Fridges are a special case among domestic appliances, in a group together with air conditioners, freezers and heat pumps.
These types of appliances have compressors on board driven by AC electric motors which have properties that challenge the use of batteries and solar.
The average fridge consumes 40 watts of electricity per hour which means 960 watt-hours of battery capacity would be needed to run a fridge for 24 hours. 960 watt-hours equates to 80Ah, so a 160Ah deep-cycle lead-acid battery discharged to 50% will be required.
Elon Musk would say you could run anything on a battery, and it’s true but it needs to be sized properly, taking into account the load characteristics.
A DC load, a purely resistive AC load or even a continuously running AC motor load are straight-forward to deal with.
Fridges and similar appliances stop and start in normal operation, which makes it tricky to calculate exactly how much power they use.
In another post I ran tests on a new fridge and an old fridge to see exactly how much current they took. Fridge technology is improving all the time, like everything else.
For example, fridge compressors start and stop to maintain the internal temperature. Each time they start the motor surge current can be 3 times more power than when it’s running, and sometimes the compressor isn’t running at all.
Older fridges run about 50% of the times, while newer models have very efficient compressors which may run 80% of the time but consume less energy.
Although my tests were interesting from a point of view of curiosity, by far the easiest way to find out how much power your fridge uses is to read it from the label.
Each manufacturer gives quite an accurate average annual kilowatt-hour figure for new fridges. You can use this to calculate battery drain.
The average energy consumed by a fridge is between 250 to 900 kilowatt-hours per year (kWh/yr).
Let’s say 350kWh/yr.
The average fridge rating in watts is about 500 watts but the amount of energy used comparing old and new fridges can be considerable.
I compared current readings for similar sized fridges, one a 12 year old and the other 1 year old. The older fridge consumed twice as much power on average than the newer one.
Why is this?
Compressors, like all moving parts, are subject top wear and need more power to drive them. Electrical components also deteriorate and the Power Factor slowly reduces over the years, making the fridge less and less efficient.
Read about Power Factor here.
It’s never a good idea running anything off a car battery unless it’s an emergency. Car batteries are designed to service cars – period!
Car batteries deliver hundreds of amps in a short time for cranking and starting auto engines. When the engine fires the battery immediately starts charging and is never discharged more than a few percent.
This is the important part – car batteries are designed not to be deeply discharged, or they’ll be ruined very quickly.
On the other hand, deep-cycle, or leisure, batteries can deliver medium currents for hours without damage. This is the type you need to run a fridge.
How long can I run a fridge off a car battery?
The average car battery has 50Ah capacity and shouldn’t be discharged more than 15%, which is:
50Ah x 15% = 7.5Ah
Converting Ah to watt-hours then:
7.5Ah x 12 volts = 90 watt-hours (Wh)
Taking the average fridge energy consumption of 350kWh/yr then each hour the fridge will consume:
350kWh /365 days /24 hours = 40 watts
This is how long I would run the average fridge from a 50Ah car battery before it’s damaged:
90 watt-hours / 40 = 2.25 hours
Can you run a deep cycle battery flat?
A lead-acid deep-cycle battery can be theoretically by discharged to 80% of it’s capacity but only 50% is recommended to maximize the life of the battery.
Battery life can be extended by 3 to 4 times by limiting the Depth of Discharge (DoD) to 50%. Of course, this means that you only have 50Ah of a 100Ah lead-acid battery available.
For this post, I’ve talked about lead-acid technology because it is still the most common, but I strongly recommend lithium iron phosphate, which can be discharged up to 95% without damage.
Conclusion – What size battery do I need to run a fridge?
I would assume that the battery in question would be recharged with solar during daylight hours, which would need to be sufficient output to run the fridge and recharge the battery ready for the night-time.
In this case the battery would need to run the fridge for about 14 hours to night-time hours and early morning/evening when solar output is tiny.
Energy consumed by the average fridge in 14 hours will be:
40 watts x 14 hrs = 560 watt-hours
560 watt-hours equates to a 50 Ah but remember you will need a 100Ah battery as you want to discharge down to 50%.
In this case a 100Ah battery will be required and it will run for about 14 hours.
Note: An inverter will be needed to run an AC fridge on a battery and they also have their losses which need accounting for.
In my experience, during testing I found that as the battery capacity moves towards 50% the inverter low-voltage alarm beeps.
The extra load of the compressor motor surge current pulls the voltage down. If it falls too much the motor will stall and may become damaged.
Recommendation: Install 50% more battery capacity than you think you’ll need and use LiFePo4.
Table – What Size Battery To Run Fridges – 200kWh to 900kWh Per Year
Battery size required to run fridge for 24 hours
Annual Fridge Energy Consumption (kWh)
12V Lead-acid deep-cycle discharge to 50% of capacity
12V Lithium iron phosphate discharge to 80%