How long will your van fridge run on battery?
The fridge is a parked van’s biggest consumer, and “100 Ah” does not tell you how long it will last: it all depends on battery chemistry and the compressor’s real duty cycle. This tool does the full calculation, standby loads included.
Runtime without charging
23h
Less than a day: without solar or daily driving, the battery will not keep up.
- In days
- 1.0days
- Average draw
- 25.8W
The method
A 100 Ah battery at 12 V theoretically stores 1,200 Wh — but you cannot use it all: discharging a lead/AGM battery below 50% brutally shortens its life, while lithium delivers 80% without complaint. That is the first correction, and the biggest.
On the consumption side, a compressor fridge does not run continuously: the compressor starts, cools, stops. Its average draw is its power times that duty cycle — 35% in mild weather, far more in summer. Add the permanent standby loads (detector, router, heater standby), often forgotten yet continuous.
Runtime is then the ratio of the two: usable energy divided by average power drawn.
In the formula: C the capacity (Ah), u the usable share (0.5 lead / 0.8 lithium), P the fridge power (W), d the duty cycle (%), a the standby loads (W).
runtime (h) = C × 12 × u / (P × d / 100 + a)
Good to know
- Never take a lead/AGM battery below 50% charge: every deep discharge costs it cycles. The calculation already builds in that limit — do not “reclaim” it by running on the reserve.
- Duty cycle depends mostly on ambient temperature and how often you open the door: a van in full sun can double the fridge’s consumption.
- Pre-chill the fridge (and its contents) on mains power before leaving: the first pull-down hours are the hungriest.
- Charging (solar, alternator) is not counted: the result is the fully parked runtime — the worst case, the one that sizes the system.
- Thermoelectric (Peltier) coolers are outside this calculation: they run continuously and draw 3–5 times more than a compressor.
FAQ
- How do I find my fridge’s real duty cycle?
-
Most reliable: a small watt-meter or Bluetooth shunt over 24 h, which gives the average draw directly. Without gear, listen: time a few cycles (running time / total time). At 20 °C ambient with the fridge at 4 °C, it almost always lands between 30 and 40%.
- Why does lithium give 60% more runtime at equal capacity?
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Because you can use 80% of it versus 50% for lead, without damage. Add a steadier voltage (the fridge cuts out later) and better charging efficiency. That is what justifies the price: 100 Ah of lithium works like 160 Ah of lead.
- My fridge claims “0.8 kWh/24 h”: how do I use that here?
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That standardised figure equals a 33 W average draw (800 Wh ÷ 24 h). Reproduce it in the tool with, say, 45 W at 73% duty — or trust your own measured power/duty pair, closer to YOUR conditions than the standard test (25 °C, door shut).
- What solar size offsets the fridge?
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Take the tool’s average draw and multiply by 24: a fridge averaging 26 W uses 624 Wh/day. A panel yields roughly 3–4 times its rated watts in Wh per summer day (real-world sun factor): 200 W of solar therefore covers this fridge, with margin in summer and just barely in spring.
- Compressor, absorption or cooler box: which for battery life?
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Compressor, no contest, for battery use: real cold at a 20–40 W average. Absorption (3-way) fridges are built for gas — on 12 V they pull 8–10 A continuously and flatten a battery overnight. Peltier boxes belong on the road, engine running.