In truth, there are too many variables to accurately count your chickens before they hatch. That being said, a beginner with a decent setup should hope for one pound of dried and manicured cannabis for each 1000 watts of high pressure sodium light. An experienced grower with the right strains might have a realistic goal of harvesting two pounds for the same grow light wattage.
Variable that effect grow room yield
You can’t simply predict that a grow room that uses 5000 watts is going to yield 5 pounds per harvest. A well designed grow room often has other systems besides lights and these use a substantial amounts of electricity. Maintaining the proper climate in a grow room may require fans, dehumidifies, air conditioning, pumps and other equipment that will draw electricity and add to your electric bill. Depending on the grow room design, location, insulation and the climate, your energy usage could range from very minimal to almost doubling that of your lights. Even a well designed room could have a substantial change in energy usage depending simply on the time of year.
The amount of vegetative growth allowed before flowering is also big variable. Some growers choose keep their plants in veg for a month or more while others flower after only a week or two of veg time. Reduced veg time will reduce the length of the growing cycle which cuts down energy consumption but may also reduce the potential yield per plant. This can sometimes be compensated for by growing more plants per square foot like Sea of Green but that’s not always practical or legal.
Strain selection is also key. Some strains yield more than other and some strains take longer than others to flower. There are strains that will reach full maturity in as little as 55 days of flowering while others may take 90 days or more.
There are modern and more energy efficient lamps like LED’s that show promise. These have the potential to reduce the energy cost devoted to lighting and they produce less heat so cooling expenses may be lowered as well. While I have yet to see anyone produce huge yields with LED’s alone, these could be an energy saving solution for veg or for reducing the wattage of HPS light during flowering. These technologies show real promise but much like solar panels, there is an open debate as to if the added start up cost is worth the potential long term savings.
Co2 enrichment can also improve yield per watt of lighting but should not be added to a grow room before the growing space is already dialed in without it. Co2 adds extra cost for the equipment and requires that tanks be regularly filled with Co2 or propane or natural gas must be burned to produce the Co2. In either case there is an extra monthly expense.
Some info from: marijuanagrowershq.com