First decision: what kind
Six shapes of grow light, drawn to scale against your 36 × 18 shelf. Four of them are the wrong shape before a single spec is compared.
Each footprint above is drawn to scale on the same 36 × 18 outline. The decision is not about brands or diodes — it is that a shelf gives you 4.5 square feet of area and almost no height, and only one form factor is built for that. Panels trade hang height for coverage, and hang height is the thing a wire tier has none of.
Every fixture, side by side
Sorted by what it costs to light your tiers. Greyed rows miss your light target, your budget or your shelf's shape.
| Fixture | Per tier | Watts | PPF | PPFD | DLI | Fits 36×18 | Your tiers | Power/yr | Verdict |
|---|
~ marks a PPF derived from wattage rather than one the seller published — 1.8 µmol/J for a budget strip, 2.6 for a Samsung-diode bar. Nine of these thirteen need it. PPFD is the average across the whole 4.5 sq ft, not the peak under a diode. DLI is that PPFD times your photoperiod — the number that decides whether a plant thrives or stretches. Power per year assumes 18.83¢/kWh, the US residential average.
The shortlist in detail
Each fixture's light, footprint, headroom and total cost measured against the numbers you typed.
Second decision: how you raise and lower it
Six ways to change the height. Two of them fail the requirement outright, and one is what every grow guide tells you to buy.
The meter on each rig is measured against the 14″ gap you entered, minus the height of the fixture the page is currently recommending. What is left is what a plant can actually occupy at the light's lowest setting — and the point of an adjustable rig is that you get all of it back as the plant grows.
Ruled out, and why
Ten that looked like a match until one measurement gave them away. The misses show where the cut line really sits.
How these were checked
What was read off a live product page, what was calculated, and what to verify yourself with a tape measure.