Efficacy is the single most important number for comparing the long-term cost of grow lights. It’s also the number manufacturers manipulate most often. This guide explains what μmol/J means, why it matters, and how to spot inflated efficacy claims.
What Efficacy Measures
Unit: μmol/J (micromoles of photosynthetic photons per joule of electrical input)
What it tells you: How many usable photons the fixture produces per watt-second of electricity consumed.
The formula:
Efficacy (μmol/J) = PPF (μmol/s) ÷ Input Power (W)
Example: A fixture producing 1,500 μmol/s of PPF while consuming 600 W from the wall:
1,500 ÷ 600 = 2.5 μmol/J
This means the fixture delivers 2.5 micromoles of photosynthetically active photons for every joule of electricity you pay for.
Why Efficacy Matters More Than PPF Alone
Two fixtures can have identical PPF and dramatically different operating costs. A 1,500 μmol/s fixture at 2.0 μmol/J pulls 750 W from the wall. The same 1,500 μmol/s at 2.7 μmol/J pulls only 556 W.
Over a year of operation (12 hours/day, 365 days), at $0.12/kWh:
| 2.0 μmol/J fixture | 2.7 μmol/J fixture | |
|---|---|---|
| Annual kWh | 3,285 | 2,435 |
| Annual electricity cost | $394 | $292 |
| 5-year electricity cost | $1,970 | $1,460 |
| Difference over 5 years | $510 per fixture | |
Multiply by 10, 20, or 100 fixtures in a commercial operation and the economics of efficacy become enormous. This is why commercial growers pay 50-100% more upfront for high-efficacy fixtures — they recoup the difference in 2-3 years and save substantially after that.
Current Efficacy Benchmarks (2026)
| Technology | Typical Efficacy | Premium Tier |
|---|---|---|
| HPS (1,000 W DE) | 1.7 μmol/J | 1.9 μmol/J (newer DE) |
| CMH / LEC (315-630 W) | 1.8-2.0 μmol/J | 2.1 μmol/J |
| Budget LED | 1.5-2.0 μmol/J (often overstated) | — |
| Mid-tier LED | 2.3-2.6 μmol/J | — |
| Premium LED (current bin) | 2.7-3.0 μmol/J | 3.0-3.3 μmol/J |
| State-of-the-art LED (lab) | — | 3.5-3.8 μmol/J |
Above 2.7 μmol/J is the threshold for DLC Premium horticultural certification. Above 3.0 μmol/J represents the current commercial state of the art at scale.
How Efficacy Gets Inflated
Manufacturers have several tricks for reporting higher efficacy than the fixture actually delivers:
1. Diode-Level vs. Fixture-Level
The most common inflation. The manufacturer adds up the rated PPF output of each individual LED diode and divides by the rated power draw. This number ignores:
- Driver inefficiency (typically 88-95% efficient — losing 5-12% as heat)
- Optical losses from any lens, secondary optics, or reflector
- Thermal derating — LEDs lose 10-20% of output when operating at fixture temperature vs. lab test temperature
- Aging — diodes start ~5% above rated at hour 0, drop 5% in the first 100 hours, then degrade slowly over thousands of hours
Real fixture-level efficacy is typically 15-25% lower than diode-level efficacy. If a manufacturer’s claim is suspiciously high (3.5+ μmol/J at consumer pricing), this is usually why.
2. Pre-Soak Output
LEDs are slightly brighter when they first power on, before reaching thermal equilibrium. Some manufacturers report this initial peak output rather than steady-state. Real-world operation is always steady-state.
3. Wrong Test Temperature
LM-79 photometric testing is conducted at a specified ambient temperature (typically 25°C / 77°F). Some fixtures are tested at lower ambient temperatures than they’ll ever see in a real grow room, artificially boosting reported efficacy.
4. Cherry-Picked Power Reading
Power consumption can be measured at the wall (highest), at the driver output (a few percent lower), or at the diodes (lower still). Reporting power at the diodes inflates apparent efficacy. Wall-power efficacy is the only meaningful number for operational cost calculations.
5. Ignoring Power Factor
For commercial operations on three-phase power, power factor matters. A fixture with poor power factor (below ~0.95) draws more apparent power from the utility than its real power consumption. Premium drivers have power factor above 0.98.
How to Verify Efficacy Claims
The gold standard: an LM-79 test report from an independent, accredited laboratory. The report will show:
- Total fixture power consumption (at the wall)
- Total photopic lumen output
- Spectral power distribution
- PPF computed from SPD
- Efficacy = PPF / input power
For our directory, AGL’s verification process does this comparison directly: we audit the manufacturer’s claimed PPF against the IES file output and the stated wall power. If those numbers don’t reconcile, the listing doesn’t get the Verified badge.
DLC Listing as a Shortcut
The DesignLights Consortium (DLC) maintains a Qualified Products List for horticultural lighting. To be DLC-listed, fixtures must:
- Pass third-party LM-79 testing
- Meet minimum efficacy thresholds (2.10 μmol/J for DLC Standard, 2.50 μmol/J for DLC Premium as of 2026)
- Provide LM-80 lumen maintenance data
- Meet flicker, EMI, and other power quality requirements
A DLC-listed fixture has been independently verified. The QPL (qualified products list) is publicly searchable on the DLC website.
Efficacy vs. Other Factors
Efficacy isn’t everything. A fixture might have excellent efficacy but:
- Poor distribution — high peak PPFD in the center, dim edges
- Inappropriate spectrum — heavy R:B ratio that stretches plants
- Bad form factor — too tall, too wide, or too narrow for your room
- Cheap driver — high efficacy at hour 0, rapid degradation thereafter
- Poor warranty — 1 year when premium competitors offer 5
Efficacy is necessary but not sufficient. Combine it with PPFD distribution analysis, spectrum review, and warranty/support terms when evaluating fixtures.
Bottom Line
Efficacy in μmol/J is the operational-cost metric for grow lights. Above 2.7 is good; above 3.0 is excellent. Be skeptical of efficacy claims above 3.0 from non-DLC-listed brands at consumer pricing — that combination usually indicates diode-level reporting rather than fixture-level reality.
For verified efficacy figures, look for the Verified badge on our directory (our team has audited the IES file against the claimed efficacy) or the Third-Party Lab Verified badge (DLC-listed or independent lab certified).