How can I check if my system needs more PSU power?

This power consumption calculator is pretty comprehensive. It will tell you the total max wattage draw for your system. You can then look up how efficient your PSU is (most are 70-80%) and get a ballpark for how many watts your PSU can actually provide.

Warning: A lot of the power calculators out there produce ridiculously overspec'ed PSU recommendations.

If you want to be precise, you need to look up the max TDP watts of

• your CPU (one chart, another chart)
• your video card (one chart, another chart)

As these components will be by far the largest consumers of power in your system... unless you have an array of ten 10k RPM hard drives inside your PC, or something else extremely unusual. I would also add that two (or more) high end video cards in SLI definitely will require a massively larger PSU than almost any other type of build.

Anyway, start with CPU and video card peak power consumption watts -- honestly, everything else is almost a rounding error beyond that. Maybe 50w for all the other parts inside your computer (motherboard, dvd-rom, hard drive) speaking conservatively. I've built HTPCs that idle at 45 watts (2008) and 17 watts (2011), so once you knock out video and CPU, it just doesn't take that much power to drive all the other bits.

Also remember that it's mostly important to have a high quality PSU rather than one with a giant number printed on the outside.