The PSU wattage is simply it's maximum rating, meaning a 1.5kW and 500W PSU will draw the same amount from the mains if powering the same PC. It also means that your PC draws less power when browsing than it does while gaming, probably in the regions of 100W vs 300W (depending on hardware). The reason to get a bigger PSU is if your computer may need more power, or simply to be on the safe side (wattage ratings can be misleading, the actual important numbers are the amount of amps that the PSU can provide on each rail). A 500W PSU should be ample for your computer, but anything greater won't be a harm.
More watts does equal more energy used (electricity companies use the unit of kWhr for the amount they charge, which equates to one kW drawn for one hour [or 0.5kW for 2 hrs etc]). As I explained above, the PSUs don't draw at their maximum wattage all the time, but rather have an efficiency. A good PSU will have an efficiency of ~90%, meaning that for every 90W the computer needs, the PSU will draw 100W from the mains.
A simple "proof" of this is that (nearly) all the energy that goes into a computer will be released as heat energy (energy must be conserved, and that is the only thing it can become within a computer). This means that if the PSU was drawing the maximum amount constantly, the computer would have a constant heat output, meaning that the room it is in would heat up the same amount regardless of whether it was being used for gaming or simply browsing the internet, which you should have noticed by now, doesn't happen.
And Harry, I am still a god
