If I purchase an iPad mini with 250MB of wifimo am I able to switch off internet from plan when I am at home and use my own wifi

I know 250MB of wifi is, well, practially none. So I was wondering if I was at my own home, as I already  have wifi, could I simply use the wifi I alreadyhave, and use the wifi from the plan when I'm away from home and not using my already purchased wifi?