By Boat Battery editorial team · 2 minute read · Editorial policy
About a day and a half for a typical 12 V marine fridge drawing 5 A at a 45% duty cycle, which is 54 Ah a day against 80 Ah of usable capacity — before the rest of the boat is counted.
Usable capacity and fridge duty cycle
The arithmetic is simple and the inputs are where people go wrong. A 100 Ah LiFePO4 bank gives you 80 Ah usable. A typical 12 V marine compressor fridge draws about 5 A while the compressor runs. The number that decides everything is the duty cycle: what fraction of the hour the compressor actually runs.
Worked 12V fridge runtime examples
In a temperate month with a well-insulated box, 30% is realistic, so 5 A times 24 hours times 0.3 is 36 Ah a day, and 80 Ah usable is a bit over two days. In a Queensland summer at anchor, with the box opened regularly and the cabin at 32 °C, 45% is closer to the truth: 54 Ah a day, so about a day and a half.
Allow for other boat loads
That is the fridge alone. Add anchor and cabin lights, a plotter, a VHF on standby and phone charging and you are typically adding 25 to 40 Ah a day on a small boat, which takes a 100 Ah bank to a single night unless something is putting charge back in.
Calculate runtime from measured demand
This is why we push people toward the calculator rather than a rule of thumb. Your fridge, your insulation, your climate and your habits move this answer by a factor of two.
Related installation planning resources
Use these notes to prepare your own measurements and questions. Check the exact equipment manuals and have the proposed system assessed for your vessel before buying or changing electrical components.
Calculate your boat battery capacity
The house bank calculator takes your loads, your nights on the hook, your solar and your alternator, and returns the bank, the DC-DC charger, the AGM equivalent and the weight you take off the boat.
Open the calculator