 150 ampere hour battery at 12 volts holds about 1.8 kilowatt hours on paper. You will never use all of it. Draining a lead acid battery flat damages it permanently, so only about half is genuinely usable, and the inverter loses another tenth turning direct current into something your appliances can use. The honest figure is closer to 0.8 kilowatt hours, which is less than half what the label suggests.
Lithium changes this arithmetic more than any other upgrade. It tolerates being taken down to twenty percent, loses less in the conversion and survives several times as many cycles, which is why the usable energy from the same stated capacity is roughly sixty percent higher.
The heavy loads are the whole problem
Look at the share column. Lights, a router, a television and a few chargers together are a modest load that a small bank will carry all evening. Add one air conditioner or an electric water heater and the total multiplies several times over, because those two appliances alone draw more than an entire house of electronics. This is why a system that comfortably runs the essentials collapses the moment somebody switches on the heater.
The practical answer is almost never a bigger battery bank. It is deciding in advance what the backup is for, wiring those circuits separately, and leaving heating and air conditioning on the generator or the state supply where they belong.
Starting surge, which no capacity figure covers
Anything with a motor pulls far more current for the first second than it does while running. A water pump or a fridge compressor can demand three to six times its running figure at the instant it starts. A bank with plenty of energy left will still trip if the inverter cannot supply that surge, which is why a system is sized on two separate things: the energy for the hours you want, and the peak the inverter can deliver at once. The runtime above answers the first. The surge is a question about the inverter rating, and it is worth asking before buying rather than afterwards.