2Be_or_Not2Be :
Ignore the comment about getting a 700W PSU; it obviously isn't the most efficient when you only have a small load of ~368W. That 500W hits the sweet spot of efficiency.
Actually the power supply sweet spot is 50% of its load. Ideal is around 40% idle and 50% at load.
http://www.tomshardware.com/forum/343495-28-power-supply-efficiency-myths
Actually the graph you linked to distorts what is actually shown. To understand the
apparent sweet spot of 50%, you first need to know that 80+ Certification starts load testing at 20% load, then again at 50%, and 100%. So a designer looking to optimize their rating would focus on the 50% load test, as it is the one which is easiest to make gains, because very light loads are harder so optimize the efficient while maintaining voltage regulation, without using larger more expensive components (i.e. large inductors and capacitors), and improvements at 100% typically involves using higher rated components which tend to be more expensive, if not larger.
Second the graph you linked to masks the poor performance of poor supplies. While good supplies are very good now (>=90% for 20-100% load for most better PSU), poor supplies drop their efficiency notably as the load decreases, but the purple on beige in the graph obscures that. The graph also only samples 'good' or better brands, it includes none of the uglies, giving an optimistic view of the performance of all power supplies found in the marketplace today.
As power consumption trends are actually
decreasing at the present time, it makes little sense to over-size a power supply more than adding a single video card (the Nvidia 980 is rated 165W, and the latest Intel extreme edition, i7-5960X is 140W).
With desktop system (not just CPU or GPU) idle loads being around 100-150W, having a massively oversized supply wastes money as the supplies' efficiency can drop quickly on loads below 20% on the supply's rating.
So you can end up consuming more power (higher operating cost) to keep the voltage within tolerate in 1000W supply that is idling at ~100W or less, than you would for a more reasonably sized 700 or 650W supply (combined with a cheaper purchase price).