Camp Power Draw Calculator
Nearly every "what size power station do I need?" answer online is a guess with a product link attached. Tick what you are actually running, say how many nights you are out, and this works it out properly — including the two things most calculators quietly skip: inverter losses, and the start-up surge that stalls an otherwise correctly sized generator.
How this is worked out
No black box. Every number above comes out of four steps, and you can check each one.
- Daily energy. Each appliance is watts × hours, which gives watt-hours. Fridges and cool boxes are entered as watt-hours per day instead, because a fridge's compressor cycles on and off — its nameplate wattage tells you almost nothing about what it consumes over 24 hours.
- Inverter losses. Anything running on mains plugs goes through an inverter, which is roughly 88% efficient. Skipping this is the most common mistake in camp power maths, and it undersizes the battery by about an eighth before you have started.
- Usable capacity. A battery's rated figure is not the figure you get to use. Lithium is taken at 90%; lead-acid and AGM at 50%, because discharging them past half ruins them quickly. This is why a 100Ah lead-acid battery and a 100Ah lithium one are not remotely the same purchase.
- Surge, for generators only. Motors and compressors pull several times their running wattage for a second or two at start-up. A generator is sized on the worst of those, on top of everything already running — which is why a generator that matches your running total on paper still stalls when the fridge kicks in.
What the calculator cannot know
The figures built in are typical draws, not measurements of your gear. Overwrite them with the numbers on your own labels wherever you have them — that is what the boxes are for. Three things move the answer more than people expect:
- Cold. Battery capacity drops in low temperatures, and a fridge works harder in a hot tent. A winter trip and a summer one are not the same load.
- Altitude and heat derate a generator. Petrol engines lose output as air thins and warms; the plate rating is a sea-level, mild-day figure.
- Anything that makes heat is in a different league. A kettle for ten minutes a day costs more energy than a full night of CPAP. If your list has a kettle, a hob or a fan heater on it, that is what is driving the answer — not the lights.
Where to go next
Once you know the number, the choice is between storing energy and generating it. Our power station reviews cover battery capacity and real usable watt-hours; the generator reviews cover running and starting watts, and how loud each one is while delivering them. If you are topping up as you go, the solar panel reviews cover what these panels actually produce rather than their peak lab figure.
Everything here is our own arithmetic, and we would rather fix it than defend it — if you have measured a load and our typical figure is wrong, tell us. More on how we work: how we test.