This is closely related to energy flux density, which is the rate of energy transfer per unit area ( measured in W / m?).
2.
Radiation pressure says " the radiation of the Sun at the Earth has an energy flux density of 1370 W / m2, so the radiation pressure is 4.6 �Pa ( absorbed ) " and twice that reflected, so the 1 meter sail would experience a force of 9.2 * 10-6 Newton.