Home Battery Backup Runtime Calculator
How long a home battery really lasts, using appliance duty cycles rather than nameplates — plus the surge check that decides whether it stays up at all.
What are you backing up?
Essential
Heating & cooling
Water
Kitchen
Comfort
Medical
Duty cycle decides this, not nameplate
A fridge is a 150 W appliance that runs about a third of the time, so its real contribution is nearer 50 W. Add up the nameplates for a fridge, a freezer, lights, a router and a furnace blower and you get 970 W. Add up what they actually average and you get 327 W. On a 13.5 kWh battery that is the difference between 13 hours and 35 — and the second figure is the true one. A calculator that sums nameplates will tell you to buy three batteries.
Runtime and surge are different problems. Runtime is energy — kilowatt-hours divided by average kilowatts. Whether the system stays up when the well pump starts is instantaneous power, and a motor draws three to eight times its running figure for the first second. An inverter that carries a house all night can still trip the moment the pump kicks in, and no amount of extra battery fixes that.
- The inverter draws power to stay awake. Thirty watts sounds like nothing and is 2.2 kWh over three days — a sixth of a Powerwall spent on nothing at all. It is trivial over an evening and substantial over a long outage.
- Discharge efficiency is not round-trip efficiency. About 94% of stored energy reaches your appliances after DC-to-AC conversion. Round-trip counts the charging losses too and is the figure that matters for cost per kilowatt-hour, not for how long the lights stay on.
- Everything restarts together when the power returns. That is the moment two motors start within the same second, which is worse than anything modelled here. Soft starters and a load manager exist for it.
- Duty cycles are typical, not measured. A fridge in a hot garage runs far more than one in a cool kitchen. If the number matters, a plug-in energy meter on the real appliance for a day beats any table including this one.
How to use
- Pick your battery, or type a usable capacity for several units.
- Tick the appliances you want to keep running.
- Read the runtime, and compare it against the nameplate figure below it.
- Check the surge tile — a motor start can trip a system that has plenty of energy.
Frequently asked questions
Why is this runtime longer than other calculators give?
Because those add up nameplate watts and this uses duty cycles. A refrigerator is a 150 watt appliance that runs about a third of the time, so its real contribution is nearer 50 watts. For a fridge, freezer, lights, router and furnace blower the nameplates total 970 watts and the actual average is 327 — on a 13.5 kilowatt-hour battery that is the difference between 13 hours and 35. Summing nameplates tells people they need three batteries when they need one.
What is starting surge and why does it matter separately?
Anything with a motor draws three to eight times its running current for the first second or so while the rotor gets moving. That has almost no effect on runtime, because it lasts a moment, and it decides entirely whether the system stays up. An inverter that comfortably carries a house all night can trip the instant a well pump starts. Runtime is an energy question and surge is a power question, and extra battery capacity does nothing for the second one.
How is the worst-case surge worked out?
The largest single motor starting while everything else is already running. That is the standard method and it is still optimistic: when utility power returns, everything that was interrupted tends to restart within the same second, which is worse than any single start. Soft starters on the large motors, or a load manager that staggers them, are what real installations use to handle it.
Does the inverter itself use power?
Yes, and it is easy to overlook. Twenty to thirty-five watts of standby draw sounds like nothing and comes to over two kilowatt-hours across three days — roughly a sixth of a large home battery spent purely on staying awake. It is irrelevant across an evening and substantial across a long outage, which is why systems that can sleep between loads do better on multi-day events.
What is the difference between discharge and round-trip efficiency?
Discharge efficiency is what you lose converting stored DC into AC on the way out, around 94 percent, and it is the figure that affects how long your lights stay on. Round-trip efficiency also counts the losses going in during charging, so it is lower, and it is the figure that matters for what a stored kilowatt-hour costs you. Using round-trip for a runtime calculation understates the answer.
Should I back up the whole house or just essentials?
Essentials, nearly always, and the reason is surge rather than energy. Central air conditioning and electric water heating are enormous compared with everything else, and a single 3 ton compressor can exceed a mid-sized inverter on startup by itself. A critical loads panel that carries the fridge, some lights, the furnace blower and the network costs less and lasts vastly longer than trying to keep everything alive.
How accurate are these duty cycles?
They are typical figures rather than measurements, and the variation is real. A refrigerator in a hot garage runs far more than the same unit in a cool kitchen, and a furnace blower in January is a different load from one in October. If the answer genuinely matters — for medical equipment, or for deciding what to buy — put a plug-in energy meter on the actual appliance for a day. That beats any table, including this one.
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