It's possible to go into the settings in the phone dialler and switch off data use. There is also a data toggle app in Preware I think.
Originally Posted by Andreas
I think the back up usually happens in the early hours of the morning for me - I don't know that there's anyway to set the time, but if you note the time (last backup time is displayed a few seconds after you open the app.) and switch of mobile data before you go to sleep, then it should happen automatically via wifi.
Another option is to simply switch off automatic back up, then reactivate to back up manually ('Back up now') when conveniently near to wifi.
Possibly, you could set something up with the Mode switcher app.