Power station runtime calculator: watt-hours, runtime and battery size

Add each appliance's watts and how many hours a day you'll run it. The calculator gives you total watt-hours a day, the battery size that covers it after losses, and how long a station of a given size should last. Use the label on each appliance, its power adaptor or its manual for the watts.

The calculator

How the maths works

Watt-hours are watts multiplied by hours. A 15W modem running for 24 hours uses 360Wh. A 2,000W kettle running for 4 minutes (0.067 hours) uses about 133Wh. Add up every appliance and you have your daily watt-hours.

A power station loses some energy turning its battery into 230V AC or regulated 12V. The calculator assumes 85% unless you change it, so a 1,024Wh battery gives about 870Wh of usable energy. That's our working assumption: makers don't all publish an efficiency figure, and real losses vary with the load. Small loads on a big inverter tend to be less efficient, because the inverter uses some power just being on.

Runtime is usable watt-hours divided by your average load in watts. The battery size you need is your daily watt-hours divided by the efficiency.

Fridges and other things that cycle

A fridge doesn't run all the time, so don't enter its compressor watts for 24 hours. Use the Energy Rating label instead: divide the kWh-a-year figure by 365, multiply by 1,000, and enter that as watts with 1 hour. For a 400 kWh-a-year fridge, that's about 1,096Wh a day. Energy Rating says the label assumes 24-hour running under standard test conditions, so real use in a hot kitchen will be higher. Our fridge in a blackout guide goes through it in detail.

For a 12V camping fridge, the manual usually gives average daily use in amp-hours. Multiply by 12 for watt-hours.

Start-up surge

Anything with a motor or compressor pulls more power for a moment as it starts. Energex says appliances with motors use three to four times their running rating while starting up. Tick the "motor" box for those rows and the calculator checks the biggest start-up, plus everything else running, against the station's rated output. If it's over, the station may trip, so stagger what you switch on.

Solar refill estimate

If you enter the watts of solar panels you'll connect, the calculator estimates hours of strong sun to put a day's use back, assuming the panels deliver 70% of their rating. That allows for heat, angle and cable losses. SolarQuotes notes Australian panels commonly run 20°C or more above the air temperature, which cuts output. Our solar pairing basics explain the voltage and current limits to check before you buy panels.

What it can't tell you

The calculator can't see your appliances, so it's only as good as the watts you put in. It doesn't check solar voltage limits, battery temperature limits or whether the station's ports can supply a given device. And it says nothing about wiring a station into your house. That needs a changeover switch installed by a licensed electrician, never a lead into a power point. For a full blackout plan, start with our cyclone season guide, and for camping, the caravan sizing guide.