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.
Look inside
Two full pages from the pack, exactly as printed. Click either to read it at full size before you decide.
What is in it
- 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
- ✓ 23 printable pages
- ✓ 10 sections, worksheets and templates
- ✓ A second PDF worked out from the numbers you enter
- ✓ No account needed, just an email address
- ✓ Instant download; link valid 7 days
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The plan picks up where the free Grow Light Calculator and Harvest Yield Estimator leave off. One gives a wattage and a daily light target but never checks the first against the second; the other runs from plants to pounds, not back. The plan works from the harvest you want to whole plants, cells from the packet and the area your seedlings fill at their peak, then sizes the light from the photons that must land on the trays, using the PPF and the PPFD map on a fixture's own spec sheet, and prices the season. Yields and light targets are averages and rules of thumb, every fixture and price in it is an example, and it covers plug-in fixtures only: any wiring is a job for a licensed electrician.