My experience with the XC40 PHEV is this. I did some experiments recently to see if petrol or electricity was cheaper. I do local running about most days, 2 to 5 miles at a time cumulatively about 10 to 25 miles a day, in normal urban traffic on the battery. I tried using the engine only on these journeys and was surprised to find I was only getting 24-28 mpg. I calculated I needed to do 42mpg for petrol to be better. Open road driving on petrol gets me 44 to 48mpg.
So on local journeys I run on battery at about half the price of petrol, on long journeys I now save the battery for the slower ends of the journey and do the main road/ motorway stuff on petrol alone.