Energy: know your units, consumption, costs

Before getting down to what you can do to generate some of your own renewable energy, I reckon it's important to have a good idea of the energy you're currently using in the house, what it costs, what you might be able to cut down on, and what might realistically be replaced by renewable sources.

If you know this already, then skip on. And sorry if it's boring, but I really believe you should be familiar with it.

Units of measurement

First the kilowatt (kW), 1000 watts: a unit of power, or a rate of use of energy.

Anything that you use to heat stuff is likely to use quite a lot of power, say a 3kW kettle, or immersion heater. Or a central-heating boiler, which might use up to (say) 20kW.

Other appliances (not for heating) are quite likely to use less than 1kW or 1000W, so working in watts (approximately):

  • Fridge, computer, TV: 200
  • Old (tungsten incandescent, or halogen) lamp: 60
  • New (compact fluorescent) lamp: 15
  • Modems, routers, etc.: 10
  • Phone chargers etc.: 3
  • All the above is talking about rates of use. What we actually pay for is the amount of energy used, and in nearly all cases the unit chosen is the kilowatt hour (kWh) - the amount of energy used if a 1kW appliance was used for 1 hour.

    Your electricity meter and bill will show kWh directly.

    Whereas gas meters measure volume: newer ones are likely to measure cubic meters (m³), while older ones measure in hundreds of cubic feet (100ft³). Your bill will show how this is converted to kWh, so (fortunately, and unlike many years ago) we end up paying for gas in the same units as electricity.

    Very approximately: 1m³ = 11kWh, and 100ft³ = 31kWh

    Know what you use per year

    I believe that the energy price comparison sites, if you don't tell them your actual consumption, will assume an average annual consumption per household of 20500kWh gas, and 3300kWh electricity. Obviously your actual figures will vary. For example if your heating was electric storage heaters, then you'd expect a lot less gas, but more electricity.

    My own figures, for one year ending 20th March 2008, were 5674kWh gas, and 2809kWh electricity. Some heating was produced by burning wood, which would have reduced the gas consumption. While a significant part of the electricity went on keeping a large greenhouse above 7 degrees C.

    If you don't know your own figures already, then I'd recommend finding them out from one year's bills, and writing down the answer.

    Know the costs. Is it worth switching suppliers?

    There are plenty energy price comparison sites. One of my own favourites is (doesn't understand 'CB24' postcodes, use 'CB4' until updated). If using these sites, then my recommendations are to put in your own exact annual energy use (as above), always say that you want to pay by monthly Direct Debit (whether you really want to or not), and select results ordered by price.

    My reasons for these are that if you select any other payment options, then some of the better deals are likely to fail to be displayed, and I don't think that 'green' tariffs are usually worth the paper they're written on. So find the best deals as I say above, and only then try other options if you really want to. Beware that some of the sites seem rather reluctant to list suppliers who don't pay them a cut if you use the site to switch.

    My own supplier is the little-known Ebico. Usually among the cheapest, but definitely the top for simplicity! They charge a fixed amount per kWh of electricity and gas - no standing charge, no expensive initial units like on most deals which only 'pretend' not to have a standing charge, and they charge everyone the same, no matter how they pay - even people on pre-payment meters who normally are ripped off. Ebico are really a not-for-profit reseller of power from Southern Electric, who handle all the billing and customer services.

    I currently (June 2009) pay Ebico 3.675p/kWh for gas, and for electricity (I'm on Economy 7) it's 15.58p/kWh during the day, and 5.27p/kWh between midnight and 7am GMT. If I was on the standard electricity rate, it would be 12.75p/kWh all day.

    You can work out from those figures that Economy 7 is only a benefit if you use around one third or more of your electricity during the cheap period. Most people won't, unless using night-storage radiators, or perhaps an immersion heater timed to be on during that period. I only have Economy 7 because of the greenhouse heating.

    Cut the waste, and insulate

    Before going on to think of generating your own energy, then don't forget that it's far better not to use the energy in the first place. So (most importantly) insulate if you haven't already. First loft, then cavity wall, then any of the other measures. Get those low-energy light bulbs in place - no excuses about 'not looking nice' or anything - we're here to save energy. I despair of the fad for halogen downlighters and the like in great numbers - one sees single rooms with several hundred watts of lighting just for 'fashion' when 50 watts at most of compact fluorescents would be sufficient. Avoid the pointless 'standby' and turn off anything not needed. Turn the heating down a bit, and off completely in unused areas.

    Generate you own

    OK - finally - if you want to collect some energy from the sun, move on to Solar Panels. And please don't contemplate silly little roof-mounted wind turbines. I'm all in favour of huge wind turbines on windy sites, but little roof-mounted toys in non-windy areas are a complete waste of time and money.

    [ Clarke Brunt's Home Page | Renewable energy ]

    Author: Clarke Brunt (
    Last modified: 4th June 2009