[TomF]

With the Kindle 3G (Wi-Fi & 3G), when you turn wireless on it will search for a previously set up Wi-Fi connection. If it finds one, it will turn 3G off and use the Wi-Fi connection. If it doesn't find one it will leave 3G on. You can't explicitly turn on one or the other and you can't explicitly turn off one or the other. If wireless is turned off, both 3G and Wi-Fi are turned off.

3G uses more battery power than Wi-Fi. So the best power savings will be to leave Wi-Fi off. If you have a wireless connection in your home and set it up on your Kindle, then leaving wireless on when in your home will automatically use the Wi-Fi connection and give you better power consumption than using the 3G connection.

In terms of what Amazon claims for battery usage, for both the 3G and the Wi-Fi (only) models they claim up to one month with wireless off. For the Wi-Fi model they claim up to three weeks with wireless turned on. For the 3G model they claim up to 10 days with wireless turned on. It is clear from this comparison that the best power savings is with wireless off. For the 3G model, connecting to your home network every time you turn wireless on should get you nearly the three weeks of battery usage claimed for the Wi-Fi model.