Abstract This paper deals with the detailed analysis of evacuated tubular collector (ETC) connected in series in the terms of thermal energy and exergy gain. The study have been done by considering four type of weather conditions (a, b, c and d type) for the climatic condition of New Delhi, India. The maximum outlet temperature increases from 53.91 °C to 129.23 °C and the useful daily thermal gain increases from 3.29 kW h to 6.34 kW h for as the number of collector increases from two to six, at constant mass flow rate ( m f ) = 0.002 kg/s. For a mass flow rate ( m f ) = 0.002 kg/s, the temperature difference between inlet and outlet ( Δ T ) increases (55.52–133.29 °C) as the number of collector increases from 2 to 12 and then becomes almost constant. Monthly gain in thermal energy and exergy are also evaluated by considering four number of collectors connected in series and at constant mass flow rate of 0.002 kg/s. The characteristic equations of ETC have also been developed. Instantaneous thermal efficiency decreases from 53.5% to 34.4% as the number of ETC increases from two to six.