大咖福利影院

That completely depends on what the landlord has set the card reader on... for electricity to be calculated you use kwh (kilowatt hours, what many refer to as 'units'), and with these readers the landlord can usually set how much is charged per unit, i.e. per kwh. But if the card costs 20 euro and you get only 15.50 euro's worth, I'd say there's something not quite right... apart from the fact that these readers are not really legal, as enemalta doesn't allow landlords to resell electricity (in other words, the card reader meters are not 100% legal...), but that's another story...
But to get back to your question, ask how many kwh (or units) you get with one such card, and then find an online calculator to estimate your use...

PS for those who might point out residential vs domestic rates, I don't know the OP's situation, but if he's just renting for 2 months I doubt this will be his primary residence, and as far as I'm informed聽 residential rate only applies if it is...

The apartment seems belonging to a historical monument in Malta and not to a private landlord.
Do you think I should still ask, as you recommended earlier, how many Kwh I am getting with lets say E20 card?聽 聽Thank you

I would definitely ask how many kwh you get, irrespective of who the landlord is.