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This is a subset of “how do I run this load” but since it comes up frequently it will be addressed here as a separate case.
The fridge specs (or reviews) will report the power required when the compressor is running. This is usually something like 45w-60w. We will use the 60w example from here on.
The compressor only runs as needed. It will run more in hot ambients and less in cool ambients. Until you observe it in actual conditions we will assume it runs 1/3rd of the time.
60w x 0.33 duty cycle x 24 hours = 480Wh required each day to run the fridge. We will use the 480Wh example from here on, and assume this is the only load. In real life the fridge will be only one part of your daily power requirements.
The battery bank capacity is dependent on a few factors:
Storing 480Wh of energy would require:
The capacities above assume the bank is fully charged each day (“one day of autonomy”). If you can only count on getting a full charge every 2 days then multiply the Ah required by 2, every 3 days by 3, etc.
Ah requirements for days of autonomy with LFP:
Note: the math is simple with LiFePO4, since it is not affected by partial states of charge. With lead banks larger capacities require stronger charging setups.