@Vivien192 I would charge the cost of electricity times 1.2 which will make you competitive with publicly available charging sites.
How you meter the electricity is a mystery to me - I assume you might have to rely on the honesty of the car driver? I am sure you will know better if you own a charger and an electric car!
It’s actually really easy to collect payments on your EV charger using an app called Tap Electric. As long as the charger is “OCPP 1.6” compliant (most are), you can add your charger, set a tariff, connect your bank account, and start getting paid immediately. It’s a free service for hosts — they charge a transaction fee to guests. I believe this currently works for hosts in UK, and some countries in Europe.