The necessity of energy in a house consists of three parts: Heating, hot water and household electricity.

Hot water consumes about 3500 – 4500 kWh annually depending on the family size and so on (4000 kWh is approximately 75000 litre hot water the same as about 200 litre per 24 hrs). Hot water will be analysed and discussed more in a later chapter.

Household electricity (lights, kitchen appliance, TV, computer and so on) is about 4500 to 5500 kWh annually in a small house, the variations can be big. For example if you have an old freezer or a car engine heater it can increase your power usage substantially. This is also the case when using a ventilation system with bad fans which could result in an increase of 1500 to 2000 kWh annually.

According to the Swedish National Board of Housing, Building and Planning a newly build house heating necessity should be about 9000 to 10000 kWh annually and 2500 kWh for ventilation heating. The rest are so called transmission lost, meaning heat leaking thru floor, walls, windows and doors.

Old small houses often have a poor insulation and are often without a recycling of the ventilation heat. This could easy result in an increase of the double or more, about 20000 to 25000 kWh annually. In larger houses it is not unusual with a heat necessity of about 30000 to 40000 kWh annually.

The location of the house is also of course of matter. In the north of Sweden Kiruna the heating necessity is 60% larger then a house in south of Sweden Malmoe.

When using oil for heating you have to remember the efficiency. One cubic of oil gives about 10000 kWh but due to loses when burning it the amount decreases to about 7000 kWh per cubic. Thus a house with a consumption of 14000 kWh electrical heating needs two cubic of oil for the same consumption.

A buildings necessity of bought energy for heating differs from the buildings loses. The important total energy balance in the building comes from the considerable addition of heat from household appliance, people in the building and the sun. This free heat gives nearly 15% differences between bought heat and buildings lose in old houses. In a newer house it could give nearly 15%, in extreme cases with houses with windows turned southwards it could give up to 25%.

With water floor heating circulating continuously during the year a larger percent can be achieved. The heat is then transferred to the different rooms and moved around as the floor works as a heat absorber in warmer rooms. This so called division of heat reduces the need of bought heat with 10% or more.

A common way of calculating the heat necessity of a building is to indicate the need at one degree Celsius temperature difference between indoor and outdoor. A modern small house with an area about 110-120 sqm has a need of about 70W/K due to transmission. This means that for keeping one degree Celsius higher temperature indoor compared to outdoor you need 70 W. As follows you need 70×40= 2800 W when there is a 40 degrees difference between outdoor and indoor.

The need for ventilation air is 25 W/K if you have heat recycling otherwise it is 50 W/K. Further more you need 5 W/K for air leakages thru badly insulated walls and even more if the ventilation system is badly done (see further information on the chapter Heat recycling).

The total sum comes to 100 to 125 W/K for a modern small house. In older houses the need can be significantly higher. The easiest way of checking is if you have electrical heating, see the chapter Measuring H N.

The entire energy consumption in a building depends on the addition of heat and way of living and not only on the condition of the building. Thus it is important to know the buildings necessity of heat when doing energy calculations or energy saving calculations for a building.

It is especially important to know the need when calculating the size of the heat pump, as it is important to get the optimal size. Note that it is no good business buying a too large heat pump.