The most important knowledge to have regarding the house energy need is the energy needed to heat the house for different outside temperatures. As soon as that knowledge is available, it is possible to compare this years energy usage with previous years, adjusted for the difference in temperature. It is also possible to calculate heatng needs and compare different heating alternatives and their probable savings.
We have measured the electricity needed by the heat pump, and the electricity used by the electric heater, and the remaining other electricity used by our house (we have separate electricity meters for the electric heater and for the heat pump, showing how much they use separate from the household energy). The measuring has been done over many years, and for different temperatures outside. Based on this, I have came to an energy need curve which I believe is not perfect, but at least good enough to be used for calculating heating costs with different alternatives för heating.
The below is the energy need curve I have produced for my house. I am currently using it for the calculations I do. However, the calculations are done on a day to day basis. The energy need is depending on how warm the walls are (when temperature is going downwards, the already stored heat in the walls helps with the heating). Thus I am using a formula which is behind the curve below, but also takes into account the temperature change. I am still collecting data on the energy need, and will at a later time adjust the curve (which is affected by the changes done to the house isolation and so on).