Air source heat pump unlike water and geothermal source heat pumps are widely used in sanitary hot water production by virtue of the relative ease of harvesting, low grade aero-thermal energy and less complexity in operating and maintaining the heat pump unit. This research focused on performance monitoring of 1.2 kW air source heat pump, retrofitting a 200 L high pressure storage tank and operating under first hour heating rating for seven different climatic conditions. A data acquisition system was designed and built to measure the desired predictors of the power consumption and coefficient of performance of the ASHP water heater. A robust mathematical multiple linear regression models were built and were used in the modeled blocks in the simulation application developed in Simulink of MATLAB. Results indicated that the maximum coefficient of performance was associated with the maximum power consumption of the ASHP water heater. Finally, the simulation application could also be used by ASHP manufacturers and the energy saving company to quantify the energy reduced when geysers are retrofitted by ASHP. Optimization using constrained linear least squares solver in the optimization toolbox was also used to eliminate errors in the measurement from inclusion into the mathematical models. (C) 2014 Elsevier Ltd. All rights reserved.