Abstract: The efficiency of an irrigation system on a property is determined as a difference between the actual water usage at a property and the irrigation water need of the plant on the property. Data for the property are received including at least the location and irrigation area of the property. A reference evapotranspiration value for the property is determined using the solar radiation data derived from the location of the property. The irrigation water need of the property is then calculated from the reference evapotranspiration value and area. The irrigation water need is compared to the actual water use at the property to determined the efficiency of the irrigation system operating thereon.