By Boat Battery editorial team · 2 minute read · Editorial policy
Use measured daily energy, not compressor current multiplied by 24. Fridge demand changes with temperature, ventilation, insulation and how often the lid or door opens.
Compressor current and duty cycle
A compressor fridge normally cycles. Its running current tells you the demand while the compressor is working, but not how many hours it works in a day. Manufacturer consumption figures are a useful starting point only when you understand the test conditions. A hot cabin and warm food loaded at departure may create a very different first-day result.
Measure daily fridge energy
Measure a full day with your usual thermostat setting. Note cabin temperature, how well the condenser can shed heat and whether another appliance shares the monitored circuit. Record the first cooling period separately from an ordinary day with already chilled contents. If the monitor only reports the whole bank, temporarily account for other loads in the log rather than attributing all consumption to the fridge.
Convert watt-hours to amp-hours
For an illustrative calculation, 600 Wh over a day is about 47 Ah at a nominal 12.8 V before planning allowances. That is an example, not a consumption prediction. Two days at the same demand would require 1,200 Wh for refrigeration alone. Add lighting, communications and other loads before applying your chosen usable-capacity margin.
Reduce refrigeration demand
Look for demand reductions before buying more storage. Keep required ventilation clear, inspect door seals and precool provisions when practical. Do not alter refrigerant systems or electrical protection yourself to chase a lower number. Re-measure after any improvement; the resulting daily figure is more valuable than a guessed percentage saving.
Record with the daily reading
- Thermostat setting and ambient conditions
- Whether contents started warm or chilled
- Condenser airflow and door seal condition
- Measured watt-hours over 24 hours
Frequently asked questions
Does a larger battery make the fridge more efficient?
No. More storage extends runtime but does not reduce the fridge’s daily energy demand. Check ventilation, seals and settings as well as capacity.
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.
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