In times of low solar activity, it may be necessary to boost the water temperature in the tank with the aid of an electric element, or gas booster. Some systems allow for boosting from a fuel (wood) heater in the winter, using a wetback coil arrangement.
The type of collector material is of great importance. Ideally this material (or design) must absorb as much heat energy as possible, whilst reflecting or radiating as little heat as possible to maintain high efficiency. Many different materials, often called selective surfaces, are in common use. The materials include:
or the most efficient (but costly) type being a matt black piping collector in a vacuum (evacuated tube). These evacuated tube types are almost 100% efficient in the conversion process.
Insulation of both the tank and collector is critical for
performance to reduce all heat losses