A battery for your fridge: calculate Wh and check start-up before choosing

Collect these details about your fridge

1. Model and power supply: 230 V AC plug or 12/24 V DC input. Check the accepted voltage range and connector.
2. Daily energy: Wh measured over a representative period, or an explicitly labelled estimate.
3. Running power: compressor watts and any other simultaneous load.
4. Start-up: initial power or current and its duration, if available in reliable documentation. If absent, they remain pending.
5. Runtime and charging: how many days or hours you need to cover without relying on mains, vehicle or sunlight.

W express power at a moment; Wh express accumulated energy. In a thermostat-controlled fridge, compressor running hours may differ from the hours it stays connected. Include any relevant consumption between cycles.

How to obtain daily consumption

A household refrigerator's European label shows annual consumption in kWh; its QR code gives model information in EPREL. This is a starting point, not a measurement in your kitchen or a description of start-up. A hypothetical label of 180 kWh/year equals 180 × 1,000 ÷ 365 ≈ 493 Wh/day. That is not 493 W of power.

If you measure accumulated energy with suitable equipment, record duration, ambient temperature, temperature setting and door usage. A reading of a few minutes may miss complete cycles. For a DC fridge, measure using the supply you will actually use; do not simply transfer a measurement taken with another adapter.

DC example: two days without charging

A hypothetical 45 W fridge with eight cumulative compressor hours per day. Eight hours does not mean it is disconnected for the rest of the day. Planning assumptions: 95% DC efficiency and 90% usable fraction of nominal capacity. These are not measured performance figures for all stations.

Daily energy in the fridge

45 W × 8 h = 360 Wh

Estimated battery energy for two days

360 Wh ÷ 0.95 × 2 = 757.89 Wh

Estimated minimum nominal capacity

757.89 Wh ÷ 0.90, rounded upwards: 843 Wh

Adding lighting or changing hours or efficiency changes the result. The battery must start at the planned state of charge. This example does not add measured DC standby consumption; include any relevant consumption between cycles in the energy budget.

AC example: eight compressor hours, socket available for 24 hours

Another hypothetical fridge consumes 120 W over eight cumulative hours per day. It must remain connected to 230 V for all 24 hours so the thermostat can restart it. We calculate one day without charging: this is not the previous DC case.

Assumptions: 85% AC efficiency, illustrative inverter standby consumption of 15 W over 24 hours and 90% usable fraction. Replace them when you have data applicable to your equipment.

Daily energy in the fridge

120 W × 8 h = 960 Wh

Battery energy for that load

960 Wh ÷ 0.85 = 1,129.41 Wh

Assumed inverter standby consumption

15 W × 24 h = 360 Wh

Estimated daily battery energy

1,129.41 Wh + 360 Wh = 1,489.41 Wh

Estimated minimum nominal capacity

1,489.41 Wh ÷ 0.90, rounded upwards: 1,655 Wh

The inverter's 360 Wh are battery consumption: do not divide them again by AC efficiency. Do not reduce 24 hours to eight because the compressor rests; the socket must remain available between cycles. For two days under the same assumptions, estimated nominal capacity would be 3,310 Wh.

Check start-up and connection separately

An assumed 15% power margin gives 52 W continuous for the 45 W DC example. A hypothetical start-up factor of 3.5 gives 45 × 3.5 × 1.15 = 181.125 W, rounded to 182 W. The 120 W AC example would require 138 W continuous and a hypothetical 483 W start-up.

The factor is not a rule for every fridge and start-up duration is unknown. If you do not know your compressor's actual start-up demand, keep it unknown in the calculator. These examples do not certify compatibility.

For a DC fridge, AC inverter power does not validate the 12 V socket. Check per-port and shared limits, voltage, current, connector and cable; a continuous limit does not prove a transient margin. For an AC fridge, check start-up at normal voltage: special modes for heating loads do not replace that check. Also confirm that the output does not switch off automatically when the load drops between cycles.

Charging and the next decision

First calculate the period without recharging. We have not deducted solar generation in these examples. A nominal panel does not deliver constant maximum power: location, shade, weather and electrical compatibility change available energy.

If consumption is missing, measure it or improve the estimate before choosing a size. If start-up or port limits are missing, resolve compatibility before buying. An energy calculation does not establish your fridge's connection or start-up compatibility.

Calculated electrical runtime does not certify food preservation: monitor storage conditions and follow the instructions for the fridge and its contents.