You need to assemble some key information;
-what does it say in your lease about electrical costs because your landlord may bounce this back to you saying that it was upto you to look after it (i.e.) its your meter so you should have been watching it.
-how will you know how much you've been over paying? You can't just estimate your share of the 500euro bill. Get yourself an Owl Electricty Monitor and start working out your own consumption ASAP.
-you mention that other tenants pay the landlord for their 'share' of electricity. Where does this money go? Can you get proof from the tenants of their payments to the landlord?
-when you say the landlord knew or should have known about the metering, you need to get proof of this and the previous question is key. i.e if he was estimating the bill to other tenants and not passing the money to you, then thats pretty good.
Mathepac suggestion of shutting off the power is brilliant, but before you do this, get an estimate of what you are owed by the landlord (from the Owl) and present him with the bill, then change your locks and shut off the power when you aren't using it. That'll concentrate the landlords mind on settling what you are owed.