The formula
Worked example
A 45 W fridge running 35% of the time uses 45 × 24 × 0.35 = 378 Wh a day, or 31.5 Ah at 12 V. A 100 Ah lithium battery (90 Ah usable) would run it for nearly three days with no charging. In 30 °C weather, when the duty cycle might reach 55%, that drops to under two days.
How to cut fridge consumption
- Give the compressor's vent space and airflow. A fridge that can't get rid of heat runs much longer.
- Pre-cool the fridge and its contents on hook-up or while driving.
- Set it a few degrees warmer: 4–5 °C is fine for most food.
- Open it less often, and keep it full. Bottles of water help hold the cold.
- Insulate around a built-in fridge, and keep it out of direct sun.
Absorption (3-way) fridges
Caravan and motorhome 3-way fridges are very inefficient on 12 V, often 100–150 W continuously. Run them on gas or hook-up when parked; 12 V is designed for use while driving only.
Frequently asked questions
How many amp hours does a 12V fridge use per day?
A typical 40–50 L compressor fridge uses 25–45 Ah a day at 12 V, depending on the weather and how often you open it.
How long will a 100Ah battery run a 12V fridge?
Usually 2–3 days for LiFePO4 (90 Ah usable) or 1–1.5 days for lead-acid (50 Ah usable), with no charging.
What solar panel do I need for a 12V fridge?
About 100–200 W of solar keeps a compressor fridge going in summer. In winter you'll need another charging source.