Energy - The Numbers

Posted on 8th February, 2012

If you are planning any eco-building scheme, then you will place green energy somewhere near the top of your list. It is very easy to simply look at the descriptions issued by the manufacturers of renewable energy production systems, but how do you equate the figures with what you actually need? This is a question that I am frequently asked and there is no other way to answer it than to use …

Simple Mathematics …  The following is simply an exercise in statistics and does not address the ethical issues of choosing energy supply, but all the same it is a task which must be tackled before any sustainable building can be successfully designed. For those of you who grow pale at the sight of calculations – then accept my apologies; but the concept is an essential element in good eco-design, and if need be, you ought to obtain balanced professional guidance.

Energy Units … You should calculate energy requirements by using units of energy – kilowatt hours in terms of time (kWh/day) – in other words the amount of power used over 24 hours. In the UK, the total amount of energy consumed * by each person is reckoned to average about 125 kWh/day, but this figure obviously covers all power usage. Depending on the source of information, it is estimated that the average UK home uses electrical power of between 3300 kWh/year (or just over 9 kWh/day) and 4700 kWh/year (about 13kWh/day). Northern Ireland’s domestic consumption for 2007 averaged 4400 kWh/year (12 kWh/day). Ireland’s consumption per capita is roughly equal to the UK. Check your electricity bills to see how you compare.

To give an example of how appliances measure up, if your home uses an average of 500 W (0.5 kW) then it will consume 12 kWh/day. If left switched on all day, an energy-efficient (11W) light bulb uses (0.011 x 24) = 0.264 kWh/day and a desktop computer using an average of 125W will consume (0.125 x 24) 3 kWh/day. It is not so simple to calculate the power consumption of some appliances just by checking out the wattage since power demands can fluctuate, e.g. fridges use power according to their ‘chill’ setting in relation to the ambient room temperature. Don’t forget that other non-electrical appliances such as stoves or oil burners also need to be included.

Renewable Energy … Renewable-power generating appliances need to be compared to see what they can achieve for you. Roof-mounted micro-wind turbines are probably not very effective in providing for your overall electrical needs. It has been estimated that a “600 W” micro-turbine mounted on a typical roof in England generates only 0.04 kWh per day – about 7 per cent of the power used by an 11 W light bulb.In contrast, roof-mounted solar water heaters make good sense. Even where the sunlight levels achieve only about 30 per cent, a 3-square-metre panel can supply on average about 3.8 kWh per day (i.e. perhaps half of a typical family’s hot water).

Other technologies are often viewed as ‘less green’, such as heat pumps; but if they drew power solely from renewable sources (i.e. wind farms, etc) then they would be a logical environmental choice. Some of the newer ones can deliver as much as 4.9 kWh of heat in the form of hot air or hot water, using 1kWh of electricity to do so.A final thought… before paying out money to provide energy, first put into action ways of reducing your power requirements.

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