I'm a bit confused, isn't the point of a PHEV that the battery charge is optional?

Optional, but a full charge saves around $10 of gas depending on the PHEV.

How much does a full charge of electricity cost in order to save $10 of gas?

My Volvo XC60 costs about £1.50 to fully charge(at home) and easily saves ~£6 in fuel cost for the amount of miles you can cover. I drive daily and fill up every 2-3k miles on average, it's a massive saving over a regular car(for me anyway).

But even if it didn't save us any money, it's still worth charging as driving in EV mode is just such a luxurious experience, it's completely quiet inside and that's what elevates the whole experience. If you don't care about it, why buy/rent an PHEV in the first place lol.

That is impossible to say in a general way but generally speaking: significantly less.

On our PHEV minivan, the electric range is ~30mi, and costs ~$1.50 in electricity (12 kWh usable, ~13c/kWh). On gas, it's ~$3. (30 mi/g, $3/g). Some places you'll save substantially more (cheaper electricity, more expensive gas) or less (vice versa).

Spotted the Pacifica driver ;-)

And yeah the calculation varies, here in Québec it's about $1.20 CAD to charge it at home(10¢/kWh), and about twice that to charge it at a public L2 charger (1$/hour). I save about 4 liters of fuel with that. Fuel right now is ~1.60 so I end up saving around $5 everytime I charge.

Adds up pretty quick, and the vehicle is actually cheaper to purchase initially because of the government subsidies. Makes me wonder every time I see a gas Pacifica...

Sounds like you'd have to do 7 full PHEV battery cycles to save the upthread $10. (I'm pretty sure your math is correct and upthread claim is...optimistic.)

PHEVs tend to have 10-20 kWh batteries.

Gas costs about 2€/litre here. So if we go by that, you'll get 5 litres for 10€. That'll take the average gasoline powered family car around 100km.

The current price for electricity is 0,07€/kWh. My car uses 15kWh per 100km on average, which costs me 1,05€.

That's 90% cheaper and as such a complete no-brainer on which to choose.

Same goes for PHEVs, driving on electric is just so much cheaper it's not even funny.

When your electricity costs 0,07€/kWh, I can see it being cheaper.

Mine is currently around ~0.45€/kWh which takes the shine off it quite a bit, and makes gas a much more appealing option.