Heat is the largest energy end-use globally, accounting for 50% of the total energy consumed and 40% of total global carbon dioxide emissions in 2018. Nevertheless, despite the notable efforts to increase renewables and sustainability, the global share of renewables in heat end-uses has not improved in two decades. Under this context, solar thermal technologies such as solar water heaters (SWH) are crucial in the clean energy transition. Governments react to such efforts by enacting public policies to enlarge and sustain SWH markets. However, current methods to measure these policies' effectiveness restrict the SWH policy analysis and design usefulness.This work proposes a novel policy-outcome effectiveness indicator: The Solar Water Heater Effectiveness Indicator (SWHEI) based on equipment deployment and the solar-energy potential for each country. Policymakers could use this indicator, constructed with publicly available data, to test solar water heating policy options' feasibility, as the SWHEI identifies cases where policies are not adequate to solve market failures.