You can’t take your car out of the equation, that’s the point. That same 2600kwh of car charging at 25p/kwh is is £666, at £7.5p it’s only £200.
So as long as your day time usage doesn’t go up by £466, you are better off. Or another way to look at this is your average rate, as long as that’s below the standard rate, you are quids in.
What car and charger do you have/getting? If it’s on the octopus intelligent go list, they offer better rates than Eon. They take control of your car or charger and charge it by the time you set in the app. While the car is charging, all your house energy is 7p when your car is charging and it’s always 7p for 6 hours overnight.
So if octopus charge your car from 15:00-16:00 (as thy did my car today) your house energy is also 7p in that period.
Standard (not intelligent) Go is a bit more expensive but is a fixed off peak tariff.
PS. This is a perfect example of why people often get solar after getting an EV as that covers the more expensive day rate most of the year. Add a battery, it’s the whole year. If you add enough solar, you can get your bill down to zero or less. Octopus paid me £300 last year but it was an exceptionally sunny year.