This paper investigates the deterioration in the performance of thermosiphon flat plate solar water heaters (SWH) with water side scaling. The study presents the analytical and experimental variation of mass flow rate of water with scale thickness in risers of conventional solar flat plate water heater for different electrical power inputs (covering the full range of solar incident radiation up to 1 kW/m2). This information is extended further to determine the drop in efficiency characteristics represented by the Hottel–Whillier–Bliss (H–W–B) constants for full–fledged SWH. To simulate scaling in risers in the absorber plate of a SWH, an artificial method of coating has been used to create single pipe riser of different uniform scale thicknesses. Four such risers are created with scale thickness of 0 mm, 0.7 mm, 1.7 mm, 2.7 mm, and 3.7 mm. The observed drop in mass flow rate through the range of risers between 0 mm and 3.7 mm scale thickness is 58.5% for the thermal input power (supplied through electric heating) of 129.5 W (corresponding to a solar incident radiation of 980 W/m2). In comparison, the analytical results show a corresponding drop of 70.12%. A comparison of the coated riser with a cut tube of an actually scaled riser indicates excellent matching of thermal conductivity. The divergence between experimental and analytical mass flow rate in the case of a riser of the highest scale thickness, viz., 3.7 mm, is the lowest because of increased pressure gain in the flow region together with higher temperature than predicted by the general equation. The experimental data of various energy parameters from the single tube scaled riser studies are matching with analytical values for the different input electrical power levels (corresponding to the different solar radiation levels). As identical conditions are used in the experimental analysis, the results for risers of various scale thicknesses and electrical power inputs are applicable to corresponding full–fledged SWH.