The intensity of electromagnetic waves from the sun is 1.4 kW/m^2 just above the earth’s atmosphere. Eighty percent of this reaches the surface at noon on a clear summer day.
Suppose you model your back as a 27cm & 51cm rectangle.
How many joules of solar energy fall on your back as you work on your tan for 0.70h?
80% of 1.4 = 1.12 kW / m²
Area = 0.27 * 0.51 = 0.138 m²
Power = 1.12 * 0.138 = 0.154 kW
Energy = Power * Time
Time = 0.7 * 3600 = 2520 seconds
Energy = 0.154 * 2520 = 389 Joules