Paul, please elaborate on the "contactor closing around midnight". Is that some kind of energy management program?
As Mike has outlined, it's the "Economy 7" tariff, which is fairly common here for electrically heated homes.
We have a dual-rate meter (i.e. two sets of dials, selected by a small solenoid, or a modern electronic equivalent), and a timeclock switches the meter over to the cheaper rate for 7 hours each night (hence the "Economy 7" name). It runs midnight thru 7 a.m. winter, 1 a.m. thru 8 a.m. in summer.
At the moment, my rates are 10.31 pence per kW/h day rate and 4.65 pence night rate, so well under half price.
As well as switching the meter to night rate, the timeclock also energizes a contactor which supplies power to a completely separate distribution panel for the storage heaters. Again, as Mike said, these contain elements inside thermal bricks so that they are "charged" with heat overnight at the low rate. A thermostatic damper control then allows that stored heat to be radiated during the day.
