Portable power station size calculator

How the calculator works

Three different limits decide whether a power station can handle your setup. Most people only look at the first one.

Capacity: how long it lasts

For each appliance we multiply its average power by the hours you use it, then add everything up. That gives watt-hours (Wh) per day.

Wh needed = (average watts × hours × quantity, summed) × days ÷ 0.85

The ÷ 0.85 is there because you never get the full rated capacity out of the wall sockets. The inverter that turns battery power into household AC power loses roughly 10–20% as heat, and many units hold a little back to protect the battery. If you only run 12V or USB devices, losses are smaller.

Fridges, freezers and sump pumps cycle on and off, so their average draw is much lower than their running draw. A full-size fridge might pull 150 W while the compressor runs but average about 60 W across a day.

Continuous output: what it can run at once

This is the number in watts (W) on the spec sheet, often written as "AC output 1,800 W". It has to cover everything that might be running at the same moment. We add up the running watts of every item and add 20% headroom, because running an inverter at its limit for hours makes it hot and loud.

Output needed = sum of running watts × 1.2

There is one exception. Most power stations can surge to about twice their continuous rating, so if your list includes a motor with a big start-up draw, we raise the output figure to at least half the surge you need (see below). Otherwise a small 300 W unit would be recommended for a fridge it can't actually start.

Surge: the first second

Anything with a motor or compressor (fridge, freezer, sump pump, air conditioner, furnace fan) needs several times its running power for a moment when it starts. If the power station's surge rating is too low, it shuts off the instant the fridge kicks on, even though the fridge only needs 150 W after that. We assume one motor starts at a time while everything else is already running.

Surge needed = sum of running watts + (largest start-up watts − its running watts)

Solar

We assume 4 peak sun hours a day and that panels deliver about 75% of their label rating in real conditions (heat, angle, cloud). Winter, shade or northern locations can cut that in half.

The appliance numbers we use

These are typical figures. Your appliance can be quite different. The label on the back or bottom usually lists watts, or volts and amps (watts = volts × amps). For the most accurate figure, a plug-in power meter costs about $20 and shows real use over a day.

*Assumes the pump runs about 10 minutes an hour during heavy rain. In a storm it can run far more. CPAP figures vary with pressure setting and humidifier level; check your machine's manual and talk to your equipment supplier before relying on battery power for medical equipment.

Worked examples

Keeping a fridge, router and phones going through a one-day outage

Fridge 60 W × 24 h = 1,440 Wh. Router 15 W × 24 h = 360 Wh. Two phones and two LED bulbs add about 140 Wh. Total about 1,940 Wh, ÷ 0.85 = about 2,280 Wh. That points to a unit in the 2,000–3,000 Wh range with at least 1,300 W surge so the fridge compressor can start while everything else is on.

CPAP for a weekend of camping

CPAP with humidifier at 75 W × 8 h = 600 Wh a night. Two nights = 1,200 Wh, ÷ 0.85 = about 1,410 Wh, so a 1,500 Wh unit. Turning the humidifier off roughly halves that. A 300 W output unit is plenty; capacity is what matters here.