Remember your max efficiency is going to be around 50-70% of the advertised PSU wattage.
Modern PSUs should have a much higher efficiency. E.g. beQuiet say this about their 950W Dark Power:
The very high efficiency of up to 93% reduces performance loss (waste heat) and also saves power, as attested by the 80PLUS Gold certification.
Others are quite similar.
That 93% efficiency figure will be at a particular load, take a look at the efficiency graph for the beloved Corsair AX1200 (also 80+ gold) for an example:
http://www.corsair.com/ax1200.html[edit] I see where I was unclear, what I should have said is that the PSUs max efficiency will be at 50-70% of its maximum load, not that it would only be 50% efficient. 80+ means
at least 80% efficient over all loads.