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: