Past exam of the physics B course of the University of Cambridge / 2013 / Paper 2 / Section C / C12 / i / Solution

The coldest day has outside temperature $-5^{\circ }\mathrm{C}$, so the indoor–outdoor temperature difference is 25 K. If $K$ is the building's heat-loss coefficient, the required heating rate is ${\dot{Q}}_H=25K$. For direct electrical heating,This sets the reference electrical cost for the same delivered heat and tariff.