WHEN IS 200W ACTUALLY 250W?

A 500W PSU uses 500W of power at full load, right? Nope, the work the PSU does in transforming, rectifying, and smoothing has its cost, in heat principally. This efficiency is expressed as a percentage. If your rig draws 200W, a typical 80 percent efficiency PSU draws 250W from the wall to do it. The efficiency is not constant—running flat out or at very low levels is more inefficient. The sweet spot is at a load of 40–60 percent. A good quality PSU tends to have a flatter efficiency curve.

In the early days, efficiencies were low; ATX 2.01 only required 60 percent at a 20 percent load. To improve things, the 80 Plus program was launched by the American Council for an Energy-Efficient Economy in 2004. To earn the logo, a PSU must be a minimum of 80 percent efficient at loads of 20 percent, 50 percent, and 100 percent. Further standards followed: 80 Plus Bronze takes us to 85 percent efficiency at 50 percent loads, and 82 percent at 20 and 100 percent loads. Silver added 3 percent and Gold another 2 percent. As we approach 100 percent, it is harder to engineer. In 2009, the first Platinum PSU appeared, pushing efficiencies to 90, 92, and 89 percent. The highest current rating is Titanium—it appeared in 2012, and manages 92, 94, and 90 percent.

Efficiency costs. You need more and better quality components. Electricity costs, too. If your rig is on for long periods, some calculations may be required to pick the most cost-effective PSU. A 500W Gold PSU over a Bronze may cost an extra $50, but only earns an extra 5 percent in efficiency, which may take years to get back in savings. However, high efficiency also gives a good indication of build quality.