Key takeaways
- Use average draw (25–45W), not the compressor’s peak rating.
- Usable energy at 80% DOD: 1.0 / 3.2 / 2.0 / 4.1 kWh.
- A 120V fridge through an inverter adds losses and a startup surge.
- Measure your own fridge for 24 hours before trusting a table.
1. Planning table
Approximate runtime at 80% depth of discharge. Real numbers move with ambient temperature and duty cycle; treat these as a starting point, then measure.
| Average draw | 12V 100Ah | 12V 314Ah | 24V 100Ah | 24V 200Ah |
|---|---|---|---|---|
| 25W | ≈41 h | ≈129 h | ≈82 h | ≈164 h |
| 35W | ≈29 h | ≈92 h | ≈59 h | ≈117 h |
| 50W | ≈20 h | ≈64 h | ≈41 h | ≈82 h |
A “100W” fridge does not draw 100W all day. The compressor runs maybe 30–50% of the time; average draw is usually 25–45W. Use a energy monitor or the battery app’s current log to find yours.
2. What changes the answer
- Ambient temperature: a 35°C van is much harder than a 20°C one.
- Insulation and door openings: every opening adds compressor time.
- Fridge type: compressor fridges are far more efficient than thermoelectric coolers.
- Inverter: a 120V fridge adds 10–20% loss and a startup surge.
- Battery temperature: cold reduces usable capacity; charge is blocked below 0°C.
3. Measure it in one day
Put a energy meter between the battery and the fridge for 24 hours, or log the app’s current while the fridge cycles. Divide Wh by 24 for average watts, then divide usable Wh by that number.
4. How to extend runtime
- Pre-cool the fridge on shore power before a trip.
- Keep it out of direct sun and improve ventilation around the compressor.
- Open the door less; use a fridge blanket in hot weather.
- Add solar or a second battery before adding a bigger fridge.
Common questions
Can a 100Ah battery run a 12V fridge overnight?
Usually yes. At 35W average, a 100Ah pack gives about 29 hours at 80% DOD. If the fridge is efficient and the night is cool, one night is comfortable; hot weather and a 50W average leave much less margin.
Does a 120V fridge use more battery?
Through an inverter, yes — add 10–20% for conversion losses and check the startup surge. A native 12V compressor fridge is usually the more efficient choice for a van.