Why is heater voltage important?

Heater voltage is a common question that relates to single-phase heaters; why Herschel states 240v as the rated voltage limit of its single-phase heaters, and whether this is adequate (high enough) given total supply voltage in the UK can legally reach 253 volts.

The regulation on the matter is clear enough: that the input power of a heater must not exceed 5% of its rated power (as it has implications on product longevity, reliability and safety, quite apart from unnecessary extra running cost). We explain more why heater voltage is important here.