When on wifi on your Android phone, you may have noticed that once you switch off your screen, your wifi goes to sleep and wakes up to connect only once you wake up the screen. In the interim period when the phone wakes up and the wifi is yet to connect to any available network, you are using the data over your network.
There is a setting to make the phone not put the wifi to sleep every time you switch off the screen and avoid the network data usage. To enable this go to
SETTINGS -> WIRELESS & NETWORK -> WI-FI SETTINGS.
Here, press the MENU button and choose
ADVANCED and click on Wi-FI SLEEP POLICY.
Here you can change the setting to let the wifi sleep with the phone on remain on when powered or remain on always.
There will, obviously, be a compromise with battery life. I have, however, heard that WiFi uses less battery than 2G or 3G network data.