It's surprising how many people overestimate how much power they really need when it comes to designing a new PC. Spec sheets get posted on online forums everyday which show an otherwise standard single GPU gaming rig juxtaposed with an ill advised 1000W PSU. The fault lies partly with manufacturers of processing components which recommend insane wattages for their gear, and partly with the cheap PSUs which fail at the very proposition of being loaded with half their rated power output. Put simply, many components overstate their PSU requirements because they assume you're using a dodgy power supply and other components with a large power draw.
There's also the fact that many people see 80% efficient GPUs as meaning you can only achieve 80% of the rated power draw. For instance, a 500W PSU would be calculated as 0.8 x 500 = 400W. That's simply not true. What would happen is that to supply the rated 500W, the PSU will have to draw 625W from mains power. It's a matter of wasted energy rather than lack of it.
If you're running an overclocked dual GPU rig, chances are that a high quality 650W PSU like the OCZ ZT Series 650W will be more than enough. With its 80PLUS bronze rating the efficiency levels can be as high as 85% depending on load. The PSU performed fine under load, with the 12v rail dropping to 12.233v from 12.294v when stressed by Prime95/FurMark, 12.272v with Prime95 only, and 12.246v with only FurMark. We didn't detect any fluctuations during the tests.
A 140mm load controlled fan keeps the unit cool without compromising silence.
The ZT Series 650W offers a completely modular design and isn't too bad to look at. Although not quite as efficient as PSUs in higher price brackets, we'd be lying if we didn't say we were pleased with the performance and welcome lack of fan noise.