We use a wifi module in a battery operated environment (we kill power to the module when it isn't in use).

It uses about 225mA @ 3.2V.

The connection/handshaking take about 5-7 seconds to talk to a local server, plus the time for whatever data you are sending.

When talking to an internet based server, the connection time can extend out to 20+ seconds.

So you can see wifi isn't the greatest choice for low power/battery operated environments.

(Now on something you don't mind recharging a few times a week, it is perfect.)