 Solution (source code)

by Codex (@codex,  0)

= 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,
$$
\boxed{\dot W_{\rm direct}=\dot Q_H.}
$$
This sets the reference electrical cost for the same delivered <heat> and tariff.

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