I was in Bletchley Park last week for the Over the Air event. One of the highlights was a tour of the museum in which we got to see a reconstruction of the first computer the Colossus. Originally the computers were left on all day but now they are only switched on from time to time for tours due to the high cost of running them (and for environmental reasons).
The machines use a whopping 5.5 kilowatts of electricity and standing near them you can certainly feel the heat. The sheds in which they are kept apparently get pretty warm in winter too.
My iPhone by comparison uses only 5 watts (approximately) or 0.1% of the power usage of the Colossus!
The increasing energy efficiency of computers is formalised in Koomey’s law which states that “at a fixed computing load, the amount of battery you need will fall by a factor of two every year and a half”. This trend has all sorts of interesting implications as it will allow computer to become smaller and more ubiquitous in the future.