Guide · Power · 7 minute read
Sizing a power station
Power stations are sold by the big number on the front, which is watts, the most they can push at once. The number that decides whether your fridge is still cold on day two is watt hours, the size of the tank. Here is how to work out the tank you need.
The formula
Watts a device draws, times the hours it runs, is the watt hours it uses. Add up every device you want running, add 20 percent for the losses inside the station, and that is your daily number.
What things actually draw
| Device | Draw | Hours a day | Watt hours |
|---|---|---|---|
| Fridge freezer, modern | 150 W running, cycling a third of the time | 24 | 1,200 |
| CPAP without humidifier | 40 W | 8 | 320 |
| Phone, charged twice | 20 W | 2 | 40 |
| LED lantern | 5 W | 6 | 30 |
| Wifi router and modem | 15 W | 24 | 360 |
| Chest freezer | 100 W, cycling a quarter of the time | 24 | 600 |
A fridge, a CPAP, two phones, lights and the router come to just over 1,950 watt hours a day. With the 20 percent, call it 2,400. That is one day from a 2,000 watt hour station plus a little, or three days from a 2,000 watt hour station with a 400 watt solar panel in decent sun.
Three sizes, three jobs
Under 500 watt hours
Phones, lights, a radio and a CPAP for a night. Fits in a backpack. Will not run a fridge.
1,000 to 2,000 watt hours
The fridge for a day, plus everything small. The size most households should own. Recharges from a wall in about two hours and from a 400 watt panel in a long day.
Over 3,000 watt hours
Fridge and freezer for two days, a space heater for a few hours, or a well pump in bursts. Needs solar to be useful past day three.
Two things the box will not tell you
- Surge. A fridge compressor draws three times its running watts for a second when it starts. A station rated 300 watts will trip; look for 1,000 watts or more of output for any fridge.
- Cold. Lithium batteries lose capacity below freezing and will not charge at all. Keep the station inside and charge it inside.
Size the tank for the fridge. Everything else is a rounding error.
Pair it with food that needs no power
The less you cook electrically, the longer the station lasts. Freeze dried meals need a kettle of hot water, which a camp stove makes in three minutes for a few cents of fuel. Save the watt hours for the fridge.
