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.

• compressor fridges are a part-time load; as a rule of thumb we can assume the compressor will be running 1/3rd of the time.
• compressor fridges don't take a lot of current
• but they will consume Wh (Watt-hours) over a 24-hour period
• you must make power to charge the bank
• and have enough bank to make it through periods where charging is absent

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:

• how deeply the chemistry can be discharged (~50% for lead, ~80% for LiFePO4)
• nominal voltage (12.0v for lead, 12.8v for LiFePO4)
• days of “autonomy” (discussed separately)

Storing 480Wh of energy would require:

• ≥80Ah of lead (480Wh / 0.5 depth of discharge / 12.0v nominal)
• ≥47Ah of LiFePO4 (480Wh / 0.8 depth of discharge / 12.8v nominal)

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 (“two days of autonomy”) then multiply the Ah required by 2, every 3 days by 3, etc.

Ah requirements for days of autonomy with LFP:

• fully charging every day = 47Ah, as above
• fully charging every other day = 94Ah
• fully charging every third day = 141Ah
• etc

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.