Perhaps the most obvious way of finding out just how much electrical energy is being used by an appliance is to turn everything else in your house off and take two meter readings exactly one minute or ten minutes apart. This will give you an approximate figure which you can then use to calculate its power demands over longer periods based on your own usage estimates.

This is not an option most people would be willing to go for, as it is a lot of hassle, and it is usually a good idea to leave appliances such as freezers and fridges on all the time.

If you are happy with an approximation, then you might just want to take the easy route and work it out from the figures given on the appliance itself, either printed directly onto the body of the device or on a label near the power inlet. If it is a DC device with an external transformer, then these figures will be both on the unit itself and on the body of the transformer.

Although the label may only tell you how many amps a device uses in normal operation, this figure can be used to work out the power rating simply by multiplying it by the mains voltage.

If you are using a DC powered device that requires an external transformer, then you need to multiply the number of amps by the stated DC voltage, rather than the mains voltage, so if it says INPUT 10V, 2A for instance, then the power rating will be ten volts multiplied by two amps, which works out as being twenty watts.

Usually, appliances are designed to be able to work within a set range of voltages, to compensate for differing electrical supply conditions. So an appliance that is set up to work with a 240 volt supply can actually accept any mains voltage from 220 to 240 Volts.

Even if you suspect that your mains voltage is somewhat less than the maximum of 240v, it is still a good idea to use this figure in any energy calculations that you are doing, as a voltage drop can be a temporary state of affairs.

Devices designed for the international market can often work with a huge range of AC voltages ranging from 100v to 240v, and often have a switch to select between them. When calculating power usage, always use the maximum voltage used in the country you are in(240v in the UK) if you want an accurate result.