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Caution: Electricity can be dangerous. Beware of any exposed wires. Work on any electrical apparatus or wiring should only be carried out by a competent person.

General
The methods described below are all based on non invasive principles in that wiring does not need to be disconnected. However as with all things electrical care must still be taken at all times. If at all unsure seek qualified advice rather than making guesses. There is a common misconception that the power used is the voltage (rms) multiplied by the current, this is not always true for AC where the result must also be multiplied by the power factor (usually between 0.8 and 1) which is the reason that multiplying the quoted voltage and current will not always give the quoted power. Another point to remember is the mains voltage of 230V has a tolerance and varies with time of day, load and distance from the substation which means that small variations in power consumption will occur.

Total Electricity Consumption
The best way is to keep a record of electricity used is to check the electricity meter reading every few weeks, trying to keep as regular as possible, calculating the amount of electricity used per day. To work out the amount of electricity used subtract the previous meter reading from the current meter reading and then divide by the number of days between readings. This will give the number of KiloWatt Hours of electricity used. Keeping a record of the daily use will give a good indication as to whether electricity consumption is decreasing or increasing. To reduce short term measurement variations in the daily rate checking every three to four weeks works best. To make this simpler and remove the need for manual calculations you can use EnCalcE an electricity use calculator which is free for non commercial use.

Appliance Power Measurement
The simplest way to measure the power used by plug in appliances is to use a plug in power meter which is used by plugging the power meter into the socket and the appliance into the power meter. This set up can be used to measure the power used under different conditions such as in use and standby. When measuring low powers e.g. standby consumption the accuracy is liable to be quite poor but should be consistent. You can purchase a simple plug in power meter from Maplin for only a few pounds.

High Power Measurement
This type of measurement can only be done using a Clamp Meter if no wiring is to be disconnected. Usually useful for appliances over 3KW rating and for measuring total power used at the main input. Beware of the point about power factor above which may lead to apparent measurement inconsistencies since these meters measure current not power. These meters work by measuring the small magnetic field generated by a wire carrying current. Read the instructions carefully before using, the most common error is clamping the meter around both live and neutral rather than just one of the wires. These meters become relatively inaccurate at low currents but can be useful for helping to trace the hidden power consumers. Also remember at low currents the current drawn can increase when an item is switched off due to power factor variations from the appliance and cabling lengths.