The method
Runtime = usable battery amp-hours ÷ the fridge's average draw (amp-hours per hour at 12 V). This is the method Dometic publishes for its CFX fridges. Three details make the difference between a good estimate and a dead battery:
- Average draw, not rated input. A compressor cycles on and off. We use each maker's published average draw where it exists, measured at a stated outside and inside temperature.
- Temperature gap. The compressor works harder the bigger the gap between outside air and the fridge setting, so we scale the published figure by that gap, and add a penalty below freezing because compressors get less efficient as they run colder (fitted to EcoFlow's published fridge and freezer figures). A freezer at 0°F on a 95°F day uses roughly two and a half times what a fridge at 37°F does.
- Usable capacity. Lead-acid and AGM batteries should only be drained to about 50%, lithium NMC about 80%, LiFePO4 about 90%; power stations lose some energy in conversion, so we use 85% of their rated Wh.
Solar is estimated as panel watts × peak sun hours × 0.75 to allow for controller, wiring and panel-angle losses. For the background, read how long a battery runs a 12V fridge, fridge power consumption explained and what size solar panel you need.
| Battery | 70°F outside | 85°F outside | 100°F outside |
|---|---|---|---|
| 100 Ah AGM / lead-acid | 3.1 days | 2.1 days | 39 hr |
| 50 Ah LiFePO4 | 2.8 days | 46 hr | 35 hr |
| 100 Ah LiFePO4 | 5.6 days | 3.8 days | 2.9 days |
| 1,000 Wh power station | 4.4 days | 3.0 days | 2.3 days |
Usable capacity: lead-acid/AGM 50%, LiFePO4 90%, power station 85%. At 85°F the same fridge needs roughly 80 W of solar with 5 peak sun hours to break even. Your fridge, your settings: run the calculator.
A worked example
Dometic publishes an average draw of 1.03 Ah/h for its CFX3 45 at 32°C (90°F) outside and 4°C (39°F) inside. Take an 85°F day with the fridge at 37°F. The temperature gap shrinks from 28°C to about 26.7°C, so the estimated draw is 1.03 × 26.7 ÷ 28 ≈ 0.98 Ah/h, or about 283 Wh per day.
A 100 Ah LiFePO4 battery gives about 90 Ah usable, so it runs the fridge for 90 ÷ 0.98 ≈ 92 hours (about 3.8 days) with no charging. To replace283 Wh a day with 5 peak sun hours, you need 283 ÷ 5 ÷ 0.75 ≈ 75 W of solar, which is why a 100 W panel is the usual minimum and 150–200 W gives margin for clouds. These are illustrative numbers from one published spec; your fridge, loading and weather will differ.