If the pump is constantly running it's either faulty or the water is getting out somewhere. An underground leak, perhaps?
Or perhaps there's no pressure in your pressure vessel, if there's one installed - that would cause the pump to cycle constantly (and you would see your water pressure at the tap "pulsing" as the pump comes on and off).
But to be honest, even if your pump was running constantly, I can't see it causing a huge ESB bill as these pumps don't typically consume a lot of power.