Energy Basics

Photograph of John Twibell
John Twibell worked for many years for the UK Government Forensic Science Service before starting a private forensic practise. He developed specialisms in fire, explosion and electrical investigations (including electrocution and electromurder)

A beginners guide to energy saving Energy units

First of all we must understand a little about the units of energy used by the energy supply industry to charge us for what we use, and how this relates to the various items that consume energy in our homes. I will try to do this in a very simplistic way without going into the underlying physics

Electrical items are rated in watts (W), which is a measure of the rate at which the item uses energy or does work for us. In order to work out how much energy we have actually used, we need to know how long the item has been actually running and the electricity meter in the house works this out for our entire consumption.

The watt is a relatively small unit and in practice the supply companies charge us on the basis of the number of kilowatt hours (kWh) used. 1000 watts (1000W) equals one kilowatt (1kW). The old fashioned one bar electric “fire” operated at the rate of 1kW (1000W) and if one bar was used for an hour it would use 1kWh of electricity, supplying this energy as heat. Similarly an appliance rated at 250W would consume 1kWh if run for 4 hours, and a 100W light bulb would use 1kWh if run for 10 hours.

Although gas is measured in volume units (cubic feet) the calorific value of the gas per unit volume is known and the gas supplier will convert the consumption to kWh units for comparison purposes.

The big energy users

Drying and space heating are the biggest users of energy. Water has a high latent heat of vaporisation which means that a very large amount of energy must be put in to turn water into water vapour. Thus it takes a large amount of energy to dry clothes, so tumble driers should be avoided – try to dry clothes outside on a line wherever possible. The UV present in sunlight also helps to freshen clothes and kill bacteria.

Electric fan heaters, convector heaters or radiant heaters usually run at up to 2 kW and if run continuously would consume about 2 units per hour. However such heaters are usually thermostatted and thus only produce heat and consume energy when the thermostat dictates that the temperature is too low. Immersion heaters in hot water tanks are usually rated at 3kW and would consume 3 units per hour, but these are also thermostatically controlled.

Electric garden power tools such are surprisingly high users and even a small lawnmower is likely to consume at least one unit per hour of operation. Shredders and chainsaws may use two units per hour.  

Energy Myths

One argument that is often used to the detriment of energy saving is that “It takes more energy to restart an appliance than to keep it switched on” This is patent nonsense and I am not sure where it originates. It may derive from the fact that when many electrical items are switched on, they require a higher start –up current than when they are already running. The resistance of the filament of an ordinary light bulb is very low when it is cold but when at its high running temperature the resistance is very much higher. Thus when first switched on it takes about 10 times its running current, but the filament heats up so rapidly that the current falls within fractions of a second to the operating current. Effectively this means that energy will be saved if the bulb is switched off for longer than a few seconds. The same is true of most electrical appliances.   (There is a downside to excessively switching light bulbs in that the high start up current stresses the filament and you may have observed that bulbs often blow at switch–on.)

Cutting Electricity consumption – tackling the hidden consumers

When considering your electrical consumption, don’t ignore devices such as adaptors and chargers that are left on day and night. Although they may each have a low consumption together they can add considerably to your bill over a period. You can check your devices with a small plug-in type energy monitor.

Many people now use portable handsets for their house phone. We have a twin handset system, the base unit consumes 8 watts (W) continuously and the second handset charger 6W. We also have an answerphone running at 6W. Thus our house phone system consumes 20W, which over 24 hours amounts to 480Wh, or almost half a unit (kWh) per day.

The laptop computer consumes 6W when hibernating and the printer consumes 10W when idle. The broadband router runs at 13W and the internet phone at 9W so the dormant computer system is still consuming 38W (over 0.9 unit per day.)

TVs, Radios and other devices on standby.

Most modern audio-visual devices have a remote control on/standby facility. Our small TV consumes 9W on standby and the aerial amplifier consumes 10W. The digibox and the VCR consume 6W and 12W on standby, thus, if left in the standby mode our TV system would consume about 37W, (almost 0.9 unit per day).

The biggest surprise to me was that our mains digital radio consumes almost as much when switched off at the on/off button on the set (14W) as when operating (17W).  This amounts to a unit every three days even when the radio is apparently off! Satellite decoders are also reputed to have a relatively high consumption when in the standby state. Users of both types of device may be advised to keep them on continuously as programming updates can be transmitted at any time, but I’m afraid that our digital radio is now switched on and off as required via the mains switch!

Cutting standby consumption

The simplest way to save energy/money is to switch off as many of these items as possible when not in use. We need one phone running at night but do we really need three and an answerphone? Most people supply their TV or computer systems via four way extension leads and thus four items can be controlled at once by the mains switch.  You could use a timeswitch to switch things off overnight. They have a negligible consumption (less than 1W) and can be over ridden if necessary. You could also use a remote control switching device operated from a hand held controller. Energy monitors and standby savers can be obtained from various outlets including Kaieteur in Sidmouth.

Sid Valley Energy Action Group (SVEAG) is trying to reduce power consumption by promoting energy efficiency.  It may be possible to replace older devices with more efficient modern ones but that would involve a different trade off in energy and cost terms.

Dr John Twibell, SVEAG November 2008