Most people buy a power station by picking a capacity that sounds reassuring. That's how you end up with a 288 Wh unit that won't run a fridge overnight — the one I tested ran a fridge for five hours — or a 4 kWh unit for a weekend of phone charging.
The alternative takes about ten minutes and gives you a number.
1 · List what you'll actually run
Not what you might run. What you will.
The most common sizing error is including a kettle "just in case". A 1,500 W kettle for ten minutes is 250 Wh — a quarter of a 1 kWh unit for one cup of tea. Decide now whether that's a real requirement, because it changes the answer enormously.
The worked example is a two-night car-camping trip:
- 12 V compressor fridge
- Two phones
- One laptop, evenings
- LED lantern
- CPAP machine
2 · Find the watts
The number on the label is usually the maximum draw, not the average. For anything with a compressor or a thermostat — fridges, coolers, heaters — the running average is far lower because the unit cycles on and off.
Typical figures, which vary by model:
| Appliance | Rated watts | Running average | Notes |
|---|---|---|---|
| 12 V compressor fridge | 45–60 W | ~45 W | Cycles; average is what matters |
| Full-size household fridge | 150 W | ~52 W | About 35% duty cycle |
| CPAP, humidifier off | 30–40 W | ~35 W | My ResMed averaged about 23 W |
| CPAP, humidifier on | 60–90 W | ~70 W | Roughly doubles it |
| Laptop | 45–90 W | ~60 W | Higher under load |
| Phone charging | 10–20 W | ~15 W | Per device |
| LED lantern or strip | 5–20 W | ~15 W | |
| Router and modem | 10–20 W | ~15 W | Continuous |
| Electric blanket | 60–100 W | ~80 W | |
| Microwave | 700–1,200 W | Full draw | No cycling |
| Electric kettle | 1,200–1,800 W | Full draw | No cycling |
| Induction burner | 1,200–1,800 W | Full draw | No cycling |
| Coffee maker | 800–1,200 W | Full draw | No cycling |
- Rated watts
- 45–60 W
- Running average
- ~45 W
- Notes
- Cycles; average is what matters
- Rated watts
- 150 W
- Running average
- ~52 W
- Notes
- About 35% duty cycle
- Rated watts
- 30–40 W
- Running average
- ~35 W
- Notes
- My ResMed averaged about 23 W
- Rated watts
- 60–90 W
- Running average
- ~70 W
- Notes
- Roughly doubles it
- Rated watts
- 45–90 W
- Running average
- ~60 W
- Notes
- Higher under load
- Rated watts
- 10–20 W
- Running average
- ~15 W
- Notes
- Per device
- Rated watts
- 5–20 W
- Running average
- ~15 W
- Notes
- Rated watts
- 10–20 W
- Running average
- ~15 W
- Notes
- Continuous
- Rated watts
- 60–100 W
- Running average
- ~80 W
- Notes
- Rated watts
- 700–1,200 W
- Running average
- Full draw
- Notes
- No cycling
- Rated watts
- 1,200–1,800 W
- Running average
- Full draw
- Notes
- No cycling
- Rated watts
- 1,200–1,800 W
- Running average
- Full draw
- Notes
- No cycling
- Rated watts
- 800–1,200 W
- Running average
- Full draw
- Notes
- No cycling
3 · Multiply by hours
Watts × hours = watt-hours. That's all it is.
The two-night trip, over 48 hours:
| Load | Average draw | Hours | Watt-hours |
|---|---|---|---|
| 12 V fridge | 45 W | 48 | 2,160 |
| CPAP, humidifier off | 35 W | 16 | 560 |
| Laptop | 60 W | 6 | 360 |
| Phones ×2 | 15 W | 8 | 120 |
| LED lantern | 15 W | 10 | 150 |
4 · Add it up
Two observations, and both are typical.
The fridge is nearly two thirds of the budget. It's the only thing running all 48 hours. In almost every camping budget, refrigeration dominates — so if you can live out of a cool box for one of the two nights, you halve your requirement.
The CPAP is the second-largest item, and it would be the largest with the humidifier on — 1,120 Wh instead of 560.
5 · Correct for the conversion loss
Here's the step almost everyone misses, and it's the reason this site exists.
A power station rated at 1,000 Wh doesn't give you 1,000 Wh. I drained eleven units from full to shutdown on the same 300 W load, with a meter at the AC outlet. They delivered between 80.1% and 92.1% of their ratings, and most landed between 84% and 90%.
So you can't buy a unit rated at your requirement. You have to divide:
Rated capacity needed = watt-hours required ÷ 0.85
For the example: 3,350 ÷ 0.85 = 3,941 Wh rated.
If the unit you're considering has a measured figure, use that instead of the 0.85 estimate. It's always better than an assumption, and it's why I publish a measured figure for every unit I've tested.
The second correction: idle draw
There's one more loss that doesn't appear in any load table: the power station's own consumption with the AC inverter switched on. I measured it on every unit, with nothing plugged in.
On a 2 kWh unit at 22 W, that's over 500 Wh a day — more than a CPAP. If your loads are light and intermittent, switch AC off when you don't need it, or run 12 V devices from the DC outputs.
The worked example, resolved
3,941 Wh rated is a large unit — the EcoFlow Delta Pro 3 class, which delivered 3,772 Wh on my meter. For a camping trip that's excessive, and at 113 lb it isn't going in a car boot.
So revisit the assumptions, which is what the exercise is for:
| Change | New two-night total | Rated unit needed |
|---|---|---|
| As listed | 3,350 Wh | 3,941 Wh |
| Fridge one night only | 2,270 Wh | 2,671 Wh |
| Add 200 W of solar, one good day | 2,270 Wh | ~1,700 Wh |
| Cool box instead of fridge | 1,190 Wh | 1,400 Wh |
With a fridge for one night and a pair of 100 W panels, you're into the 2 kWh class — the Jackery Explorer 2000 v2, which delivered 1,702 Wh on my meter, or the Bluetti Elite 200 V2 at 1,902 Wh.
Without the fridge at all, a 1 kWh unit covers it comfortably.
Two things capacity doesn't tell you
Continuous output. Capacity is the size of the tank; output is the size of the pipe. An 800 W unit won't run a 1,000 W kettle however full it is. Check the continuous watts against your largest simultaneous load. I held every unit at its rated continuous output for ten minutes, and all eleven held.
Surge. Motors — pumps, compressors, power tools — draw several times their running wattage for a fraction of a second. A fridge averaging 52 W may pull 800 W to start. In my step-up tests, overload protection tripped at between about 1.5 and 2 times the continuous rating — 580 W on the 300 W Anker C300, 8,800 W on the 6,000 W F3800 Plus. Don't plan a load right up to a published surge figure.
Sizing for solar
If you'll recharge from panels, run a second calculation — your daily use against your daily input.
Panels rarely produce their rating. I assume 75% of nominal in clear conditions, so two 100 W panels deliver about 150 W. Over a seven-hour useful solar window that's roughly 1,050 Wh a day.
If your daily use is 1,675 Wh, two panels don't keep up. You need four — and then you have to check your unit will accept four. In my testing every unit accepted within 6% of its stated solar ceiling, so the ceiling on the spec sheet is the one to plan around. Several cap at 400 W.[1]
More on this in solar recharge in practice.
FAQ
What size power station do I need?
Add up your loads in watt-hours, then divide by 0.85 to correct for conversion loss. For a fridge, phones and lights over one night, roughly 700–900 rated watt-hours. For two nights with a CPAP, closer to 2,000. For a house through an outage, 3,000 and up.
How do I calculate watt-hours?
Watts multiplied by hours. A 45 W fridge for 24 hours is 1,080 watt-hours. For anything that cycles — fridges, coolers, thermostats — use the running average, not the rating on the label.
Why divide by 0.85?
Because a power station delivers less than its rating. The inverter loses energy converting DC to AC, the battery management system reserves some to protect the cells, and the unit's own electronics consume some. On my bench, eleven units delivered between 80.1% and 92.1% of their rating.
Should I size up for battery degradation?
If you're planning for a decade, yes. Packs are rated to retain 70% or 80% of capacity after their rated cycle count — check which, because manufacturers advertise the same cycle numbers against different retention figures.[2][3]
Is it better to buy one big unit or two small ones?
One big one, for total energy. In my testing larger packs delivered more of their rating — 92.1% on the 4 kWh EcoFlow Delta Pro 3 against 80.6% on the 288 Wh Anker SOLIX C300 — and the 2 kWh Jackery carried more usable energy per pound than any 1 kWh unit. Two units make sense for redundancy or splitting the carry, not for capacity.
Once you have your number, the full ranking lists every unit I've tested by measured usable capacity, so you can match it to a real product rather than a rating.
Sources
- Jackery. Explorer 2000 v2 — specifications. Manufacturer spec · accessed 5 September 2026
- Jackery. Explorer 1000 v2 — specifications. Manufacturer spec · accessed 5 September 2026
- Anker SOLIX. SOLIX C1000 Gen 2 — specifications. Manufacturer spec · accessed 5 September 2026