When there is no sun, there are 2 options:

a. Because the boiler is insulated with a 5cm thick polyurethane insulation, the sanitary water inside the boiler stays hot for many hours and as such you can continue using it even at night or during a cloudy day. This thermal inertia depends on many factors such as: initial temperature of the water in the boiler, outside air temperature, usage. For this reason, it is better to install a bigger boiler, when possible, in order to use the water mass to better retain heat during non-exposure periods.

b. Inside the boiler there is a back up electric resistance with thermostat which will heat or keep the water hot to the set temperature once sun/night will not be able to heat the water.