Self-paced physical work in the heat tends to be associated with a stable working heart rate, integrating the strain of the physical work and the thermal climate. It follows that work rate decreases in-line with the severity of heat stress, in order to maintain a stable heart rate. With climate change, increased environmental heat stress will thus be met with reduced physical work capacity (PWC%), an effect which has global economic implications. Understanding the independent effect of solar radiation on human performance is critical when developing an empirical model of PWC% in future climatic scenarios. The aim of this study was to document how the effect of solar radiation on physical work capacity changes as a function of the air temperature, humidity, and level of clothing insulation. 14 young adult males (7 semi-nude, 7 full body coveralls) walked for 1-hour at a fixed heart rate of 130 b/min, in seven air temperature (25 to 45°C) and relative humidity (20 or 80%) combinations, with and without solar radiation (800 W/m-2 intensity using solar spectrum lamps). A total of 172 trials (90 semi-nude, 82 with coveralls) were completed in this study. The cumulative net energy expenditure above resting metabolism was calculated based on the treadmill speed and grade. To determine PWC%, the net kilojoules of work in each heated condition was expressed relative to that achieved in a reference condition without heat stress (15°C, 50% relative humidity). The impact of solar radiation on PWC% during heat stress depended on the air temperature, humidity, and clothing. At 20% relative humidity in semi-nude, solar radiation had only a marginal impact on PWC% at air temperatures ≤40°C (< 5% PWC loss). In dry conditions but with protective coveralls, solar radiation consistently lowered PWC% at all air temperatures >25°C by 10 to 15%, indicating increased vulnerability from solar radiation when wearing protective clothing. At 80% relative humidity, solar radiation decreased PWC% similarly between semi-nude and clothed (10 to 20% PWC loss). The absolute loss in PWC% was predicted by the change in mean skin temperature, in both solar and non-solar conditions. Thermal indices which do not account for solar radiation (i.e. natural wet bulb, humidex) over-estimate PWC% in outdoor working scenarios. In summary, solar radiation had a different effect on PWC% depending on the biophysical aspects of the environment, and if clothing was worn. Solar radiation had a marginal impact in dry conditions, unless protective clothing was used. Solar radiation reduced PWC% in humid conditions regardless of the clothing ensemble. The study contributes to the development of a comprehensive empirical model which aims to predict PWC% in future climate scenarios.