I just found this out - and I'm amazed at the stupidity of Google for having this setting so hidden no one would find it.
I use Wi-Fi 90% of the time - at home and at work. Generally using Wi-Fi takes a ton less battery than using 3g since you don't have to have the radio connection for data usage.
But did you know that the base setting for Droids have Wi-Fi turn off once your screen goes off?
You heard me right - when the screen goes off so does your wi-fi connection. It turns back on when you turn the screen on. This means when the screen is off you are using far more energy using 3g.
I changed the setting today and my phone is still 80% after 7 hours. Unheard of.
Want to know how?
Wireless & Networks
THEN press the four line button on the bottom of your phone. (the one next to your home button)
Click Wi-Fi Sleep Policy
Change it to NEVER.
Yeah, who would think.