It is the most frequent question, and the one most often answered badly. You often read “45 W × 10 h = 450 Wh”. That calculation is wrong, and it leads people to buy twice as much as they need.
The factor everyone forgets: the duty cycle
A compressor fridge does not run all the time. It cools down to its set point, stops, and starts again when the temperature rises. Over a night it actually runs between 25 and 40 % of the time, depending on insulation, outside temperature and how often the door is opened.
The correct formula is therefore:
Power (W) × duration (h) × duty cycle = watt-hours
For a camping fridge rated at 45 W, over 10 hours of night, with a 35 % duty cycle:
45 × 10 × 0.35 = 158 Wh
Not 450. The difference is not a detail: it decides which station you end up buying.
What makes that duty cycle vary
The outside temperature, first, and strongly. At 15 °C overnight, the compressor barely runs. At 30 °C in a closed van that has been heating up all day, it can run more than half the time — consumption nearly doubles.
The set point next. Going from 5 °C to 2 °C costs a lot of energy for a debatable benefit on a few days' shopping.
The load, finally, and counter-intuitively: a full fridge uses less than an empty one. Cold mass holds its inertia, air does not. Water bottles do the job perfectly well.
The figures worth remembering
- One night (10 h), mild weather: 120 to 180 Wh
- One night in real heat: 250 to 350 Wh
- A full 24 hours, mild weather: 300 to 450 Wh
- A full 24 hours in real heat: 600 to 800 Wh
A Volta 1000 and its 614 Wh therefore hold a night of fridge comfortably, even a hot one, with room to spare for lighting and two phones. Over 24 hours in summer, you need to move up to the Volta 1600 or plan a recharge.
Do not forget the 15 % of losses
The figures above are what the appliance uses. The station, for its part, does not give back 100 % of its stated capacity: the inverter takes its share on the way. Allow 85 % usable — the deliberately cautious figure we use in the runtime table on every product page.
A 614 Wh station therefore gives back around 520 real Wh. A hot night at 300 Wh uses a little over half of it.
The habit that changes everything
Cool the fridge down while you are driving, off the alternator, rather than once you have parked. An appliance already at temperature when you switch off uses noticeably less overnight than one switched on at bedtime. It is free, and it is often worth a hundred watt-hours.