Whether you’re buying a PSU as part of a
whole new system, or as an upgrade for an existing one, there are no point in
buying a PSU that’s far beyond your needs. PCs are remarkably power-efficient
these days, and their PSUs are actually most efficient at 50 per cent load.
It’s usually best to aim for a PSU that offers between 200-300W spare
capacities under very heavy use in your particular system, where very heavy use
means placing the graphics card and CPU under considerable strain.
A high-end dual –GPU graphics card, such as the GeForce GTX 690, requires a
lot of power when it’s working flat out, particularly if it’s partnered with an
LGA2011 CPU
This is actually quite rare in many games,
as they’re usually either GPU or CPU dependent, while’s carrying out tasks such
as image editing or video encoding usually place the CPU under far more load
than the GPU. With an extra 200-300W at your disposal, you’ll have plenty of
headroom if you add another graphics card, a more powerful CPU or if you want
to try your hand at overclocking. The PSU will be quieter too - the closer it
gets to its maximum load, the faster its fan will have to spin to keep it cool.
For example, if your PC has a heavily
overclocked Intel Core i5-3570Kand an NVidia GeForce GTX 670 2GB, we measured
this to draw around 375W using the small ft. test in Prime 95 to max out the
CPU, and Unigine Heaven to push the graphics card to its limits. This is a
worst-case scenario, as no game will put your PC under this much load.
Factoring in our 200-300W spare power rule, you should be aiming for a PSU
that's rated between 550W and 650Wfora system such as this.
A system using an AMD A8-3870K, with its integrated GPU running at full
pelt, only draws 160Wof power from the mains
A lower-end system, such as an Intel Core
i3-2100 with an AMD Radeon HD6870 1GB, only draws 225W in the same stress tests
- so for this, you'll only need a PSU of 450-500W to provide ample headroom for
your system. We found that only the most expensive systems, such as an Intel
Core i7-3960X Extreme Edition over clocked to 4.7GHz with an NVidia GeForce GTX
690 4GB, need anything close to a 1kW(1000W) PSU. All the graphs below show
maximum power usage using our stress tests in a typical system with case fans,
an SSD and a CPU cooler.
