Christian McArdle wrote:

Electric heaters are effectively 100% efficient at point of use.

A common myth... OK the efficiency is close to 100%, but power lost due
to voltage drop in the wiring is often higher than people realise. The
wiring regs allow up to 4% voltage drop, so /in extremis/ 4% of the
energy you pay for may go to waste. It all depends on whether the heat
from the cable can be counted as "useful."

Example - take a 3 kW immersion heater fed via 20 m of 2.5 mm^2 cable.
Assume Uo = 240 V, to reflect reality rather than nominal values. The
resistance of the heater element will be 240^2/3000 = 19.2 ohms, and the
loop resistance of the cable is about 0.3 ohm. The same current flows
through both, so the fraction 0.3/19.2 of the metered power heats the
cable - IOW the efficiency of heating the water is 98.5% not 100%, and
about 45 watts is wasted heating the cable.

--
Andy