Graeme Bathurst, of independent energy consultancy TNEI, suggests 7p/kWh as a reasonable estimate(http://news.bbc.co.uk/2/hi/uk_news/magazine/4900188.stm)
Find out from your latest bill from your electricity supplier how much you are paying for a kilowatt-hour. Multiply that charge by 1.38, so if the charge is 20 cents then 20 x 1.38 = 27.6 cents.
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