The Right-Size Grow Light Plan
From the harvest you want to the photons your seedlings need: plants, cells, the peak area, the hours, and a fixture checked against its own spec sheet
GardenerPlace's Grow Light Calculator gives a wattage and a daily light target, and its Harvest Yield Estimator runs from plants to pounds. Neither checks the thing you are about to buy. A watt is electricity going in, not light landing on a leaf, and the light is usually sized for the tray you sow in rather than the flats your seedlings fill after potting up. The Right-Size Grow Light Plan does that arithmetic. In the worked example, 70 lb from four crops is 19 plants and 25 cells, one flat at sowing and three at the peak, 4.93 sq ft. At 16 hours a day the seedlings' target of DLI 12 needs 209 umol/m2/s, so 96 umol of photons a second must land on the trays. The calculator's 59 W reaches that at 2.0 umol/J but not at 1.5; one fixture whose center reading looks generous averages DLI 10.9 across the trays, while another's corners still reach 12.4. Running 16 hours instead of 12 lets a 65 W fixture do the job of an 87 W one for the same electricity. Every figure is computed by code from the site's calculators and the definitions it states; the fixtures, maps, harvests and prices are examples, and yields and light targets are averages, not promises. It covers plug-in fixtures only and gives no wiring advice. Included: 7 steps, 3 printable working pages, a light-target table for every stage, a quickstart, and a personalised sheet worked from the numbers you enter in the calculators.
What’s inside
- The Round-Up Rule: the harvest you want, back to whole plants, with the half line
- The Packet Rule: cells to sow from each packet's germination rate
- The Peak Rule: size the light for the flats after potting up, not the day you sow
- The Hours Rule: DLI, PPFD and why a longer day buys a smaller fixture
- The Photon Floor Rule: efficacy from the spec sheet, and why lumens do not count
- The Average Rule: reading a maker's PPFD map by its average and its corners
- The Plug-In Rule: the season's running cost, and a safe plug-in setup
- Working pages: seed-starting plan sheet, fixture comparison sheet, light log
- PPFD and watts per sq ft for every stage at 12, 14, 16 and 18 hours
- A quickstart that closes the score loop, plus a personalised sheet from your calculator inputs
Run one of the free calculators first and buy from that page, and this sheet is computed from your own figures. Buy from here and the pack still arrives complete — the sheet simply has no numbers of yours to work from yet.
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