[bvalle]

If 80% of the solar irradiance at the top of the atmosphere reaches the surface, then the irradiance at the surface is0.8 * 1.4 kW/m^2 = 1.12 kW/m^2Now multiply by the surface area to get the power1.12 kW/m^2 * 0.3m *0.5m = 168 WattsA Watt is a Joule/s, and in 1 hour there are 3600 seconds.168 Watts * 3600 s = 604 800 joules.