Does anyone know if power supplies constantly take as much energy as they are rated for? For example, is a 500 watt power supply always using the same amount of power or does it vary based on your pc's needs?

It only draws as much as you're actually using. Most power supplies are surprisingly efficient.

Read any decent graphics card review these days and they actually take a watt meter to the thing and tell you how much power the different graphics cards use at idle to maximum load. Usually in the 150watt to 400watt range depending on the card of course.