@MattyM Just after posting my last response I remembered that I did an experiment on this last year with our PHEV, so I can give you a real world example of how not charging a PHEV affects mpg.
We have had our PHEV (Tucson 4WD) for almost 7 months now, and in that time have driven 4,000 miles. For 3,600 of those miles I charged the car after every time that I used it. The total costs (petrol plus charging costs) over those 3,600 miles work out as 13.15 pence per mile, which if you equate that into an ICE car mpg equivalent based on current petrol price is 46.6 mpg.
For the 400 miles that I deliberately didn’t charge the car, allowing the car to regenerate the battery using the petrol engine whilst it was also powering the car, the running costs were 18.17 pence per mile, which converted to an mpg equivalent is 33.7 mpg. So by not charging the PHEV at all, running costs increased by 28% compared to charging it after every time I used it. I should add that I am on a standard electricity tariff with no cheap off-peak period, so for someone who has cheaper electricity than me, their pence per mile cost (if charging regularly) will be a lot lower than mine (especially if they do a greater proportion of local journeys than I do), so that gap would be even wider!
That’s why I maintain that unless you charge the car after every time you use it, a PHEV can be a costly mistake.
