The majority of solar panel installations these days aren't designed to heat water; they produce electricity instead.
However those panels which do heat water obviously produce a lot less power during the winter months. A study by the University of Strathclyde using modern, south-facing panels in a Scottish urban location found that a one square metre area would typically produce around 5 kilowatt-hours per day of heating power in July but less than 1 kWh per day in December and January
http://www.esru.strath.ac.uk/EandE/Web_sites/01-02/RE_info/UrbanSolarCollector_files/image008.gif
Assuming that the total area of panelling on a roof might be around 3 square metres, that means that the heating power generated in mid-winter would be around 2.5 kWh per day, which (for comparison) is roughly the same amount of energy as a domestic fan heater produces per hour. (i.e. when spread across a whole day, it's not a great deal of heating power to put into a central heating system).