Ive just realised that skimask has already given me the formula for that.

E = IR, P = EI, therefore P = IIR
I should have realised but i was thinking of the formulas on the LED page. They also use ohms law so it must be the same kind of thing but as i understand it they work out the wattage based on the current required by the LEDs.

Im gonna read through all 3 ways of doing it again and see if i can understand exactly whats happening in all of them. I can see that the one you just gave me and the one skimask gave me are practically the same just written a little different.