We are about to order one.
According to Honest John, B4 averages 37.4mpg.
https://www.honestjohn.co.uk/realmpg/volvo/xc40-2018/b4
There are much better cars of mpg is a real issue for you, such as the Qashqai E-Power.
There is also an XC40 page as well as XC40 forum you can ask this question on.
https://www.xc40forums.co.uk/viewtopic.php?t=3277