the further one travels, the less one knows

With Copenhagen's energy summit getting fired-up, we are all thinking about the climate a little bit more at the moment.

One of the popular discussions is around new forms of transportation power, such as electric cars. I'm a believer in 'treading lightly' but I suspect some of the calculations will need refinement for electric cars to become mainstream.

The 'rule of thumb' equation for electric cars consumption goes something like 60mph = 20KWh. In other words, a car travelling at 60 mph for an hour uses 20 Kilowatts of power - roughly 10 fan heaters or 333 60 Watt light bulbs. As a quick cross check, the Toyota Prius peak output is between 43KWh and 73KWh, depending on model - higher than my figures.

To keep things simple, I'll assume that the 60 miles represents the equivalent of an average car's daily travel (ie 20KW) but that it only gets recharged on working days. Thats 220days x 60miles = 13,200 miles per annum or 220days x 20KWh = 4,400KWh per annum. 13,200 miles seems about right as an average.

Now for the average power consumption of a house. I looked this up on a US web-site. US homes have more central heating and are quite appliance rich, so their government statistics are a handy reference. Its around 960 KWh per month, according to the average of all of the States.

So a home uses 12months x 960KWh per annum (call it 12,000KWh) for simplicity. A single car uses 4,400KWh per annum.

Or around 1/3 of a home.

UK has around 25million homes and 33million cars. Lets make that 1.3 cars per household. That's average 5,700KWh per annum. Or an average of around a half a home of charge per night.

Now put the car electricity onto the household power grid. Most people would recharge at night. Say between 7pm and 7am. 12 hours. Thats 12 hours to deliver half of a house's average daily consumption.

Looks to me as if thats pretty much full continuous load.

So does that mean the whole grid needs uprating to support eCar charging? How big is the carbon footprint to do that? Maybe we will find out over the next few days - or perhaps it will take a three year study?