Measuring your own fridge's daily use

A maker's number is a starting point. Your fridge, your weather and your habits make the real one. Measuring takes one day and a meter.

Choose a method

• The fridge's own display or app. Some fridges show it. Dometic's app graph plots the inside temperature and the energy use in Ah/h¹. It only shows energy use when the fridge runs on DC power¹. Check your manual for what your model shows.
• A plug-in energy meter. It sits between the wall socket and the fridge's mains lead. It only works for a fridge you run from a mains socket, for example while pre-cooling at home. It won't read the battery side.
• A battery monitor. This counts the amps going in and out of the battery, so it reads the real draw from the battery.
  • Some work with a shunt, a very low resistance placed in the circuit. Large currents are measured indirectly via the voltage drop across it².
  • Fitting one means working on battery cables. See the safety section below.
• A clamp-on ammeter. One household guide describes this as an electrician's tool that clamps around one wire³. It gives a reading at one moment, not a day total.

Running a clean test

• Pick a normal day. Same fridge setting, same contents, same shade.
• Pre-cool and load first. If you want your steady-running figure, let the fridge settle before you start. The routine is on the packing page.
• Zero the meter or write down the starting reading. Note the time.
• Use the fridge normally. Open the lid as you would. Don't hold back, or the result won't match real trips.
• Stop after a full day. Whole days cover the on-off cycling. The compressor switches itself on and off to hold the set temperature, even though the fridge is powered all day³.
• Write down the extras. Air temperature, shade or sun, the setting, and how often it was opened. These explain the number later. See why use changes.

Reading the numbers

• Amp-hours. One amp-hour is a steady one amp flowing for one hour⁴. A battery monitor usually counts these.
• Watt-hours. A plug-in meter usually counts these, and a kilowatt-hour is one kilowatt of power for one hour⁵. A watt-hour is a thousandth of that.
• Hourly figure. Divide the total by the hours of the test.
• Daily figure. Multiply the hourly figure by the hours in a day, or just use the full-day total.

Then compare with the spec sheet. Remember that a maker's figure is for given ambient and internal temperatures¹. If your test was hotter, a higher result is no surprise.

Traps that spoil a reading

• Start-up surge. A meter shows about three times more current when starting a motor than when it's running smoothly³. Don't use that first spike as your figure. Take a reading while the device is running³.
• A one-hour test. The compressor cycles, so a short test can catch it on or off. Use a full day.
• Mixed loads. If the monitor also sees lights or a phone charger, you'll read too high. Measure the fridge alone, or note what else was on.
• Different temperatures between tests. Compare like with like.

Safety

• Plug-in meters. Use one only as the maker describes, on a standard socket. Don't open it, and don't use a damaged one.
• Clamp meters. Make sure you're familiar with how to use an ammeter safely³. Follow its manual.
• Battery monitors and shunts. Fitting or swapping one means taking battery cables on and off. Switch the battery off at its isolator first and follow the maker's instructions. If you're at all unsure, a licensed auto electrician should fit it. It isn't routine DIY.
• Mains wiring. Permanent mains wiring and grid connections need a licensed electrician in Australia. Never try them yourself.
• Lithium batteries. Follow the maker's charging and storage limits, and don't use a damaged battery. Flammable electrolytes inside lithium-ion cells make fire or explosion a real hazard⁶.

Once you've got your own daily figure, put it into the battery runtime sum.