There are too many factors affecting the home heating consumption to get it all down.
For example, if you like me have an electric furnace and a heat pump, and measure the consumption for both of these, it will still only provide a portion of the true picture of the heating consumed by your house..
- Measuring the energy used by the electric furnace is straight forward, assuming there are no big differences by furnaces, you can directly translate it to what amount of heating energy that is needed.
- Measuring the energy used by the heat pump is a bit more difficult. Knowing what is used is not the same as knowing what amount of energy it contributes with to the house. In order to know how much heating is used for your house, you need to know the COP of the heat pump. And the COP is varying with the outside temperature, and the forward heating water temperature.
- Comparing the energy used by the above to the total energy used in the house will give you a difference. That difference is not irrelevant, since parts of the energy used in access to the direct heating measured above is actually used to heat your house (a very large part of the power consumption in electrical devices like lamp bulbs, computers, TVs, ... are coming out as additional contributed heat inside the house)
- Another part of the heating cost, the hot tap water, is to a large extent flushed out outside the house
- On top of that is yourself and your family. Even people contribute with heat. An empty house will need more energy for the same heating than a house with a lot of people in it.
In the same way as above, measuring the things affecting the energy needs of the house at different temperatures is not easy. Measuring the outside temperature is easy, but also there is a lot of additional uncertainties
- When the temperature is starting to go downwards, the house itself (the walls and so on) as well as the ground, contains a lot of stored heat energy. This means that after a long period of high temperature, a downward going outside temperature will need demand less additional heating energy than the same outside temperature would after a long period of colder outside temperature.
- The amount of sun ray heating is not measured by a thermometer, but contributes heavily to the heating of the house. The efficiency of the sun rays in the very same weather conditions vary over the year.
- The wind speed, as well as the wind direction, affects how much the house is cooled down from the outside air even if sun and temperature are constant.
- Also humidity and other factors add to the above uncertainties
Now knowing the uncertainties in measured energy used and needed for the house heating, we can still add to the uncertainties. For example a heat pump is in itself depending on a number of factors affecting what effiency it has.
Thus, knowing how much energy is used by the heat pump, and the same time knowing what the outside temperature was when it was running, does not provide you with enough information regarding how much energy it produced.
- The temperature of the medium in the heat pump has an effect
- The pressure of the medium in the heat pump has an effect