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Use this free power station size calculator to find how many watt-hours (Wh) you need for a blackout. Tick the devices you want to keep running, set how many hours of backup you want, and we will estimate the capacity and surge you need — plus the best-fit unit for renters.
Tick what you need to power
How the calculator works
Capacity needed = total running watts × hours ÷ 0.85 (to allow for inverter losses). We also flag the peak surge — the brief spike when a motor or compressor starts — because that, not the running watts, is what trips an undersized unit. For the full method see how to size a power station for a blackout, and to sanity-check the results, what a power station can run and what 1000Wh runs in a real apartment.
New to this? Start with the complete backup-power guide for renters or the buying guide.
Frequently asked questions
How many watt-hours do I need for a power outage?
Add the watts of everything you must run and multiply by the hours of backup you want, then add ~15% for conversion losses. Most renters land between 500Wh (essentials) and 1500Wh (fridge + full day).
Why does surge matter more than running watts?
Fridges, microwaves and AC units draw a brief spike 2–3× their running watts at startup. If the unit cannot supply that surge it shuts off, even if it has plenty of capacity.
Is this calculator accurate?
It uses typical wattages for a solid estimate. For exact figures, check the label or nameplate on your own devices — actual draw varies by model.
Size up, not down
When your result sits between two units, choose the larger — headroom means longer runtime and no surge trips. For most apartment setups the Bluetti AC180 covers the widest range of needs.