Charging only the electricity used by a particular room - that will be undercharging the tenant.

There will be electricity used in the common area - the lights in the living hall, the kettle in the kitchen, the fridge and so on.

If you install sub-meter, how to do it so that it can be fair ?

If you installed submeter to each room, maybe the bill calculation can be like this :-

1. Each tenant has to pay the amount charge according to the submeter for the room + average common area charge.

2. How to calculate common area charge :-

common area charge = Total bill - room1 - room 2 - room 3

Average it for 3 rooms = common area charge / 3

Do you think the tenants will find it fair to calculate this way ?