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Sprout & Spectrum

Winter Daylight Loss, and What to Add Back

Two effects multiply: the day gets shorter and each hour gets weaker. That compounding is why January feels so much darker than the hours alone suggest.

By Scooter M. — An enthusiast who's genuinely into this. I read the manuals, compile the published specs, and do the math. No lab coat.

Published · How we pick

A gray winter window seen from inside with a plant on the sill

Photographs on this site are licensed stock and are illustrative of the fixture type. We do not own the units we write about.

How this page is funded: the buy links here are Amazon Associates links, and we earn a commission on qualifying purchases at no extra cost to you. We have not been paid to feature anything on this page, we accept no free product, and commission plays no part in the ranking. Full disclosure.

We have not tested these units. Everything below comes from the manufacturer’s own published specifications, or from arithmetic we have shown our working for. Where a figure is not published, we say so instead of estimating one. Read the method.

Quick picks

Efficacy is photons out per joule in. Where a manufacturer publishes it we quote theirs; where they publish photon output and wattage but not efficacy, we divide and label it as ours; where they publish neither, the cell says so rather than guessing. Tap a row for the full write-up.

#ProductBest forEfficacyPrice
1
Linked LED strip lights mounted under a wire shelf above trays of seedlings
Barrina T5 4ft Grow Light Strips
The default seed-starting bar for good reasons — 4 ft long, linkable in series, cheap per foot — and the reason we built a coverage calculator, because Barrina publishes no photon data whatsoever.
A windowsill runNot published
2
A square white LED grow panel hanging above leafy seedlings indoors
Spider Farmer SF1000
Publishes PPF to two decimal places and an efficacy figure that actually divides out of it. That internal consistency is rarer in this category than it should be.
A window that gets nothing2.5 umol/J
3
A screw-in LED grow bulb in a desk lamp aimed at a potted houseplant
SANSI 36W BR30 Grow Light Bulb
The rare grow bulb that publishes both PPF and a PPFD figure with the measurement distance attached, which is the only reason it can be sized honestly.
One specimen plant1.82 umol/J (ours)

The reasoning

Everyone knows winter days are shorter. What is less obvious is that the shortness is only half of it, and the two halves multiply rather than add. That compounding is why a room that felt bright in September feels genuinely dark in January, and why plants stall harder than the change in day length alone would predict.

The two effects, and why they multiply

Effect one: fewer hours. Day length in the northern US drops from roughly fifteen hours around the summer solstice to around nine at midwinter. That alone is about 40% fewer hours of any light at all.

Effect two: weaker hours. The sun sits much lower in the sky, so its light passes through more atmosphere and arrives at a shallower angle. Add the heavier average cloud cover of a northern winter and each remaining hour delivers substantially less than a summer hour would.

Because daily light integral is intensity × hours, the two effects multiply. Cutting hours by 40% and intensity by 50% does not leave you at 55% of summer; it leaves you at 30%.

How the compounding works

Take a plant receiving 300 µmol/m²/s for 12 useful hours in June: DLI = 300 × 12 × 3600 ÷ 1,000,000 = 13.0 mol/m²/day. Comfortably in the high-light band.

In January the same spot receives 150 µmol/m²/s for 7 useful hours: DLI = 150 × 7 × 3600 ÷ 1,000,000 = 3.8 mol/m²/day.

That is 29% of the summer figure, and it has moved the plant from the high-light band to the bottom of the low-light one. The illustrative intensities here are our assumption for demonstration, not measurements — but the multiplicative structure is exact, and it is the point.

Sizing the supplement

The good news follows directly from the same arithmetic: because the two terms multiply, you can close a large deficit by adding to whichever term is cheaper. Hours are cheaper.

Deficit (mol/m²/day) = target − what the window supplies
Supplement PPFD needed = deficit × 1,000,000 ÷ (supplement hours × 3600)
Closing a 4 mol deficit

Target 8 mol/m²/day for a medium-light plant; the winter window supplies about 4. Deficit = 4 mol/m²/day.

Supplementing for 10 hours a day: PPFD needed = 4 × 1,000,000 ÷ (10 × 3600) = 111 µmol/m²/s.

Over a 2 × 2 ft area (0.372 m²) at 0.8 uniformity, the fixture needs to produce 111 × 0.372 ÷ 0.8 = about 52 µmol/s. That is inside a single 36 W grow bulb — the SANSI BR30 publishes 65.6 µmol/s.

Running cost: 36 W × 10 h × 30 ÷ 1000 = 10.8 kWh a month, about $1.98 at 18.34 cents per kWh.

Two dollars a month to move a plant from stalled to growing is the most favorable arithmetic anywhere on this site, and it is the reason the winter supplement is worth doing even for people who are not otherwise interested in grow lights.

Practical points that follow from the math

  • Extend the day at both ends. Two hours before sunrise and four after sunset adds six hours of supplement without competing with daylight you already have.
  • Move plants toward the glass in winter and back in spring. Intensity falls off very fast with distance from a window. Three feet closer is often worth more than a fixture.
  • Clean the windows. Unglamorous and genuinely effective; a winter’s film on the glass is a real percentage.
  • Expect less, not nothing. Reduced winter growth is normal and healthy. The goal of supplementing is to keep a plant ticking over, not to force summer growth in January.

For the fixtures, see the best grow lights for winter. For what a whole season costs, the winter running-cost guide works it out fixture by fixture.

Every pick in full

Linked LED strip lights mounted under a wire shelf above trays of seedlings
01

Barrina T5 4ft Grow Light Strips

LED bar

A windowsill run

The default seed-starting bar for good reasons — 4 ft long, linkable in series, cheap per foot — and the reason we built a coverage calculator, because Barrina publishes no photon data whatsoever.

What Barrina publishes

Power draw
20 W per fixture; an 8-pack draws 160 W (source)
Spectrum
5000 K (source)
Dimming
No; on/off switch per fixture, linkable up to 8 in series (source)
Dimensions
4 ft strip, 120-degree beam angle, 120 V AC (source)

What Barrina does not publish

  • PPF in umol/s — for any strip in the range
  • PPFD at any stated distance
  • Photon efficacy in umol/J
  • LED count per strip, and rated lifespan

Who should skip it

Skip it if you need to know what you are getting before it arrives. There is no published PPF, no PPFD and no efficacy figure, so nothing about its output can be verified in advance.

A square white LED grow panel hanging above leafy seedlings indoors
02

Spider Farmer SF1000

LED panel

A window that gets nothing

Publishes PPF to two decimal places and an efficacy figure that actually divides out of it. That internal consistency is rarer in this category than it should be.

What Spider Farmer publishes

Power draw
100 W +/-5% at 100-277 V AC (source)
PPF (photon output)
249.21 umol/s (source)
Efficacy
2.5 umol/J (source)
Stated coverage
2 x 2 ft core, 3 x 3 ft maximum (source)
Spectrum
650-665 nm, 730-740 nm, 2800-3000 K, 4800-5000 K (source)
Diodes
Bridgelux (source)
Dimming
Yes, dimming knob (source)
Dimensions
32.5 x 29.0 x 5.9 cm (source)
Rated life
55,000 hours (source)

Our arithmetic

  • Does the published efficacy check out against the published PPF?: Yes — 2.49 umol/J computed against 2.5 umol/J published
    249.21 umol/s / 100 W = 2.4921 umol/J. The published 2.5 umol/J is that number rounded. Not every brand's two figures agree like this, which is exactly why we divide them.
  • Average PPFD across its 2 x 2 ft core footprint: about 536 umol/m2/s
    2 ft x 2 ft = 4 sq ft = 0.372 m2. 249.21 / 0.372 = 670 umol/m2/s at perfect uniformity; at a 0.8 uniformity factor, about 536 umol/m2/s.

Who should skip it

Skip it if raw output is what you are buying — the TS 1000 delivers 38% more photons per second, at the cost of 50 W more draw.

A screw-in LED grow bulb in a desk lamp aimed at a potted houseplant
03

SANSI 36W BR30 Grow Light Bulb

Grow bulb

One specimen plant

The rare grow bulb that publishes both PPF and a PPFD figure with the measurement distance attached, which is the only reason it can be sized honestly.

What SANSI publishes

Power draw
36 W (source)
PPF (photon output)
65.6 umol/s (source)
PPFD
265.58 umol/m2/s at 1 ft (source)
Spectrum
4000 K, 400-780 nm, CRI close to 100 (source)
Rated life
25,000 hours (source)

Our arithmetic

  • Efficacy, computed from the published figures: 1.82 umol/J
    65.6 umol/s / 36 W = 1.82 umol/J. That is roughly a third less efficient than the panels on this page — the price of a screw-in form factor.
  • Electricity used at 12 hours a day for 30 days: 12.96 kWh a month
    36 W x 12 h x 30 d / 1000 = 12.96 kWh.

What SANSI does not publish

  • Beam angle
  • Fixture dimensions

Who should skip it

Skip it if you are lighting more than one plant. 65.6 umol/s is a small amount of light spread over a cone — it lights a single specimen, not a shelf.

Questions people actually ask

+Why do plants stop growing in winter indoors?
Because the daily light total falls much further than the change in day length suggests. Fewer hours multiplied by weaker hours compounds — cutting hours by 40% and intensity by 50% leaves you at 30% of the summer total, not 55%.
+How many extra hours of grow light do plants need in winter?
Enough to close the gap between what your window supplies and the plant's target. Ten hours of supplement at a modest intensity closes a 4 mol per square meter per day deficit for a typical houseplant, which is inside what a single 36 W grow bulb delivers.
+Should I run the grow light during the day or at night?
At the ends of the day. Running it before sunrise and after sunset extends the photoperiod, which is what you are short of; running it at midday adds to light the plant already has and buys much less.
+Is reduced winter growth a problem?
No — it is normal and healthy for most houseplants. The aim of supplementing is to keep a plant ticking over rather than sliding backward, not to force summer growth rates in January.

Sources

Read next

Where this sits

This page belongs to Grow Lights for Houseplants and Winter. Every figure on it is either quoted from a named published source or derived in view — the full method is on our methodology page, and how the site is funded is on the affiliate disclosure.