According to Toyota, our new 2020 Prius Prime PHEV gets around 4.3L / 100km of city driving. We will use this number since it is not too far off of the combine driving number of 4.4L / 100km. This means at the time of writing this post, the current fuel price at our neighbourhood pump is at $1.15 / L. If you do some fancy math, the Prime will yield us 20.2km per dollar invested at the pump (20.2 km/$).
As depicted by the chart on the right, in Ontario we have three tiers of charging rates. The Prime in the winter can do about 35km with a 9kWh battery. The exact numbers are 40km / 8.8kWh, but this is perfect condition, and we use some battery for heating the vehicle. This will yield us the following:
So by comparing the above numbers, it makes perfect sense to charge the vehicle during Off and Mid Peak hours, and not so much during On-Peak hours. However the On-Peak comparison is so close that if the mileage rating was at 4.5L/100km then it is a wash.
With a bit more fancy math, you can actually calculate how much does gas have to cost per Litre before On-Peak charges make sense. This turns out to be around $1.24/L.
Hopefully you find this information helpful.