18 hours of light = 64,800 seconds
624 μmol/m2/s from the PAR meter
Micromole μmol/s (micromole) is the unit in which P.A.R (Photosynthetic Active Radiation) is expressed. This unit indicates how many micromoles per second a light source emits. μmol/s is one-millionth of a mol (10−6 mol).
624x64,800=40,435,200