Lamp Needs Calculator.
Use the Lamps Needed Calculator, © Neil Mattson, Cornell University 2015, to estimate lamp needs for your greenhouse. The calculator will open in Excel. Fill your information in the yellow highlighted fields. Do not modify any boxes or go into the pink fields. Note: the placement of lamps should be determined by a lighting professional for light uniformity in the greenhouse. Lamps needed may be somewhat more if you want uniform lighting edges of the growing space and to account for spillage of light at the edges.
US Daily Light Integral Maps
When thinking about supplemental lighting, it is important to know how much sun light you have in your location. Joanne Logan (University of Tennessee) and Jim Faust (Clemson University) developed these interactive maps of the average daily light integrals (DLI) throughout the US. You can find monthly averages for the DLI for any location. Keep in mind that this is the DLI outside. To calculate the DLI inside your greenhouse, you need to know how much of the sunlight is transmitted into the greenhouse. Light transmission depends on the glazing material of your greenhouse, how old it is, and how clean it is. Transmission can range anywhere from 35% to 80%. Here is a short video on light transmission, produced by the E-Gro team.