Colossus
I went to Bletchley Park today - http://www.bletchleypark.org.uk/
One of the exhibits there is the Colossus machine – regarded as the World’s first computer. Imagine a machine that has 2 rows of electronics each 5 racks wide by 1 rack high. All of this delivers the computing power of a pentium 2 chip and consumes 7.5KW of electricity.
The machine uses 1500 valves (some still from the original 1943 machine) and gives off a tremendous amount of heat
If you are in the area I recommend a visit – its quite staggering to compare where we are today with this first machine