A study on the effect of zinc oxide-water nanofluids on the thermal performance and efficiency of a flat plate solar collector is presented. The effect of altering mass fraction, mass flow rate and fluctuating solar irradiance on thermal efficiency and viscosity as a measure of thermal performance was evaluated. A series of experiments were carried out on an existing pumped solar water heater using mass fractions of 0.05%, 0.1% and 0.15% zinc oxide-water nanofluid at mass flow rates of 0.0556 kg/s, 0.0972 kg/s and 0.2778 kg/s. It was found that the use of Zinc Oxide-Water nanofluid improved the thermal performance of the flat plate solar collector by 30% when the system was run at 0.15% mass fraction and at a mass flow rate of 0.0978 kg/s. This is a very important finding that can assist in optimal use of solar water heaters in Botswana.