We are actually talking about heat density. It's the number of kilowatt hours per square meter of ground. So the dense areas are most interesting for district heating, as the distribution system is a large investment. So the distribution grids for small houses become quite costly. Also, due to the expansion of nuclear power in the seventies, we had an excess of electricity in the seventies and eighties, which was put out to heat small houses. They are not as easy to convert to district heating. More often they are direct heated, as we don't have the water system for distribution within the houses. That's the problem. It takes some time.
On April 23rd, 2009. See this statement in context.