What Size Power Station Do I Need for a Winter Outage?

Sizing a power station comes down to two numbers, and most people only look at one. Get both right and you’ll know exactly what you’re buying before the power goes out.

Watts vs watt-hours: the two numbers that matter

• Output, in watts (W) — the most power the unit can deliver at once. If your devices draw more than this together, the power station shuts off. Motors like furnace blowers and fridge compressors also need a brief surge when they start.
• Capacity, in watt-hours (Wh) — how much energy is stored. This decides how long things run.

A useful rule: output decides what you can run; capacity decides for how long.

The formula

Capacity needed (Wh) = average draw (W) × hours ÷ 0.765

That 0.765 accounts for the reserve the battery keeps (we assume 90% is usable) and inverter losses when converting to household power (we assume 85% efficiency). It’s the same assumption our winter runtime calculator uses.

“Average draw” matters because many appliances cycle on and off. A fridge’s compressor might draw 150 W when running but average under 50 W across a day. Your fridge’s EnerGuide label gives you the annual kWh — divide by 8,760 to get its average watts.

Three worked examples

These use typical numbers. Swap in your own from nameplates and EnerGuide labels.

1. Essentials only — lights, internet, phones

Load	Average draw
Wi-Fi router + phone charging	25 W
A few LED lights	30 W
Total	55 W

For 12 hours: 55 W × 12 ÷ 0.765 ≈ 860 Wh. A unit around 1,000 Wh covers a night comfortably. Output needs are tiny — almost any power station works.

2. Essentials + the fridge

Add a fridge rated 400 kWh/year on its EnerGuide label (about 46 W on average): total ≈ 101 W.

For 12 hours: 101 W × 12 ÷ 0.765 ≈ 1,580 Wh. You also need at least 1,000 W of output to handle the compressor starting. A ~1,000 Wh unit gets you partway there — or all the way if you cycle the fridge rather than running it nonstop. Here’s how.

3. Essentials + fridge + furnace blower

Add a furnace blower drawing 450 W that runs about half the time on a cold night (225 W average): total ≈ 326 W.

For 8 hours overnight: 326 W × 8 ÷ 0.765 ≈ 3,400 Wh. Output should be 1,500 W or more to cover startup surges.

That’s more than any single ~1,000 Wh unit holds, which is why people who want furnace backup usually go one of three ways:

1. An expandable power station with add-on batteries.
2. Run the furnace in shifts, letting the house cool a little between runs.
3. Pair a power station with a generator run safely outdoors in daylight — see power station vs generator.

Also check whether your furnace has an ECM (variable-speed) blower — they often draw far less than older motors, which can cut the capacity you need dramatically. Our furnace guide shows how to read the nameplate.

Don’t forget the cold

Every number above assumes the power station sits in a heated room. A cold battery delivers noticeably less energy, and most lithium units won’t recharge below 0°C. Some stop working entirely around −10°C. Check the limits for each model in our cold-weather spec database.

Quick sizing cheat sheet

What you want to run	Output (W)	Capacity (Wh)
Phones, lights, router overnight	300+	500–1,000
Add a CPAP overnight	300+	500–1,000
Add a fridge for ~12 hours	1,000+	1,000–1,600
Add a furnace blower overnight	1,500+	3,000+ or expandable

Ready to compare models? Plug your own devices into the winter runtime calculator, or see our picks for the best power stations for a winter outage.