The size of the system is influenced by the properties of the house. In electrically-heated detached houses, it is sensible to aim for 30 % output of the overall consumption. That is, with an annual consumption of 15,000 kWh, the optimum size for a solar energy system is probably about 6 kWh, which generates about 5,000 – 5,500 kWh per year in Southern Finland. If there is a separate heating system in the house, such as district heating, oil heating and/or cooling air conditioning, over 50% output of the overall consumption of electricity can be reached for. Enlargening the size of the system does not make the household self-sufficient, as the production peak is during the summer and from November to January, the output is close to zero, anyway. If the system is over-dimensioned, it creates excess production in the summer and still, in the winter, almost all of the electricity must be bought from an electrical network company.