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OffgridCalc

Power Station Calculator: what size do I need?

1,000 Wh or 2,000? And will the outlet run a kettle? This calculator answers both at once: it adds up your devices including the losses at each output, factors in the days you want to go without recharging plus a reserve, and names the matching size class – along with the continuous and surge power your unit has to deliver.

Because power is where most bad purchases happen: a large capacity is useless if the output can't run your strongest device.

Devices

Rated power from the label, actual runtime per day, and whether the device is on the AC outlet (1) or on 12 V/USB (0).

Your needs

How long should the power station last without an outlet?

Weekend = 2. With solar recharging you need less.

What comes in daily (solar, car, hookup). 0 = none. The solar & off-grid calculator works out the solar yield.

Buffer for cold days, extra devices and ageing. 20 % is typical, 30 % in winter.

Advanced settings

Power stations typically deliver 85–92 % of the rated capacity at their outputs.

Inverter losses, typically 82–88 %.

Considerably more efficient than the inverter, typically 90–95 %.

Result

Recommended power station

2,048 Wh

calculated requirement: 2,032 Wh · lasts 2.4 days

Requirement versus recommended size2,032 / 2,048 Wh
Required continuous power
65 W
strongest single device
Required surge power
135 W
inrush current at switch-on
Consumption per day
762 Wh
incl. 77 Wh conversion losses
Calculated capacity needed
2,032 Wh
for 2 days incl. reserve
Runtime of the recommendation
2.4 days
with 2,048 Wh
Everything at once
120 W
sum of all devices
  • “Cooler” briefly pulls 135 W at start-up. The power station's surge rating has to be above that.
  • If everything really runs at once it's 120 W. That rarely happens in practice – but the outlet should handle it.
Show calculation
  1. 1Cooler: 45 W × 10 h via 12 V/USB = 489 Wh
  2. 2Laptop: 65 W × 3 h via AC = 229 Wh
  3. 3LED lights: 10 W × 4 h via 12 V/USB = 43 Wh
  4. 4Daily consumption: 762 Wh (of which 77 Wh conversion losses)
  5. 5Required capacity: 762 Wh × 2 days × (1 + 20%) ÷ 90% = 2,032 Wh
  6. 6Matching size class: 2,048 Wh – lasts 2.4 days
  7. 7Required power: 65 W continuous, 135 W briefly at start-up

 

How it's calculated

A power station has to meet two requirements at once – and most bad purchases happen because only the first one gets checked:

  1. Energy (Wh): is the capacity enough for as many days as you want to go without an outlet?
  2. Power (W): can the unit actually run your most demanding device?

A 2,000 Wh power station rated at 300 W has plenty of energy but won't run a kettle. Conversely a 2,000 W output is little help if it's empty after four hours. That's why this calculator works out both separately.

daily consumption [Wh] = Σ (power × runtime ÷ output efficiency) capacity [Wh] = (daily consumption − recharge) × days × (1 + reserve) ÷ usable fraction
762 Wh/day × 2 days × 1.2 ÷ 0.9 ≈ 2,032 Wh → recommended: the 2,048 Wh class

Why the output port changes consumption

Devices on the AC outlet run through the inverter and cost an extra 12–18 %. On the 12 V or USB output it's only 5–10 %. The same laptop therefore draws more from the power station on its wall adapter than on USB-C.

Continuous power, surge power, simultaneity

Compressors (coolers, fridges) and motors (pumps, fans) draw a multiple of their rated power at start-up – only for seconds, but the power station has to supply it.
FigureWhat it meansHow to size it
Continuous powerWhat the unit can deliver indefinitelystrongest single device
Surge powerBrief inrush current at switch-oncompressor or motor × 2–3
SimultaneityTotal if everything runs at oncerarely happens in practice
The 20 % reserve covers what's hard to predict: cold nights, an extra device, cell ageing. If you're planning tight, use 30 %.

Worked example

A weekend in the van without hookup: compressor cooler, laptop on its AC adapter, LED lights and two smartphones – two days without recharging, 20 % reserve.

Inputs

  • Devices:
  • Kühlbox: 45 W × 10 h/day × 0 1/0
  • Laptop: 65 W × 3 h/day × 1 1/0
  • LED-Licht: 10 W × 4 h/day × 0 1/0
  • Smartphone laden: 20 W × 2 h/day × 0 1/0
  • Days without recharging: 2 days
  • Recharge per day: 0 Wh
  • Safety reserve: 20 %
  • Usable fraction of capacity: 90 %
  • AC output efficiency: 85 %
  • 12 V / USB output efficiency: 92 %

Result

3,072 Wh

Recommended power station

Required continuous power
65 W
Required surge power
65 W
Consumption per day
805 Wh
Calculated capacity needed
2,148 Wh
Runtime of the recommendation
3.4 days
Everything at once
140 W

Calculation

  1. Kühlbox: 45 W × 10 h via 12 V/USB = 489 Wh
  2. Laptop: 65 W × 3 h via AC = 229 Wh
  3. LED-Licht: 10 W × 4 h via 12 V/USB = 43 Wh
  4. Smartphone laden: 20 W × 2 h via 12 V/USB = 43 Wh
  5. Daily consumption: 805 Wh (of which 80 Wh conversion losses)
  6. Required capacity: 805 Wh × 2 days × (1 + 20%) ÷ 90% = 2,148 Wh
  7. Matching size class: 3,072 Wh – lasts 3.4 days
  8. Required power: 65 W continuous, 65 W briefly at start-up

The variables explained

Devices: power, runtime, output
Rated power from the label, actual runtime per day, and whether the device is on the AC outlet or on 12 V/USB.
Days without recharging
How long the power station should last without an outlet, solar or the car. Weekend = 2; a week of boondocking = 3–4 with solar.
Recharge per day (Wh)
What comes in daily – from a solar panel or driving. Reduces the required capacity directly.
Reserve (%)
Safety margin for cold days, extra devices and ageing. 20 % is a good starting point.
Usable fraction (%)
Power stations deliver 85–92 % of the rated capacity at their outputs (BMS reserve and converter losses).

Common mistakes

  • Looking only at watt-hours: a kettle needs 1,500 W of continuous output – many units fail there regardless of capacity.
  • Forgetting inrush current: a 45 W compressor cooler briefly pulls 135 W at start-up.
  • Using the full rated capacity: of 1,000 Wh on the label about 900 Wh reaches the outputs.
  • Running everything on AC: the inverter costs 12–18 % and idles on top. Cooler and lights on 12 V save noticeably.
  • Counting the cooler at 24 hours: it cycles – 8–12 hours of actual runtime in summer.
  • Planning for the best day: three overcast days in a row are normal if solar is meant to do the recharging.

Assumptions and limits

  • All devices run the same hours every day; seasonal variation is not modelled.
  • Recharging is assumed to be spread evenly across the days. How much solar actually delivers is what the solar & off-grid calculator is for.
  • Inrush factors come from the presets (compressors ×3, motors ×2). Custom devices are calculated without a surcharge – the data sheet takes precedence.
  • The recommended sizes are common market classes; individual manufacturers differ.
  • Temperature effects on capacity are not included: below 0 °C lithium batteries deliver noticeably less and often cannot be charged at all.

Frequently asked questions

What size power station do I need for a weekend in a van?

With a compressor cooler, laptop and lights you land at roughly 700–800 Wh per day. For two days without recharging plus a 20 % reserve that's about 2,000 Wh – the 2 kWh class. With a 200 W solar panel adding 600 Wh a day, the 1 kWh class is enough.

How many watt-hours do I need per day?

Power × runtime, device by device. Frugal use with lights, phone and laptop: 200–400 Wh. With a compressor cooler: 600–900 Wh. With a coffee maker, kettle or induction cooktop, quickly 1,500 Wh and more.

Is a 1,000 Wh power station enough for a fridge?

For a small 120 V fridge at 60 W running 8 hours that's around 565 Wh a day including inverter losses – so the usable 900 Wh lasts about a day and a half. A 12 V compressor cooler is far more efficient.

What do continuous power and surge power mean?

Continuous power is what the unit can supply indefinitely (e.g. 1,000 W). Surge power is a brief reserve for inrush currents (often double). Your strongest device must stay below the continuous rating, its inrush below the surge rating.

Can I run an electric kettle from a power station?

Only with at least 1,500 W of continuous output. Energy-wise it's harmless – six minutes of boiling is 150 Wh – but many smaller units shut down under that load. Check the continuous rating, not the capacity.

Is the 12 V output worth it compared with AC?

Yes, clearly. The inverter costs 12–18 % of the energy and keeps idling. Cooler, LED lights and USB devices connected directly to 12 V or USB-C save 10–15 % of daily consumption.

How big should the reserve be?

20 % is typical. If you travel in winter, plan close to the limit or want the unit to last years, use 30 %: cold reduces usable capacity and cells age.

Power station or a fixed battery in the vehicle?

A power station is plug-and-play, portable and needs no installation – but costs more per watt-hour and has higher conversion losses. A fixed LiFePO4 battery with a charge controller and inverter is cheaper per kWh and more efficient, provided the wiring and fusing are done properly.

Sources and background

  • Manufacturer data sheets of power stations (usable capacity, continuous and surge power, solar input)
  • The solar & off-grid calculator on this site for the daily solar recharge