Guide + Tool — Power

How to Size a Portable Power Station: The Only Math You Need

Power station marketing runs on one big number. Sizing runs on three small ones: what your devices draw, how long they run, and the losses nobody prints on the box. This page does the arithmetic in the open — then hands you the calculator so you never have to trust a headline again.

Last updated November 4, 2026 · How we choose

Step 1 — Separate the two numbers every listing blurs

Watts (W) are a rate: how much power flows at this instant. Watt-hours (Wh) are an amount: one watt flowing for one hour. A station has both — an output rating in watts and a capacity in watt-hours — and they answer different questions. Output: "can it run my microwave at all?" Capacity: "for how long?"

The classic trap is a station advertised as "3,000 watts!" whose capacity is 1,000Wh. It can start almost anything in your house — and run a 150W fridge for about six hours. Both numbers, every time, or walk away from the listing.

Step 2 — List your loads honestly (duty cycles included)

For each device: watts × quantity × hours per day = watt-hours per day. The honest part is the hours. A refrigerator's compressor cycles — roughly a third of the day in a temperate kitchen, more in a hot garage — so a 150W fridge is a ~1,200Wh/day load, not 3,600Wh. A CPAP is the opposite trap: it runs all night, every night, and the heated humidifier can triple its draw (30W without, 60–90W with, on typical published figures).

Step 3 — Apply the two corrections

The 0.85 factor. Batteries are DC; your house runs on AC; converting costs energy, and so do the station's own electronics. Plan on getting about 85% of nameplate capacity as usable AC power. A 1,024Wh station is an ~870Wh station in real life. Any sizing that skips this step is a disappointment scheduled for your first outage.

The reserve. Add 25% on top of your daily need. Loads creep, batteries age, and the outage never ends when the utility app says it will. Size = daily Wh ÷ 0.85 × 1.25.

Worked example — the winter outage basics

Fridge (1,200) + CPAP with humidifier (480) + lights (100) + two phones (40) + router (240) = 2,060 Wh/day. Corrected: 2,060 ÷ 0.85 × 1.25 ≈ 3,030 Wh suggested. That's a 3.6kWh-class station (EcoFlow DELTA Pro) for one full day with reserve — or a 2kWh station if the CPAP drops its humidifier and the fridge door stays shut (about 1,700 Wh/day, ≈2,500Wh suggested with reserve; the 2kWh class covers it at roughly a day with discipline). This is why the calculator matters: the honest answer is usually one class smaller than the fear purchase, and one class bigger than the bargain.

Step 4 — Check output against your worst moment

Add up everything that could run at the same instant. Fridge starting surge plus a microwave plus a coffee maker can spike past 2,500W — beyond a 1kWh station's 1,800W inverter, even with capacity to spare. If the combined draw beats the output rating, the station shuts down protectively and the specs page wins again. When in doubt, the answer is a bigger inverter, not a bigger battery.