The Department of Energy (DOE) mentions only one disadvantage of solar heat collectors on its " Solar FAQs" page. This is the initial expense of installing the system.
The cost is likely to be more than other kinds of heating systems, which means that any financial benefits may take many years to recoup. The DOE points out that many solar systems last from 15 to 30 years, which is advantageous in its own way but could mean that unless you stay in the same house for 15 years or more, you may not accrue any financial advantage from installing a solar heating system.
The German Solar Energy Society has a much more rigorous and comprehensive assessment of solar heat collectors. They analyze every system and component in terms of advantages and disadvantages.
The main disadvantage of glazed flat-plate collectors is relatively low efficiency when compared with vacuum collectors. Further, you need more roof space and a weight-supporting fixture for flat roof mounting.
Evacuated tube collectors, which feature a number of connected tubes linked by a collector box, are more expensive yet have a lower yield when the sun is low. In air-operated solar systems, since air has a lower heat capacity than liquid, the system requires larger duct diameters and exchanger surfaces.
Variable Energy Source
As solar thermal consultant Bob Ramlow points out, no two years are the same in terms of availability of solar energy. The sun is a variable source of energy, at least in terms of how much of the energy gets through our atmosphere and weather conditions.
This is a big disadvantage for solar heat collectors, since more it requires more heat for buildings at times when the conditions for collecting solar energy are less propitious, as when cloudy. Further, although solar PV (photovoltaic) systems can work in diffused as well as direct sunlight, shading during daylight hours, by buildings or trees, can reduce the amount of electricity generated.
Leaves gathering on the surface of the panels can also reduce their efficiency, as can bird droppings.