Power station size calculator
Two numbers decide whether a power station works for you, and they answer different questions. Capacity in watt-hours decides how long it lasts. Surge in watts decides whether it starts your appliances at all. A unit can pass one and fail the other.
This calculator accounts for the two things most sizing tools leave out — the duty cycle of anything with a compressor, and the startup surge of anything with a motor. It shows its arithmetic below so you can check it rather than trust it.
What do you need to run?
150 W running · 1200 W starting surge · runs ~35% of the time — Compressor cycles — runs roughly a third of the time
90 W running · no motor surge · runs continuously
5,029 Wh
You will consume about 3,420 Wh. Inverter losses take that to 4,024 Wh drawn from the pack, and the figure above adds a 20% reserve so you are not planning to a dead battery.
240 W continuous
Needs at least 1,290 W surge to start the largest motor while everything else is already running. A station that passes on capacity but fails here will trip the moment a compressor kicks in.
The arithmetic, so you can check it
Energy = Σ (watts × quantity × hours × duty cycle). Duty cycle is the part most calculators skip: a fridge compressor runs roughly a third of the time, so treating it as a constant 150 W load oversizes it by about three times.
From the battery = energy ÷ 0.85 — the inverter loses roughly 15% converting DC to AC.
Recommended capacity = that ÷ 0.8, leaving a 20% reserve. Lithium (LiFePO4) packs tolerate deep discharge far better than lead-acid, but planning to zero leaves nothing for a cold night or an ageing cell.
Surge = the largest single starting surge, plus everything else already running. Motors draw 2–3× their running watts for a moment at startup; resistive loads (heaters, kettles, microwaves) do not surge but are simply enormous the whole time they run.
Running watts here are typical planning figures for common household units. The nameplate on your specific appliance always wins — check it before buying.
Questions this raises
What size power station do I need to run a refrigerator?
Far less than a naive calculation suggests. A full-size fridge draws around 150 W while the compressor runs, but it only runs about a third of the time — so a day is roughly 150 × 24 × 0.35 ≈ 1,260 Wh, not 3,600 Wh. The harder constraint is startup surge: the compressor pulls around 1,200 W for a moment, so the inverter has to clear that even though the battery barely notices it.
Why does my power station trip when the fridge starts?
Because it was sized on running watts. Induction motors draw two to three times their running wattage for a fraction of a second at startup. A station rated 1,000 W continuous can be perfectly adequate for a 150 W fridge and still shut down the instant that 1,200 W surge arrives, unless its surge rating covers it.
Can a power station run a sump pump during an outage?
Sometimes, and surge is the deciding factor rather than capacity. A 1/3 HP sump pump runs at roughly 800 W but can spike past 2,200 W starting. Energy use is usually modest because the pump only runs when water arrives — but if the inverter cannot clear the surge, the runtime is zero no matter how large the battery is.
How much capacity do I lose to the inverter?
Roughly 15% converting DC to AC on a decent unit, so a 1,000 Wh pack delivers about 850 Wh at the outlet. It is also worth leaving about 20% in reserve rather than planning to a completely flat battery.
Why can't a power station run a space heater for long?
Resistive heating has no duty cycle worth the name and no efficiency trick — 1,500 W is 1,500 W, continuously. That empties a 1,000 Wh station in well under an hour. Heat is the one load where battery power is almost always the wrong answer.
Related reading
- What size power station do I need to run a sump pump?
- Portable power station vs jump starter — which do I need?
- Power stations compared
Planning figures are typical values for common household units, not measurements of your specific appliance. The nameplate on the unit always wins.