Updated:16-May-2013

Miscellaneous

Electricity Measurement Tips

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 alternating current (AC) electricity which is used for domestic supplies 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 (in the UK) has a tolerance (+10% to -6%*, which gives a voltage range of 216 to 253V and can vary with time of day, load and distance from the substation. This means noticeable variations in power consumption can occur due to voltage variation making some power results apparently a bit inconsistent.

If you want to convert electricity used into CO2 generated check out the conversion page.

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 per day. Keeping a record of use will give a good indication as to whether electricity consumption is decreasing or increasing. A suitable meter reading record sheet can be found in the record sheet section. To reduce the effect of short term 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 EnCalcEU an electricity use calculator which is free for personal non commercial use.

An alternative which requires less effort is to use an electricity consumption meter which works by attaching a sensor to the main power cable, the only down side of these is the accuracy is not as good as taking meter readings. They do have the benefit of allowing real time consumption to be measured. You can purchase a power monitor from Maplin for around £40. The reason the accuracy of these meter is not good is they measure current drawn rather than true power which means the power factor is ignored. Experience of the one we have been using in two houses is that they under read by around 10% based on comparing with meter readings. This accuracy will vary in different situations because the main cause of the inaccuracies is the constant standby consumption which will consist of a number of low power items where the power factor can be quite low. See finding the hidden consumers for more information on this subject.

How To Read The Electricity Meter

Reading an electricity meter can be somewhat confusing because there are two main types of meter with several variations of each type. Electricity meters measure electricity consumption in KiloWatt Hours (KWHrs). Below are a selection of links with information on how to read a range of different meters:

How to read electricty meters from USwitch

How to read electricty meters from Which

How to read electricity meters from Citizens Advice

How to read your electricity meter from EDF Energy

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. These plug in power meters will usually also measure voltage and current. You can work out the cost of appliances by measuring the power and then using EnCalcE an cost and cost comparison calculator which is free for personal non commercial use.

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 alternating 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.