Overcharging - Faulty Meters

If the meter is faulty, then the supplier is never going to get things right, for example, the meter could record night as day and vice versa, sometimes to the clients benefit, sometimes not.

Many meters, especially larger supplies, have a multiplier fitted.  This means that for every kWh the meter reads, you have actually used 10, 20, 50 or 100 units.  If the multiplier on the meter doesn't match the multiplier on the bill, the error could be anything from double to 100 times the amount of energy you've actually used.  Again this can work both ways, if you have a site which is using much more or much less energy than you'd expect, then checking the meter is a good place to start.