Use your actual tariff and charging records. The energy drawn from the supply is not exactly the same as the energy added to the car battery because charging has losses.
For an illustration only, 20kWh drawn from the supply at 10p/kWh costs £2; at 25p/kWh it costs £5. These are example rates, not current tariff offers, and exclude standing charges. A full charge costs more or less depending on the energy required and when charging happens.
When comparing tariffs, include the electricity used by the rest of the house. A lower charging rate can come with different daytime prices or eligibility conditions. Check the supplier’s current terms for your vehicle and charger.
Surplus solar used in the car may reduce imports, but you give up any export payment it would have earned. Public chargers have their own prices and possible session charges, so check those before starting.
Give us your vehicle, usual mileage and tariff details. We can discuss a charging setup around them without assuming everyone has the same monthly bill.