But what I would urge you to consider (as opposed to photovoltaic, which I never thought were worth it) is one or more heat collector panels. We installed these 9 years ago, they are around 4m² total of vacuum solar collector tube-type (the most efficient heat collectors). Annually we get all the hot water we need (up to 85 degrees temperature) for about 9 months of the year and central heating on top of that to the point where we need no other form of heating for around 6 months of the year (minimum 18 degrees throughout overnight, 21-22 during the day). In the summer, because we don't have anywhere else to send the heat nor do we have covers for the panels, the heat goes to the radiators - we are probably famous for having windows open. For more than 140m² floor area you might choose more panel area and vice versa, all depending on the heat retention characteristics of the house.