Lux to watts calculation formula Lux to watts calculation with area in square feet. The power P in watts (W) is equal to the 0.09290304 times the illuminance E v in lux (lx) times the surface area A in square feet (ft 2), divided by the luminous efficacy η in lumens per watt (lm/W): P (W) = 0.09290304 × E v(lx) × A (ft 2) / η (lm/W)