Nelpuni Energy · Practical example · Explained from the basics ·
How much battery does a camper fridge need for two days?
Let’s work through a simple example: how much battery a camper fridge might need for two days. You do not need to know about electricity. We explain the words first, then do the sums. The result is a guide; it cannot guarantee how long a particular fridge will run.
Three simple ideas before we start
W means watts: how much electricity the fridge needs while its motor is running. In this made-up example, it needs 45 W. Think of watts as the rate at which energy is being used at that moment.
Wh means watt-hours: the total amount of energy used. If a fridge needs 45 W and its motor runs for 8 hours altogether, it uses 360 Wh. Put simply: W is the rate; Wh is the total used over time. A battery’s capacity is also usually shown in Wh.
DC means direct current. On a Spanish label, it may also appear as CC. It is the type of electricity supplied by a battery. A household socket usually supplies AC (alternating current). You do not need to learn the electrical theory: check the label to see which current type and how many volts (V) the fridge accepts.
The example, step by step
Imagine a 45 W fridge. Its compressor is the motor that cools it: it switches on and off to keep the fridge cold. We assume that all those on-times add up to 8 hours a day. The fridge can stay plugged in for all 24 hours. We want to go 2 days without recharging and do not count any solar power.
- One day: 45 W for 8 hours = 360 Wh. That is the energy the fridge would use in one day in our example.
- Two days: 360 Wh a day × 2 = 720 Wh for the fridge. We also assume that for every 100 Wh the battery supplies, 95 Wh reach the fridge. For 720 Wh to reach the fridge, the battery would need to supply about 758 Wh.
- We do not assume we can use every last bit of the battery. We assume that 90 out of every 100 Wh it stores can be used. That is why the estimated battery size rises to about 843 Wh.
The 95% and 90% figures are assumptions used to explain the calculation, not specifications for a real battery. Change the inputs and the result changes.
Try the example with your own figures
How much battery would I need?
We use an example fridge. If you do not know the figures for yours, that is fine: leave these numbers as they are and see how changing them affects the result.
Estimated energy per day
Wh means watt-hours: the total electricity used. For example, 100 W for 2 hours adds up to 200 Wh. W is the rate; Wh is the total accumulated.
Estimated battery size
This is how much energy the battery would need to store in this example; it is not a recommendation for a specific model.
DC (direct current) is the type of electricity supplied by a battery. We assume that for every 100 Wh it supplies, 95 Wh reach the fridge, and that 90 out of every 100 Wh stored can be used. These are calculation examples, not guarantees. The calculation does not check whether the cable fits or whether the battery can start the fridge.
If a figure is not on the label or in the manual, leave the example value. You do not need to guess.
Continue with this case in the calculator →What to check before choosing
In the original example, 843 Wh is a rough guide to how much energy the battery stores; it does not choose a battery for you. If you change the calculator inputs, the result changes too. Before buying, check three things on the labels or in the manuals: that the battery has enough capacity, that the socket and cable are suitable, and that it can handle the extra demand the motor may need when starting. This calculation checks none of those connections or start-up requirements.
Do not know how many watts your fridge uses? Check its label or manual for its electrical consumption; cooling capacity is different. Annual use may be shown in kWh, or kilowatt-hours: 1 kWh equals 1,000 Wh. That is energy added up over a year, not the watts the fridge uses while running.
If a figure comes from a measurement, note how long you measured and the operating conditions. A short measurement may miss complete compressor cycles.
Next step
Use the result as a guide. Before buying, check that the cable fits and that the battery can handle the extra power the fridge needs when starting. The full guide also explains the case of a fridge plugged into household mains.