Lotus100 :

popatim:

Why are you dividing and multiplying by 100?

Isn't it much easier to just multiple outright? For example, if I have a 1000W PSU and it operates at say an 80% efficiency then isn't it just a matter of multiplication: 1000W * 80% = 800W ? In other words, in this example, I'll be getting only 800W at max efficiency?

PSU's are rated by their output

we multiply by 100 to get the result into percent form but if you understand that 80% is the same as 0.80 then we are good.

Efficency is output divided by input. E=O/I

Knowing the output and the efficiency, How do you calculate the input? = Algebra time.

1: multiply both sides by I { IE=IO/I }

2: the I's on the right cancel out leaving { IE=O }

3: Divide boith sides by E { IE/E = O/E }

4. The E's on the left side cancel out leaving { I=O/E }

So your 1000w PSU example assuming 80% efficient. I= 1000w/0.80 = 1250W Input (which is what the PSU is drawing from the wall)

Lets check our results.

E=O/I = 1000w/1250w = 0.80

0.80 * 100 (to put it into percent form) = 80%