The zero-cost figure is... um... wrong. Your central heater probably either does not use electricity (presumably using a cheaper fuel source) or is a heat pump

The zero-cost figure is... um... wrong. Your central heater probably either does not use electricity (presumably using a cheaper fuel source) or is a heat pump (and thus gets efficiency greater than 100% on the electricity by stealing heat from the ground). The real cost is still lower than the nominal cost, but it's not zero.