What should I look for when buying a grow light?

Q: What should I look for when buying a grow light?

When buying a grow light, look first for efficacy above about 2.5 µmol/J, a measured PPFD map over your tent's footprint showing 600–900 µmol/m²/s average with edges at least 70–80 % of the centre, and a real wattage at the wall. Then a dimmer, a broad white spectrum, a driver you can mount outside the tent and an electrical safety certification. Ignore lumens and 'equivalent watts'.

Short answer

Buy on three numbers: efficacy above about 2.5 µmol/J, a measured PPFD map over your footprint (about 600–900 µmol/m²/s average, edges at least 70–80 % of the average), and the real wall wattage. Then a dimmer, a broad white spectrum and a remote driver. Supported.

Why

Efficacy sets running cost and heat. Good current LEDs exceed 2.5–3 µmol/J; HPS is about 1.7 ^[1]. In cannabis lighting research, efficacy mattered more for cost-effective production than spectrum ^[2].

The PPFD map tells you what the canopy gets. Yield rose linearly with canopy PPFD in controlled trials ^[3], so a light that's bright in the middle and dim at the edges grows big centre colas and larf in the corners.

Spectrum matters less than marketing suggests. A broad white spectrum works for veg and flower ^[2]. UV gave no useful rise in yield or cannabinoids ^[4].

The numbers

Spec	Look for	Red flag
Efficacy	≥ 2.5 µmol/J, measured	not stated, or only "lumens"
PPF	stated in µmol/s	missing
Wall wattage	real draw, e.g. 320 W	"equivalent 1,000 W"
PPFD map	your footprint, stated height, average shown	centre point only, tiny area
Uniformity	edges ≥ 70–80 % of average	corners at half
Dimming	0–100 % or close	none
Spectrum	broad white, ~5–15 % blue	colour story as the main selling point
Driver	detachable, can sit outside the tent	sealed in, hot
Safety	recognised electrical certification for your country	none

What to do

Measure your canopy footprint.
Shortlist fixtures with a PPFD map for that footprint.
Compare µmol/J and actual watts; size with 25–40 W/ft² for flower.
Prefer dimmable fixtures with a remote driver.
Check the certification mark and warranty.
Confirm the circuit can carry the load under 80 % of its rating.

Common mistakes

Buying on "equivalent watts" or lumens.
Trusting a centre-point PPFD at 30 cm.
Paying extra for UV to "boost THC" (Myth).
Oversizing for a small tent with no dimmer.
Skipping certification to save money.

Follow-up questions

What is a good µmol/J for a grow light?

A good current LED grow light delivers about 2.5–3 µmol/J or more of photosynthetic photons per joule of electricity; HPS is about 1.7 µmol/J. Higher efficacy means less electricity and less heat for the same light on the canopy. Look for a figure measured in an integrating sphere and stated with PPF in µmol/s and the actual wall wattage.

What is a PPFD map and how do I read it?

A PPFD map shows measured light intensity in µmol/m²/s at points across a stated footprint and hanging height. Check that it matches your tent size, read the average rather than the centre, and compare the corners with the centre: edges should be at least about 70–80 % of the average. A map measured at 30 cm over a smaller area than advertised flatters the fixture.

Do I need UV or far-red in a grow light?

No, you don't need UV or far-red in a cannabis grow light. Controlled trials found UV gave no commercially useful rise in yield or cannabinoids and high doses cut yield. Far-red is contested: it stretches plants, and its effect on yield seems to trade off against cannabinoid concentration. Buy a broad white spectrum with good efficacy; add extras only for a side-by-side trial.

Sources

Kusuma P, Pattison PM, Bugbee B, 2020. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7:56. doi.org/10.1038/s41438-020-0283-7

Westmoreland FM, Kusuma P, Bugbee B, 2021. Cannabis lighting: Decreasing blue photon fraction increases yield but efficacy is more important for cost effective production of cannabinoids. PLoS ONE 16(3): e0248988. pmc.ncbi.nlm.nih.gov/articles/PMC7987162/

Rodriguez-Morrison V, Llewellyn D, Zheng Y, 2021. Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Frontiers in Plant Science 12:646020. doi.org/10.3389/fpls.2021.646020

Rodriguez-Morrison V, Llewellyn D, Zheng Y, 2021. Cannabis inflorescence yield and cannabinoid concentration are not increased with exposure to short-wavelength ultraviolet-B radiation. Frontiers in Plant Science 12:725078. doi.org/10.3389/fpls.2021.725078