I want to run a coolbox in my campervan that has a 12v leisure battery.

The given wattage is 35w

It would be plugged into an inverter that provides 230v and has roughly 85% efficiency

How should I calculate the amp usage so I can judge the amount of time I can run the fridge from my battery?

a) (35W/230V)*/0.85 giving 0.18 Amps

b) (35W/12V)*/0.85 givig 3.4 Amps

c) ???

  • 1
    \$\begingroup\$ The one that's actually the current going through the battery. \$\endgroup\$
    – Hearth
    May 19, 2018 at 18:23
  • \$\begingroup\$ You cannot plug a 12v coolbox into the 220v inverter. Perhaps you should do a diagram of what you mean. Also a coolerbox and a fridge or is it one and the same? \$\endgroup\$
    – Solar Mike
    May 19, 2018 at 18:24
  • \$\begingroup\$ 35W is most likely a peltier, and not a compressor cooler. Peltiers run on low voltage. Check if your cooler has a power supply inside... if you're lucky it uses a 12V peltier and a 230V to 12V power supply which would be redundant... \$\endgroup\$
    – bobflux
    May 19, 2018 at 18:28
  • \$\begingroup\$ @SolarMike Thanks I have a 12v battery but the cool box would normally run off the mains, hence the inverter. \$\endgroup\$ May 19, 2018 at 18:56
  • \$\begingroup\$ Could someone please just explain the maths and I'll worry about those other things you mentioned myself! \$\endgroup\$ May 19, 2018 at 18:59

1 Answer 1


You have to consider the voltage and the power on the battery side, to calculate the current:

$$P_\mathrm{bat} = V_\mathrm{bat} \cdot I_\mathrm{bat} \Rightarrow I_\mathrm{bat} = \frac{P_\mathrm{bat}}{V_\mathrm{bat}}$$

To get \$P_\mathrm{bat}\$ you need to know the output power of the inverter (which is equal to your load) and the efficiency:

$$P_\mathrm{bat} = \frac{P_\mathrm{load}}{\eta_\mathrm{inv}}$$

Putting everything together leads to your suggestion "b":

$$ I_\mathrm{bat} = \frac{P_\mathrm{load}}{\eta_\mathrm{inv} \cdot V_\mathrm{bat}} = 3.43\,\mathrm{A}$$


Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.