What is heat load calculation

The kilowatt hour as a benchmark for heating costs

Energy output can be measured in terms of watts. If this energy output is broken down to the hour, one speaks of one kilowatt hour. Owners encounter this unit of measurement, for example, on the billing of their electricity provider. This requires a previously agreed amount of money per kilowatt hour.

The kilowatt hour (kWh) is also suitable as a parameter when comparing the running costs of different types of heating. For an average single-family house, for example, it is assumed that 160 kWh of heating per square meter of living space are required. If you take the costs for the respective energy carrier per kWh, you can calculate how high the annual heating costs per square meter are:


Price / kWh (Cent)

Annual requirement / sqm living space

Costs per sqm / year (€)

gas7,1160 kWh160 kWh * 7.1 cents = 11.36
oil10,1160 kWh160 kWh * 10.1 cents = 16.16
electricity17,7160 kWh160 kWh * 17.7 cents = 28.32
Pellets5,5160 kWh160 kWh * 5.5 cents = 8.80

From consumption to costs

With most energy sources for heating systems, the question is how much of a fuel do I need to generate the required output of ten kilowatts, for example. The different energy sources bring different calorific values ​​as "inner" quality. This determines the amount consumed to achieve the target. The unit of calculation is the kilowatt hour for each energy source.

In addition to the degree of exploitation and the efficiency of the energy source, the purchase price in terms of quantity plus basic costs are the decisive parameters for recording heating costs. In addition to the hour unit, this calorific value can and must be combined with the quantity in some cases.

Final cost accounting

In the last calculation step, the amount of energy consumed is assigned to the market prices. Utilities and suppliers must indicate these in the same unit of measurement. The price per kWh of gas, oil, pellets or electricity is multiplied by the required quantity in order to calculate the "material costs" using the price of the kilowatt hour. Very large systems and high values, in turn, allow three zeros to be saved. When converting megawatts (mWh) into kWh, the factor of a thousand comes into force again.

Kilowatt specifies horsepower

For vehicles, the colloquial power value horsepower (PS) is still used. Mathematically, the units of measurement differ from the time reference. A car “has” horsepower, regardless of the driving and working time. The specification in watt hours quantifies the performance. James Watt, a British inventor and entrepreneur, shaped both values.

One kilowatt hour corresponds to 1.36 hp. The horsepower is converted into the kilowatt value by dividing the horsepower figure by this factor. One hundred horsepower corresponds to 73.6 kilowatts.

The performance described arises from electricity, fuel or any other mechanically effective process. Therefore, at least not always directly, all heating energy sources and systems can be recorded in this unit of measurement. Note that it is not an hourly total, but an hourly total.

When asked what is a kilowatt hour (kWh), the average value dimensions must be taken into account in practice. The prefix kilo denotes a thousand times the value. One kWh corresponds to 1,000 watts. In heating technology for private houses, the required moving power ranges from 1,000 to around 10,000 watts. The kilowatt value saves three zeros.

Comparable unit of measure

When heating energy sources are compared, kilowatt hour costs are determined. They provide information about two essential energy properties.

  1. What can be extracted from the energy delivery medium?
  2. How much usable volume does the technical system create?

Comparability arises from the time unit hour. In a heat load calculation, every square meter of enclosed living space can be assigned to a performance value. Secondary factors such as age, location and insulation situation must be taken into account. An example is based on one hundred watts per square meter. With a living space of one hundred square meters, the heating system must be able to generate and distribute 10,000 watts or ten kilowatts. This base power value is used to calculate future heating costs.