There are several things that affect the sizing of oil-filled electric radiators. The necessary size of a liquid-filled electric radiator depends mainly on two things; where the house is located and how old the house is. The following case comes from Sweden, where oil-filled radiators are quite common. The map below shows the different temperature zones in Sweden. Since there is a cooler climate in the north of Sweden than in the south, higher power per square meter is required when heating.

The tables below contain rules of thumb for how many watts are reasonable per square meter depending on where in Sweden you live and how old the house is. The values apply to normal ceiling height, ie. 2.40–2.50 m. To calculate the amount of power needed for other ceiling heights, you can expect a power requirement of 30 W per cubic meter.

Also, to consider when sizing a radiator is not necessarily to choose the smallest possible one that meets the rules of thumb. One should consider taking a size larger as the operating cost is the same as for a smaller one with the same heat demand. The difference is that the large radiator can contribute more heat during the coldest days.

How to easily calculate your home's total heating needs

dimensionering elradiatorer, temperaturzoner

Temperature zone 1

Temperature zone 1, which is the northernmost, requires the most power per square meter. Table 1 shows the rules of thumb for how much power is required per square meter depending on when the home is built. All of these values are for homes without exhaust air ventilation. If the dwelling in question has this then the rule of thumb in this temperature zone is to add 25 W per square meter.

 

Built (year) Watt per square meter
Earlier than 1960 98
1960-1975 88
1975-1980 78
Later than 1980 65

Temperature zone 2

Table 2 shows the rules of thumb for how much power is required per square meter in temperature zone 2. All these values are for houses without exhaust air ventilation. If the home in question has this then the rule of thumb in this temperature zone is to add 22 W per square meter.

 

Built (year) Watt per square meter
Earlier than 1960 89
1960-1975 80
1975-1980 71
Later than 1980 59

Temperature zone 3

Table 3 shows the rules of thumb for how much power is required per square meter in temperature zone 3. All these values are for houses without exhaust air ventilation. If the home in question has this then the rule of thumb in this temperature zone is to add 20 W per square meter.

 

Built (year) Watt per square meter
Earlier than 1960 84
1960-1975 76
1975-1980 67
Later than  1980 56

Temperature zone 4

Table 4 shows the rules of thumb for how much power is required per square meter in temperature zone 4. All these values are for houses without exhaust air ventilation. If the home in question has this then the rule of thumb in this temperature zone is to add 18 W per square meter.

 

Built (year) Watt per square meter
Earlier than 1960 72
1960-1975 65
1975-1980 58
Later than 1980 48
 

 To consider when sizing a radiator

There are certain situations where a liquid-filled electric radiator is more suitable than other alternatives. An example is an extension. The original home may have a hydronic system which means that new pipes must be drawn. To avoid this, electric radiators can be placed in the new, extended part of the building.

In some cases, there are no other heating options, for example in many summer cottages. Geo-heating is expensive to install, and district heating is only used in densely populated areas. Using biofuel requires a boiler and is therefore difficult to install after the housing has been completed. The boiler also requires some maintenance in the form of soot. In this case, an electric radiator is the perfect solution, provided the summer cottage has electricity.

It may also be that a home needs complementary heating during the coldest days of the year. An electric radiator is flexible, ie. you get the heat where you want it, and consume no electricity when it is switched off, it therefore offers a cost-effective complement to the existing heating system.

The fact that the electric radiator is a good complement to increase the comfort of passive houses has also been demonstrated in tests that have been carried out and presented on eg. Passive House Conference in Gothenburg 2013.