How to Size a Portable Power Station (Without Guessing)

Updated 2026-09-24

Every power station has two numbers that matter far more than the rest, and most buying mistakes come from mixing them up.

• Capacity (Wh) is the size of the tank. It decides how long things run.
• Output (W) is the size of the pipe. It decides what can run at all.

A 2,000Wh station with a 1,000W inverter can run a fridge for days but can't start a microwave. A 300Wh station with a 600W inverter can run a microwave — for about 15 minutes. You need both numbers to fit.

Step 1: Add up what runs at the same time

List everything you'd want running simultaneously and add the running watts. That total must be under the station's continuous output rating, with some margin. Check the label on each device — it usually lists watts, or amps (multiply amps × 120 to get watts).

Step 2: Check the biggest start-up surge

Anything with a motor or compressor — fridges, freezers, pumps, AC units, power tools — draws two to four times its running watts for a fraction of a second when it starts. The station's surge rating has to cover that spike on top of everything else already running.

This is the most common reason a station "trips" when the fridge kicks on. If a fridge runs at 150W but needs 1,200W to start, you want a station whose surge rating clears 1,200W plus whatever else is plugged in.

Step 3: Work out capacity from runtime

Now decide how long you need things to run. The formula:

Runtime (hours) = Capacity (Wh) × 0.85 ÷ Average draw (W)

The 0.85 accounts for the energy lost converting battery DC power into household AC. It's a reasonable average; small loads on a big inverter can be less efficient.

"Average draw" matters for anything that cycles. A fridge might pull 150W while its compressor runs, but it only runs about a third of the time, so it averages closer to 50W — roughly 1.2kWh a day.

Worked examples

Overnight CPAP while camping. 45W × 8 hours = 360Wh, ÷ 0.85 ≈ 425Wh. A 500Wh-class station covers one night; run the CPAP from a DC adapter instead of the AC outlet and it stretches further. See our CPAP guide.

Keep the fridge, router and a few lights going during a 24-hour outage. Fridge averages ~50W, router 20W, lights 40W for 6 hours. That's about 1,200 + 480 + 240 = 1,920Wh ÷ 0.85 ≈ 2,260Wh. A 2kWh station gets you most of the way; a 1kWh unit gets you through a long evening and night. The fridge guide goes deeper.

Van weekend with a 12V fridge, laptop and phones. 12V fridge ~20W average, laptop 60W × 4h, phones 30Wh. That's ~480 + 240 + 30 = 750Wh per day. A 1kWh station plus 200W of solar keeps up indefinitely in good sun.

Don't forget recharging

If an outage could last several days, the question becomes how fast you can refill. Solar input (in watts) is the ceiling: a station with 500W solar input and a matching array can recover roughly 2kWh on a good summer day, but only if you actually own that many panels. Stations with higher solar input ratings are worth more for multi-day outages.

Shortcut

Skip the math: the runtime calculator does all three steps for every station at once, and flags which ones can't handle your surge.