Whether you need a backup system for your solar thermal system depends largely on the type of system you choose and where you live. Solar water-heating systems almost always require a backup system for cloudy days and times of increased demand. This backup system is typically a conventional gas or electric water heater and may already be part of the solar system package. It may also be part of the solar collector, such as rooftop tanks with thermosyphon systems. For example, an integral collector-storage system (ICS) may be packaged with an instantaneous gas water heater for backup.
Most people do not need or use a backup heater with solar pool heaters. In freeze-prone climates, solar pool heaters are used mainly in summer. Spas or hot tubs are an exception. Spa owners may use their solar system to heat both the pool and the spa, but will use a backup heater to get the spa to a higher temperature. In climates where it rarely freezes, pool heaters may be used year-round.
If you decide to install solar thermal space heating, you will almost certainly need a backup system. Space-heating systems require a large collector area, significant storage volume, and a highly efficient building envelope to produce enough energy to heat your home at night and during the winter. Solar space heating usually provides 40% to 60% of your home’s space-heating needs. A backup system ensures that your house is comfortably warm during power outages, cold snaps, and extended cloudy periods. Many building codes and mortgage lenders require a conventional backup space heater.