The precise calculation is PAR Watts per unit of area in your growing space, so let's define some terms;
PAR Watts; that fraction of the energy from the light in use that is actually in the wavelengths the plants can use. For HID lamps running on magnetic ballasts and old style fluorescent lighting, that's about 30% of the draw at the wall. DE lights and 315W CMH on square wave ballasts are about 40% efficient. Crap LED starts at 40% and rarely gets better than 45%. Quality LED lights, such as those available from sellers here on RIU like Timber and HLG, can be anywhere from 50% to 60% efficient. Multiply total Watts by the percentage to get total PAR Watts. You want to shoot for 15-20 PAR Watts per square foot.
Unit of area; square foot, square meter etc. Here we usually use square feet.
Growing space; the actual canopy, not including aisles, walls, etc. Divide PAR Watts by square footage lit by the fixture.
So you want to know if your 1000W HPS (on a magnetic or digital ballast?) can cover your space? Do the math:
1000W x .3 = 300 PAR Watts
300W/16 square feet = 18.75 Watts per square foot
So if your plants will each fill up a 2x2' space, you'll fit 4 plants under your thouie.
I'll let you do the rest!
