# 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'.

- Canonical: https://genetixbay.com/knowledge/answers/what-to-look-for-in-a-grow-light.html
- Topic: Environment · Type: How-to
- Evidence: Supported · Status: Draft: awaiting expert review
- Full article: [PPFD and DLI by stage, with the evidence](https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.md)
- Also asked as: What grow light specs matter? | What does µmol/J mean on a grow light? | How do I choose an LED grow light? | Which grow light specs are marketing?
- Updated: 2026-10-31 · Licence: CC BY-SA 4.0 (https://creativecommons.org/licenses/by-sa/4.0/), attribute "GenetixBay Knowledge Base (genetixbay.com/knowledge)"

## Short answer

Buy on three numbers: **[efficacy](https://genetixbay.com/knowledge/glossary/photon-efficacy.md) above about 2.5 µmol/J**, a **measured [PPFD](https://genetixbay.com/knowledge/glossary/ppfd.md) 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](https://genetixbay.com/knowledge/glossary/led.md) exceed 2.5–3 µmol/J; [HPS](https://genetixbay.com/knowledge/glossary/hps.md) 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](https://genetixbay.com/knowledge/glossary/cola.md) 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](https://genetixbay.com/knowledge/glossary/cannabinoid.md) [4].

## The numbers

| Spec | Look for | Red flag |
|---|---|---|
| Efficacy | ≥ 2.5 µmol/J, measured | not stated, or only "lumens" |
| [PPF](https://genetixbay.com/knowledge/glossary/ppf.md) | 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

1. Measure your canopy footprint.
2. Shortlist fixtures with a PPFD map for that footprint.
3. Compare µmol/J and actual watts; size with 25–40 W/ft² for flower.
4. Prefer dimmable fixtures with a remote driver.
5. Check the certification mark and warranty.
6. 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](https://genetixbay.com/knowledge/glossary/thc.md)" (*Myth*).
- Oversizing for a small tent with no dimmer.
- Skipping certification to save money.

## Related

- Full article: [PPFD and DLI by stage](https://genetixbay.com/knowledge/environment/ppfd-dli-by-stage.md)
- [What wattage LED do I need for my tent?](https://genetixbay.com/knowledge/answers/what-wattage-light-for-my-tent.md)
- [Why don't lumens matter?](https://genetixbay.com/knowledge/answers/lumens-vs-ppfd.md)
- [LED or HPS?](https://genetixbay.com/knowledge/answers/led-vs-hps.md) · [Light spectrum](https://genetixbay.com/knowledge/environment/light-spectrum.md)
- Terms: [photon efficacy](https://genetixbay.com/knowledge/glossary/photon-efficacy.md) · [full spectrum](https://genetixbay.com/knowledge/glossary/full-spectrum.md)

## 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

1. Kusuma P, Pattison PM, Bugbee B, 2020. From physics to fixtures to food: current and potential LED efficacy. Horticulture Research 7:56. https://doi.org/10.1038/s41438-020-0283-7
2. 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. https://pmc.ncbi.nlm.nih.gov/articles/PMC7987162/
3. 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. https://doi.org/10.3389/fpls.2021.646020
4. 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. https://doi.org/10.3389/fpls.2021.725078