Seed Shelf Lights

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Seed starting shelf light calculator

Count the fixtures a shelf needs, see how many seed trays fit, and estimate what a season of running the lights costs. Change any number; the picture and results update as you type.

Shelf
Lights
Read it off the fixture's label. The default is a placeholder.
Our planning rule, not a standard. See how the numbers work, below.
Trays
Defaults suit a typical 1020 flat. Measure yours.
Running cost
Use the rate on your bill. The default is a placeholder.
48 in24 in

Top view. Pink bars are fixtures, brown rectangles are trays.

Fixtures needed
2
2 rows of 1 fixture, each 48 in
Trays that fit
4
Total power
80 W
10 W per sq ft
Season cost
$11.76
78.4 kWh over 70 days at 14 h/day

Your numbers are saved in the page address, so you can bookmark or share this setup. Nothing is sent anywhere.

How the numbers are worked out

  • Rows of lights = shelf depth divided by the depth one row covers (default 12 in), rounded up. We allow a 5% tolerance, so a 12.5 inch shelf still gets one row. The 12 inch default is our own planning rule based on examples like two 4-foot fixtures over a 24 inch deep shelf. It is not an extension standard.
  • Fixtures per row = shelf length divided by fixture length, rounded up, again with 5% tolerance.
  • Trays that fit = whole trays in either orientation. The default 21 x 11 inch size is a typical outside size for a 1020 flat. Measure your own, because trays vary.
  • Total power = fixtures times watts per fixture. The defaults of 40 W for a 4-foot fixture and 10 W for a 2-foot bar are placeholders; use the labels on yours.
  • Season cost = watts x hours a day x days of use / 1000 gives kWh, and kWh times your price gives cost.
  • Watts per square foot is shown for comparison only. Vendors quote very different figures for seedlings, and watts measure what a light draws, not how much usable light reaches the plants.

What this calculator cannot tell you

It counts fixtures and space. It does not measure light intensity at the seedlings, which depends on the fixture, its spread and how high it hangs. Check the manufacturer's information, and see the guide for the hanging heights and hours that university extension sources give.

Questions

Is two 4-foot lights per shelf enough?

For a 24 x 48 inch shelf of seedlings, that is the layout the University of Saskatchewan describes. It also says fruiting plants grown to maturity may want up to four lights.

Can I use 2-foot bars instead?

Yes. Pick "2 foot" above and the calculator will use more bars per row. Oklahoma State notes that fluorescent tubes lose light near their ends, so check coverage details for short LED bars.

Should lights stay on all day and night?

No. Extension sources recommend about 14 to 16 hours a day, and UConn notes plants need some darkness.