Abstract: There is disclosed a partially or wholly electric fluid heater (100) arranged to heat fluid in a first circuit, the fluid comprising heating fluid or tap water. The heater (100) comprises: a first electric heating element (108) arranged to heat fluid in the first circuit; and a DC power supply (120) arranged to at least partially power the first heating element. The DC power supply has a capacity of at least 1 kWh, and optionally at least 5 kWh.