How Long Will My 12V Fridge Run on a Battery?
The fridge is usually your biggest load. Here is how long the battery will carry it.
The answer depends on three numbers: how much usable energy is in the battery, how much the fridge uses each day, and what else you run.
1. Usable energy in the battery
Usable Wh ≈ Ah × volts × the share you can safely use.
- Lithium (LiFePO4): often 80 to 90% usable. Check the maker's figure.
- AGM: about 50% if you want it to last.
A 100Ah lithium at 12.8V with 90% usable: 100 × 12.8 × 0.9 ≈ 1,150Wh.
2. The fridge's daily use
A compressor fridge cycles on and off. Watts × 24 × the share of time it runs.
A 45W fridge running 35% of the time: 45 × 24 × 0.35 ≈ 378Wh a day.
3. Days without charging
1,150 ÷ 378 ≈ 3 days, with nothing else running. Add lights, pump and phones and it drops.
4. Hot weather changes it
On hot days the compressor runs more. The same fridge at 50% run time uses 540Wh a day, which cuts the time to about 2 days.
5. Make it last longer
- Park the fridge in shade with airflow around the vents.
- Pre-cool food and drinks at home.
- Keep lid openings short.
- Set it a degree or two warmer if food safety allows.
6. Check your own numbers
Use the 12V Fridge Power Draw Calculator and the Battery Runtime Calculator.
Work on 240V AC wiring, including inverter outputs and mains inlets, must be done by a licensed electrician. For high-current DC gear, follow the maker's installation manual and have an auto electrician check your work if you're unsure.
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