How to buy a solar water heater

The important components of a solar water heater are black coated absorber and the hot water storage tank.


There are two different types of collectors.  They are non-focusing or flat plate collector and focusing type. The non-focusing type is simple and widely used for home applications while the focusing type makes use of a second heat transfer fluid that can withstand high temperature.

Flat plate collector

This has the absorber surface area same as the aperture area through which the sunlight enters.  The non-focusing type is of two types.  They are solar flat plate collector and the recent variation, evacuated tube collectors.


In the flat plate collector, the black coated surface contains channels for the flow of water and transfers heat to it.

In the case of the evacuated tube collectors, black coated tubes sealed in an evacuated glass tube acts as the absorber and they are arranged side by side very closely covering the total surface area of the solar flat plate configuration.

Water flows through the black coated tube and is heated up by absorbing the solar radiation .  The number of panels required can be determined by calculating the total hot water requirement of a family and then the hot water output needed per unit time.

No. of panel required

For the calculation of the required number of panels the following details are required and they can be used to select the correct collector size.  As the solar water heater is modular in nature,  with the standard  sizes of the collector being one or two square meters, they can be arranged  suitably.

Parameters and typical calculation

a)      Sum up the hot water requirement in litres = Y  (Bathing, Washing, etc)

b)      Average solar insolation (W/m2)   =  I ( for example 900 W/m2)

c)       Duration of sunshine (Hours) = H

d)      Atmospheric temperature of the place = TA

e)      Hot water temperature required = TW

f)       Specific heat capacity of water (Cp) = 4.816 J.Kg-1K-1

g)      Thermal energy in water heated = Y x 4.816 x (TW – TA)

h)      Solar energy received = I x T x A x efficiency of collector

i)        (T- transmittance of the cover glass of the collector

A-     Absorptance of the absorber plate)

Mass flow per unit area of the collector (Kg.m2) = (solar energy received x3600 )/( Thermal energy in the water heated )

For the same output of thermal energy in a reduced time period, the number of collectors can be increased.