The percentage you see in the battery stat isn't the amount of battery being used by the app, it's the percentage of the amount used that's used by that app.
IOW, if the total drain on the phone is 5mA, and wifi is using 20%, it's using 1mA. If you could see the stats on every app that has run since you took the phone off the charger, they'd lways add up to 100% - even 5 minutes after you unplugged the charger. The amount of power being used is always 100% of the amount of power being used.
The question is whether the battery is dying after a few hours, or is it lasting the normal amount of time between charges?
If you go to Wifi in Setting, press the menu and go to Advanced, you'll see "Always allow scanning". Turn that off if you always know where you'll be using wifi (and turn it on by hand). (And don't be upset that it's scanning when you look at your wifi settings - it thinks you want to connect to a wifi signal, so it's scanning for one. It'll stop when you close that screen.)