Also "Mains voltage in the UK is 230v so Watts = 13 x 230 = 2990 watts" HOLY SHIT YOU GOT SOME BEASTY PSU O_O! you can run a nuclear power plant with that much electricty :C.
whats that supposed to mean?
230V is the voltage,
Wattage is a product of voltage and the max current your device can take( P=U*I) ,
my question:
With that calculation you are saying that your voltage is 230V (U) (which would be right) and your electrical current would be 13A (I), where you got that from?
You basically calculated the max. wattage (P) of a circuit with a 13A fuse, which hasn't anything to do with a device which is running on it.
real on topic:
The wattage of a device is stated on a label on the device itself, get your information from there, theres no way of getting it another way :s