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.
You may not want to do this however, as apart from the inconvenience of it all, you probably do not want to be turning off your fridge or freezer unnecessarily.
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 trying to figure out the power demands of a DC device, remember to use the DC voltage stated on the transformer rather than the mains voltage in your calculations.
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.
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