Supplemental Lighting Calculator

Fixture Type (LED, HPS, CMH) is recorded for your own reference, but the calculator already asks for each fixture's actual Watts per Fixture and Fixture PPFD Output directly -- those two numbers fully capture how efficient a specific fixture is. Adding a separate technology-based adjustment on top would double-count what you've already told the calculator through those fields.

Fixtures Needed is the larger of two separate requirements: physical coverage (about 16 sq ft per fixture) and light output (enough combined PPFD to hit your target). At typical settings the coverage requirement is usually the larger of the two, so Growing Area -- which sets coverage directly -- moves Fixtures Needed more than Target PPFD does. Push Target PPFD high enough, though, and the light-output requirement can take over as the binding constraint.

A fixture with a higher PPFD rating delivers more usable light per unit, so fewer of them are needed to reach the same total light-output target -- this only reduces the light-output side of the calculation, though, so Fixtures Needed can never drop below the coverage-based minimum set by Growing Area.

Monthly Electric Cost is built from total power draw (Fixtures Needed times Watts per Fixture) times the hours the lights run, times your Electricity Cost. Both Watts per Fixture and Electricity Cost scale the final cost directly and proportionally -- doubling either one roughly doubles Monthly Electric Cost, holding everything else constant.

It doesn't model fixture mounting height, beam angle, or how evenly light is actually distributed across an irregularly shaped canopy, nor does it de-rate for fixture age or lumen depreciation over a bulb's service life -- all of which affect how much of the rated PPFD genuinely reaches the plants versus what the fixture's spec sheet claims.