I have just bought Orange (UK) Micro SIM for my iPad, because 5p/ per MB cheap and pay as I use iPad plan.
I have noticed that maybe iPad use Cellular data before Wi-Fi start??
My iPad is in my house and always use Wi-Fi at home connected, still if I check the usage/ cellular Network Data/ received...I am using about 1Mb a day. I checked usage with Orange. Yes I am paying. It is connected to Orange but connected to Wi-Fi too. I thought use wifi all the time if connected.
But seems like check Cellular data first?
Why?
Maybe push Notifications or something use Cellular data before Wi-Fi start?? or when iPad is sleeping?
It's not expensive but I may try Cellular data OFF at home all the time.
I know that I can make Cellular data OFF.
And only WiFi maybe at home and when I am out side make Cellular data ON.

Yes, if your paying for the amount of data you use turn the data off.

It should always use wifi first if available, but remember if you have data turned on its still connecting to the cell network and communicating with it every so often just to make sure your still connected.

That's I thought.
Thanks.