WattWise

Power Station Size Calculator

Add the appliances you want to run and get the exact capacity and output you need — surge watts included. No sign-up, no guesswork.

Appliance
Qty
Hrs/day
Watts
Running power
0 W
Output needed (with surge)
0 W
Energy per day
0 Wh

Pick a power station capacity, then see how long it runs each appliance.

Wh capacity

Runtimes assume ~85% usable capacity (inverter loss). Fridges/freezers cycle on and off, so real-world runtime is often 2–3× the steady-draw figure.

💡 How we make this free: when you buy a power station through links on this page we may earn a commission, at no cost to you. We recommend by capacity and price tier — never by who pays us — and you can use the math above to check any model yourself.

How to size a portable power station (the simple version)

There are two numbers that matter, and people constantly mix them up:

1. Output / running watts — “can it turn this on?”

This is the inverter rating, measured in watts (W). It must be higher than the combined running watts of everything you plug in at once — and crucially, higher than the surge of any motor-driven device. A fridge that runs at 150 W can spike to 600–800 W for a split second when its compressor kicks in. If your unit's output can't cover that spike, it trips off.

2. Capacity / watt-hours — “how long will it last?”

This is the battery size, measured in watt-hours (Wh). Runtime is simply capacity ÷ device watts, minus losses. A 1000 Wh station running a 100 W device lasts roughly 1000 × 0.85 ÷ 100 ≈ 8.5 hours. The 0.85 accounts for the inverter converting DC battery power to AC — you never get 100% out.

The recipe

Add up each appliance's watts × hours per day to get your daily watt-hours. Divide by 0.85 for losses, then add ~20% headroom so you're not running the battery flat every day (which shortens its life). That's your target capacity. The calculator above does exactly this.

Rough capacity guide

Popular sizing guides

Frequently asked questions

What size power station do I need?

Add up the watt-hours you use per day, divide by about 0.85 for inverter losses, and add 20% headroom. Phones and laptops: a 300Wh unit. Weekend camping: 500–1000Wh. Fridge or CPAP overnight: 1000–2000Wh. Home-essentials backup: 2000–5000Wh.

How long will a 1000Wh power station run a refrigerator?

A full-size fridge draws ~150 running watts but cycles on and off, averaging ~50–70W over an hour. So a 1000Wh unit (~850Wh usable) realistically runs a fridge ~12–17 hours, even though the steady-draw figure at 150W is ~5–6 hours.

What are surge watts and why do they matter?

Motors and compressors (fridges, pumps, tools, AC) briefly draw 2–3× their running watts at startup. Your unit's output rating must cover that surge or it shuts off — even if the running watts are fine.

Can a power station run an air conditioner?

A 5,000 BTU window unit draws ~500W running, ~1,100W surge — you need output above that surge and ~2,000Wh+ to run it a few hours. Bigger portable AC units (1,000–1,400W) need a 2,000W+ output and 2,000–3,600Wh.

Is it better to get more capacity than I need?

A little, yes — running a battery to 0% every cycle shortens its lifespan, so ~20% headroom is healthy. But oversizing wastes money and weight. Match the tier to your real daily watt-hours.