Mats Bengtsson Heat saving calculations and house energy need

House Energy Need Calculation From Consumption

Mats Bengtsson mib over the years

Calculating the house energy need

Calculating the house energy need correctly is not easy (there are so many factors affecting the need as well as the calculations). But as long as we know we are working on an estimate, and willing to accept the uncertainties that comes from the simplifications, there is a number of ways:

  • The best way is to daily measure the energy need, and the temperature that were present at that need. When you have the result from those figures (either yearly hear energy needed, or the house energy needed per degree Celcius), savings from different usages of heat pumps kan be readily estimated.
  • Second best way is to measure the same data during a number of very cold days. The colder the better. The rest of this page guides on the route for using such data to calculate your house need
  • There are more ways. One is for example to know the energy consumption during a relative cold period of the year (say a month or so), and based on that data and the below available conversions methods, use that as a base for a rough estimate of the yearly need. In order to do the last thing, you can find the rough percentage of the house heating normally needed during certain months of the year.

Collecting data needed to calculate house energy costs

Doing the data collection during many days, like a year, gives a very good knowledge of the heating need. But it can be done in shorter time. The shorter the time, the more likely errors are to creep into the calculations. One way to reduce errors is to do the measures at cold days (days needing high amounts of energy are reducing the errors by making the side factors less important).

So basically, one needs to do three things:

  1. Sum all energies used and wanted in the calculation, translating them all into one common factor.
  2. Measure the temperatures that were present during these measures
  3. Calculate the house energy need expressed as a function of the outside temperature (normally simplified as a simple dependency expressed as Watts per degree)

Summing all energies wanted in the calculation

The simplest way to sum all energies is to convert them into a common factor before summing them. An easy way is thus to just tabulate them, summing the the total for all factors. To do this, below is a table allowing a simple addition of the most common sources, and using electricity kWh as the common factor. The cells marked with a green background in the below table is fields that can be filled in, but for which the current default value most likely is good enough, why a change is not necessary to get acceptable results.

Source of energy Unit of measure Amount used kwh per unit Efficiency factor Total kWh
Oil cubic meters
wood (stacked) cubic meters
wood (not stacked) cubic meters
Pellets Metric ton
Electric heating (excluding heat pump) kWh
Heat pump consumed energy kWh
All total kWh N/A N/A N/A

Separating out household energy and warm water if not already done

If the above factors include hot water production, or household energy, these two are best split into their own parts. If the amounts are not known, some kind of guestimates have to be used. A common guess is 1200 kWh hot water per household member and year (more for teenagers and for bath lovers) and the same amount of household energy per hosuehold member and year.

Division of energy Unit of measure Amount used
Total energy used (as per previous table) kWh
Warm water production kWh
Household energy kWh
Left total heating usage kWh

Translating the energy data to house need per degree Celcius

Having the above data, very little remain in order to be able to look at house energy cost saving alternatives. Next step is:

  1. If the above data are collected for a whole year, house energy saving alternatives can already now be compared if yearly temperature data is available for the location where you live
  2. If the above data is calculated for a limited time period, or if you have the knowledge of the temperatures during the period data was collected, the house energy need curve can be calculated.

To calculate the house energy curve (Watts needed per degree Celcius outside temperature) the only thing remaining is to state the sum of the temperatures for the days when the above data was collected, and to define above which temperature house heating is not needed (for example, it is likely the house does not need any external heating to reach an inside temperature of 21 degrees if the outside temperature is 18 degrees or more).

House energy factor Unit of measure Amount used
Total heating energy used (as per previous table) kWh
Sum of all average temperatures for all days for the period above usage was measured Celcius
Number of days included in above sum of temperatures days
Outside temperature above which heating of the house is not needed Celcius
House energy curve factor Watt/degree Celcius

With the above data (energy need and house curve factor), it is possible to calculate and compare different heating alternatives to understand how different heat pumps and their COP can affect your heating costs. Just remember that not only do you have to input the energy need, you also have to define the house curve factor using the above calculated value.

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