[billrussell420657]

80% of 1.4kW/m^2 is 1.12kW/m^2Your back is 30cm x 50cm or 1500cm2 or 0.15m2Radiation hitting your back is 1.12kW/m^2 x 0.15m2 = 0.168kW or 168 watt1 joule is 1 watt sec168 watts x 3600sec/hr = 605,000 W-sec or joulesedit corrected arithmetic