How much power a 12V fridge really uses

Three different numbers get mixed up on a portable fridge. Each one answers a different question. Sort them out and the rest of this site makes sense.

The numbers you'll see

• Rated input current (amps). This is the ceiling. Dometic describes it as "the maximum current input that the unit is rated"¹.
  • Use it when you size leads and fuses.
  • Don't use it to guess daily use. The fridge won't draw this all day.
• Energy consumption (amp-hours per hour, written Ah/h). This is an average. Dometic says it is the average amp-hours the unit uses on 12V "for a given ambient and set internal temperature"¹.
  • "Ambient" just means the air temperature around the fridge.
  • "Internal" is the temperature you've set inside it.
• Watts. Some labels give watts instead of amps. One extension guide says to look for the wattage on the nameplate, which is the label stuck on the unit².

Amps, amp-hours and watt-hours in one minute

• Amp-hour (Ah). One amp flowing for one hour³. Battery sizes are quoted this way.
• Watt-hour (Wh). Power over time. A kilowatt-hour is the energy delivered by one kilowatt of power for one hour⁴, and a watt-hour is a thousandth of that.
• Switching between them. If the label shows amps but not watts, multiply the current by the voltage². For a 12V system that means amps times 12 gives watts.

Dometic's own example shows the link. It writes 48 W divided by 12 V as "approx. 3.75 A"¹.

Turning the hourly figure into a day

Take the Ah/h number and multiply by the 24 hours in a day. That's the quickest way to a daily figure in amp-hours, using the same spec figure¹.

• Dometic lists its CFX3 100 at 1.56 Ah/h with the air at 32°C and the inside at 4°C¹.
• Times 24 hours, that works out to about 37 Ah in a day¹.
• Times 12 volts, it's about 450 Wh².

This is arithmetic on one maker's figure, not a measurement. Your fridge, your weather and your settings will move it. See why a fridge's power use changes for the reasons.

Why the label current isn't the daily draw

The compressor is the motor that does the cooling. It doesn't run flat out all day. It runs, cuts out, and starts again when the inside warms up.

• Dometic's worked example for a compressor cooler takes 3.75 A and multiplies it by a running time of 22 percent¹. Their answer is under 1 Ah/h.
• So rated amps times a full day gives a number far too high¹.
• A household guide makes the same point. Fridges are on all the time but cycle on and off to hold their temperature².

That 22 percent is Dometic's example, not a promise for your fridge¹. A hot day or a full load of warm drinks pushes it up.

A quick checklist before you trust any figure

• Is the spec sheet figure for temperatures close to yours? Check the ambient and internal values next to it¹.
• Is it a rated maximum or an average? Only the average belongs in a daily sum.
• Did you multiply amps by volts, not add them?
• Does it include everything on that circuit, or just the fridge?

Wiring matters too. Before you touch a lead or fuse for a new fridge, switch the battery off at its isolator and read the fridge maker's manual. Not sure? Book a licensed auto electrician. Permanent mains-voltage wiring, switchboards and grid connection belong to licensed electricians in Australia, so leave those alone.

Where to go next: work out a battery size from the daily figure, or measure your own fridge and swap the maker's estimate for a real one.