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Sprout & Spectrum

How Many Grow Lights for a 4 × 4 Area?

Four by four is four times the area of a 2 × 2, and needs four times the photons. Here is what that means in actual fixtures.

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

The interior of a grow tent with reflective walls and plants inside

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
A flat white-light LED grow panel suspended above a tray of young plants on a wire rack
Mars Hydro TS 1000
The most photons of anything on this list, and one of only three fixtures here that publishes both PPF and efficacy so the claim can be checked against itself.
Four across a 4 × 42.3 umol/J
2
A thin aluminum LED grow board above a row of potted houseplants
AC Infinity IONBOARD S22
The highest published efficacy of any fixture here at 2.7 umol/J, on Samsung LM301H diodes, with dimming in real 20% steps rather than a vague knob.
Efficient at scale2.7 umol/J
3
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.
Cheapest per square footNot published

The reasoning

The trap in this question is that a 4 × 4 ft area sounds like “twice” a 2 × 2. It is four times: 16 sq ft against 4. Area scales with the square of the side, so the photon requirement quadruples, and a fixture advertised for a 2 × 2 does not cover a quarter of the problem — it covers exactly a quarter of the problem, which is the point.

A 4 × 4 ft area in square meters
1.486 m²

16 sq ft ÷ 10.7639. Every photon requirement on this page is that number multiplied by a target and divided by the 0.8 uniformity factor.

The requirement, at three target levels

Photon output needed for a 4 × 4 ft area

Medium (200 µmol/m²/s): 200 × 1.486 ÷ 0.8 = 372 µmol/s.

High (350 µmol/m²/s): 350 × 1.486 ÷ 0.8 = 650 µmol/s.

Seedling target (12 mol/m²/day at 14 h, so 238 µmol/m²/s): 238 × 1.486 ÷ 0.8 = 442 µmol/s.

How many fixtures that actually is

Fixtures required to reach each target over 4 × 4 ft, computed from published photon output
FixturePublished PPFFor medium (372)For high (650)
Mars Hydro TS 1000343 µmol/s2 units2 units
Spider Farmer SF1000249.21 µmol/s2 units3 units
AC Infinity IONBOARD S22about 311 µmol/s (derived: 115 W × 2.7)2 units3 units
SANSI 36W BR3065.6 µmol/s6 units10 units
Soltech Aspect Gen 250 µmol/s8 units13 units
Barrina T5 4ftNot publishedCannot be computedCannot be computed

Two things fall out of that table. First, the bulb and pendant form factors stop making sense at this scale — ten screw-in bulbs is not a lighting plan. Second, two panels genuinely do cover a 4 × 4 at a medium level, which is a smaller answer than most 4 × 4 buying advice implies.

Two panels or four?

The photon sum says two, and uniformity says think harder. Two 2 × 2 ft panels over a 4 × 4 area leaves the corners meaningfully darker than the centers, because each panel is lighting a square roughly half the size of the quadrant it is responsible for.

Four smaller panels, or a grid of bars, distributes the same photons far more evenly. If your plants are one crop that you want uniform, spend the money on distribution. If they are a mixed collection where some things want more light than others, two panels and a deliberate arrangement — high-light plants in the bright zones, foliage plants at the edges — is a better use of the same hardware.

Bars are the honest answer at this size

A 4 × 4 area is exactly where linear fixtures win. Four to six 4 ft bars, spaced evenly and run in series, put emitters across the whole area rather than concentrating them at two points. That is the same argument made in bar versus panel coverage, and it gets stronger as the area grows.

The catch, again, is data: Barrina publishes no photon output, so a bar-based 4 × 4 build cannot be verified in advance the way a panel-based one can. That is a real trade-off between coverage quality and predictability, and which side you take should depend on whether you can afford to be wrong.

What it costs to run

Running a 4 × 4 at the medium target for a season

Two Mars Hydro TS 1000 at 150 W each = 300 W total.

300 × 14 h ÷ 1000 = 4.2 kWh a day.

Over 150 days: 630 kWh.

At 18.34 cents per kWh: about $116 for the season.

The same photons from a 2.7 µmol/J fixture instead of 2.3 would draw roughly 256 W and cost about $99 — a $17 difference, which is the honest size of the efficacy effect at this scale.

Price your own configuration in the running-cost calculator.

Every pick in full

A flat white-light LED grow panel suspended above a tray of young plants on a wire rack
01

Mars Hydro TS 1000

LED panel

Four across a 4 × 4

The most photons of anything on this list, and one of only three fixtures here that publishes both PPF and efficacy so the claim can be checked against itself.

What Mars Hydro publishes

Power draw
150 W +/-5% at 100-277 V AC (source)
PPF (photon output)
343 umol/s (source)
Efficacy
2.3 umol/J (source)
Stated coverage
2.5 x 2.5 ft vegetative, 2.3 x 2.3 ft flowering (source)
Spectrum
660-665 nm, 730-740 nm, 3000 K, 5000 K (source)
Diodes
Bridgelux (source)
Dimming
Yes, 0-100% dimmer knob (source)
Dimensions
406 x 326 x 50 mm (source)

Our arithmetic

  • Average PPFD across its stated 2.5 x 2.5 ft vegetative footprint: about 470 umol/m2/s
    2.5 ft x 2.5 ft = 6.25 sq ft = 0.581 m2. 343 umol/s divided by 0.581 m2 = 590 umol/m2/s at perfect uniformity; applying a 0.8 uniformity factor gives about 472 umol/m2/s average.
  • Electricity used at 14 hours a day for 30 days: 63.0 kWh a month
    150 W x 14 h x 30 d / 1000 = 63.0 kWh.

Who should skip it

Skip it if you are lighting a bookshelf. 343 umol/s concentrated into a 2.5 ft square is far more light than a monstera wants, and you pay 150 W to deliver it.

A thin aluminum LED grow board above a row of potted houseplants
02

AC Infinity IONBOARD S22

LED panel

Efficient at scale

The highest published efficacy of any fixture here at 2.7 umol/J, on Samsung LM301H diodes, with dimming in real 20% steps rather than a vague knob.

What AC Infinity publishes

Power draw
115 W (source)
Efficacy
2.7 umol/J (source)
PPFD
1098 PPFD (no measurement distance published) (source)
Stated coverage
2 x 2 ft (source)
Spectrum
Samsung LM301H, plus 660 nm, 730 nm, 3000 K, 6500 K (source)
Diodes
272 diodes (source)
Dimming
Yes, 0-100% in 20% increments (source)
Dimensions
30.0 x 30.0 cm board, 1.1 cm thick (source)
Rated life
50,000 hours (source)

Our arithmetic

  • Photon output implied by the published wattage and efficacy: about 311 umol/s
    115 W x 2.7 umol/J = 310.5 umol/s. AC Infinity does not print PPF directly, so this is our arithmetic from two figures it does print.

What AC Infinity does not publish

  • PPF in umol/s — only efficacy is given, so photon output has to be derived
  • The measurement distance for the 1098 PPFD figure, without which the number cannot be compared to anything

Who should skip it

Skip it if you want to place it by the numbers. AC Infinity prints 1098 PPFD with no measurement distance attached, which makes that figure impossible to use or compare.

Linked LED strip lights mounted under a wire shelf above trays of seedlings
03

Barrina T5 4ft Grow Light Strips

LED bar

Cheapest per square foot

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.

Questions people actually ask

+How many watts do I need for a 4x4 grow area?
Wattage is the wrong starting point, because efficacy varies by close to a factor of two across fixtures. The photon requirement is about 372 µmol/s for a medium light level and 650 µmol/s for a high one. At 2.7 µmol/J that is roughly 138 W and 241 W respectively; at 1.39 µmol/J the same photons need nearly double the power.
+Is one light enough for a 4x4?
For a medium light level, one fixture would need to publish about 372 µmol/s, which is more than any of the consumer panels we cover. Two is the realistic minimum, and distribution rather than total output becomes the limiting factor at that size.
+Do I need to cover the corners of a 4x4?
Only if you intend to grow in them. It is entirely reasonable to accept a bright center and dimmer edges and arrange plants to match — high-light species in the middle, foliage plants at the perimeter. That is cheaper than buying uniformity you do not need.
+Are bars or panels better for a 4x4?
Bars, on distribution: a grid of linear fixtures puts emitters across the whole area instead of concentrating them at two points. The counter-argument is data, since the most popular bars publish no photon output at all, so a bar build cannot be verified before you buy it.

Sources

Read next

Where this sits

This page belongs to Coverage and Sizing Math. 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.