Would you save money by replacing your freezer?

A guide for people who don't understand electricity

Theory

This section is the hardest. If you can understand this, the rest is easy.

An electrical device uses electricity at a rate that's measured in watts (abbreviated as W). Watts are one of those odd units — like the nautical unit, the knot — which express a rate, but don't have per in their names. The higher the number of watts, the faster the device burns electricity. For example, an 8W LED bulb uses less energy than the 60W incandescent bulb it replaces.

For high-power devices, we use kilowatts (kW). One kilowatt is the same as a thousand watts. A 3kW kettle is the same as a 3000W kettle.

The total amount of energy used by a device is measured in watt-hours (Wh) or kilowatt-hours (kWh). Unsurprisingly, one kilowatt-hour equals a thousand watt-hours. Typically, your electricity company will bill you by the kilowatt-hour: each kWh will cost you a certain amount of money. You should be able to find the price by looking at your electricity bill, your electricity meter, or your supplier's Web site.

To calculate the total amount of energy used, you multiply the power of the device (in watts or kilowatts) by the length of time for which it was switched on (in hours). For example, an 8W LED bulb switched on for three hours burns 24Wh. A 1.2kWh fan heater that runs constantly for two hours burns 2.4kWh.

Should I replace my freezer?

Modern freezers use less electricity than older ones. If your freezer is very old, it may use so much electricity that you'd save money (over time) by replacing it. To make this decision, you need to know four things:

How much electricity does my current freezer use?

There are fairly cheap plugs that measure electricity consumption (both instantaneous consumpsion, in kilowatts, and total consumption, in kilowatt-hours). They sit between the mains socket and the device you want to measure. If you can get hold of a power-measurement plug, it's easy. Plug it in, make a note of the exact time and date, wait a few days, see how many kWh have been used, and then scale up to a year. (For example, if you measured for exactly a week, multiply the weekly figure by 52 to get an annual figure, because there are 52 weeks in a year.)

If you don't have access to a power-measurement plug, you'll need to do a little more work. Look in the freezer's instruction manual, on the Web, or perhaps on a little plate on the freezer itself, to see how much power it draws (in watts). Then estimate the proportion of the time for which the motor is running. One way to do this is to plan 20 times at which you'll go to the freezer over the course of a week (at different times of the day and evening) and see whether the motor is running or not. Write down the times and dates beforehand in a notebook, and put a tick or a cross by each entry when the time comes. If the motor is running 7 times out of 20 then that's about 35% of the time, so the average power consumption is 35% of the number in the instruction manual. For example, if the freezer has a 250W motor that's running 35% of the time, its average power is about 88W. Multiply that by the number of hours in a year (which is about 8760) and you'll see how many kilowatt-hours the freezer burns in a year (in our example, about 771kWh per year). You may want to reduce this figure slightly if the house is substantially warmer when you're at home than when you're out, since you won't be checking the motor when you're out.

If possible, make your measurements in the spring or autumn. If you do it in the summer, you'll slightly over-estimate the amount of energy used; if you do it in the winter, you'll slightly under-estimate.

How much does my freezer cost me to run?

This is where you need to know the price you're paying for electricity in pounds (or dollars or whatever) per kWh. Multiply this price by the number of kWh you found in the previous section and you'll know the cost of running the freezer for a year. For example, if you estimate that your freezer uses 771kWh per year and you're paying £0.52/kWh, you'll discover (sit down, please) that your freezer costs about £400 per year to run.

What would a new freezer cost to buy and to run?

Visit any white-goods shop on the Web. Find some freezers of the same size as yours. Look at the spec sheets. They should tell you how many kWh per year each freezer uses. Being cynical, I'd add 50% to the quoted values because they're bound to have been measured in ideal conditions that don't reflect real life. (You know — because marketing.)

Suppose the new freezer quotes 250kWh per year, and you're paying £0.52 per kWh. The cost of running it for a year is 250 * 1.5 * £0.52 = £195. Increase or decrease that 1.5 figure if you're more or less cynical than I am.

The purchase cost of a new freezer is right there on the Web site. Remember to add tax and the cost of delivery of the new freezer and disposal of the old one.

So should I or shouldn't I?

That depends on how fast a new freezer would pay for itself, whether you think electricity prices are likely to rise or fall in that time, whether you have the money for a replacement, and how much life you think is left in your old freezer. I can't give you a yes or no answer, but at least you now have enough information to make the decision.