This paper presents a new solar heater model which is shaped as an inflatable hemisphere. It is called SOLARBALL(C). This solar heater is above all distinguished by its small weight and volume. The final form is blown up by the user. The objective of this work was to develop a mathematical and a numerical model to analyse solar radiation and heat transfer in such a heater. The numerical model was verified by a series of experiments. After successful verification, the numerical model was used for the parametric analyses to establish the time required to heat water for different meteorological data, size of reflector and optical properties of the transparent cover, reflector and absorber surface. It was found out that typical optical efficiency and overall heat transfer coefficient of the hemispherical solar heater are between 0.45 to 0.5 and 0.6 to 1.6 W/m(2) K respectively. By standardised conditions (G(cov) = 700 W/m(2) and temperature difference T-w - T-a equal 50 degrees C) the required heating time for heating 0.33 I of water varies between 15 and 25 min according to the construction parameters.