ASHRAE, California, and DLC Just Converged on the Same Grow Light Efficacy Floor

ASHRAE approved a new efficacy floor for horticultural lighting in July 2025. California’s 2025 Energy Code raised its own floor to take effect January 1, 2026. The DesignLights Consortium delisted its Version 3.0 horticultural products on January 5, 2026, replacing them with a stricter Version 4.0. Three standards bodies, three separate rulemaking processes, and all three landed within a few tenths of a µmol/J of each other inside a single twelve-month window.

That convergence isn’t a coincidence. DesignLights Consortium’s Version 4.0 work informed ASHRAE’s new luminaire definition, and California’s update tracked similar cost-effectiveness math. If you’re specifying commercial fixtures this year, the headline percentages (31%, 47%, 8.7%, depending on which body you ask) matter less than a simpler question: who this reaches, and when.

The Changes, Side by Side

Here’s how the three requirements stack up against each other, sourced from the standards documents themselves rather than manufacturer summaries.

Read our full breakdown of Title 24’s grow light provisions for the California-specific detail. This piece focuses on the ASHRAE addition, since it’s the one most growers outside California haven’t clocked yet.

Greenhouses take the bigger jump

Notice the two percentages aren’t equal. Indoor grow spaces move from 1.9 to 2.5 µmol/J, a 31% increase. Greenhouses move from 1.7 to 2.5 µmol/J, a 47% increase, once a facility crosses the 40 kW threshold with integrated luminaires. ASHRAE’s earlier standard treated greenhouse supplemental lighting as a smaller add-on to sunlight, so it started from a lower bar. Addendum bz erases that gap and holds both space types to the same number. Greenhouse operators still running older HPS or first-generation LED supplemental fixtures have the furthest distance to close.

The Luminaire Redefinition Buried in the Fine Print

ASHRAE’s Addendum bz does more than raise a number. It redefines what gets measured. The old standard rated “light source PPE,” a bare LED or lamp figure that ignored how much of that light the fixture’s optics and housing delivered to the canopy. The new standard rates “luminaire PPE,” which accounts for the whole fixture: optical losses, thermal derating, everything between the diode and the plant.

ASHRAE built this definition on the ANSI/ASABE S640-2017 metric, the same one the American Society of Agricultural and Biological Engineers developed for horticultural lighting testing, and aligned it with DLC’s luminaire-based approach in Hort V4.0. That alignment matters more than the number itself. Under the old rule, a manufacturer could publish an efficacy figure straight from a diode’s datasheet, before accounting for how much light a cheap housing wastes as heat. Under the new definition, that gap shows up in the published spec.

For background on how PPE, PPF, and PPFD relate to each other, see our explainer on the three numbers that matter most.

Crossing the 40 kW Line

Both ASHRAE and California trigger their stricter floor at facilities running 40 kW or more of connected horticultural lighting load. Below that line, ASHRAE’s exception language keeps the older 1.7 µmol/J minimum in place.

Forty kilowatts sounds substantial until you count fixtures. Commercial LED bars in the 600 to 800 watt range are standard for flowering rooms running sole-source lighting. At 650 watts per fixture, 40 kW of connected load works out to 62 fixtures. At 800 watts per fixture, it’s 50. A single flowering room in the 3,000 to 6,000 square foot range, depending on spacing and target PPFD, can cross that threshold on its own. The exemption protects hobbyist and micro-commercial setups. It doesn’t protect a mid-size single-room cultivation facility, and that’s the kind of operation most prone to assume a national energy code change targets someone bigger.

The Floor Sits Behind the Market

ASHRAE’s own foreword to Addendum bz makes an unusual admission for a regulatory document: even at 2.5 µmol/J, the new requirement lands “near the bottom (22nd percentile) of lighting efficacies in the DesignLights Consortium’s database of horticultural LED lighting products.” Most DLC-listed fixtures on the market today already clear the new “stricter” code floor by a wide margin.

That’s by design, not oversight. ASHRAE 90.1 requirements have to pass a cost-effectiveness scalar threshold before they get adopted, meaning the standard targets a payback that works across the full population of regulated facilities, not the leading edge of what a fixture can do. A grower buying to the code minimum, rather than to what DLC’s qualified products list offers today, leaves both efficiency and rebate dollars unclaimed. Our manufacturer overview covers where current fixtures land on the efficacy curve.

Model Code Versus Enforced Code

ASHRAE 90.1 is a model code. It becomes enforceable when a state or local jurisdiction adopts it into its own building code, and that adoption runs on a separate, often slow, timeline. As of mid-2025, multiple states were still enforcing energy codes equivalent to ASHRAE 90.1-2016 or 90.1-2019, several code cycles behind. New York adopted requirements equivalent to 90.1-2022 in July 2025. Rhode Island’s updated code, based on the 2024 IECC, became effective in December 2025. Addendum bz targets 90.1-2022 and gets folded into 90.1-2025, a standard most states haven’t adopted yet.

California doesn’t wait on ASHRAE’s cycle. Title 24 runs its own rulemaking process, and its 2.3 µmol/J requirement is already in force for new permits as of this year. If you’re building or retrofitting in California, the higher bar applies now. If you’re anywhere else, check what edition of the energy code your jurisdiction enforces today before assuming Addendum bz reaches your next build.

How adoption works in practice

A state doesn’t flip a switch when ASHRAE publishes a new edition. Most states run their own legislative or agency rulemaking process to decide whether to adopt a new model code edition, hold public comment periods, and set a phase-in date for new construction versus renovations. Some states write ASHRAE 90.1 into law by reference and update it on a regular schedule. Others build their code around the International Energy Conservation Code, which tracks a similar but not identical cycle, and lag ASHRAE by a version or two. A handful of states and cities write their own energy codes from scratch and borrow provisions from both. Before you assume a new requirement applies, find out which body writes your local energy code, not just which national model it borrows from.

Checks to Run Before Your Next Fixture Order

Ask whether a published PPE figure is luminaire-level or diode-level

A spec sheet that cites only a chip or lamp efficacy number, without a fixture-level test report, isn’t giving you the figure the new codes use. Ask the manufacturer whether their published number was tested per ANSI/ASABE S640-2017 at the luminaire level, not the diode level.

Confirm current DLC QPL status yourself

V3.0 listings are being removed from the qualified products list. A fixture marketed as “DLC listed” on a product page from a year ago may no longer carry a current listing. Check the QPL search tool at designlights.org rather than trusting the marketing copy.

Rebate eligibility runs through DLC, not ASHRAE or Title 24

Most utility rebate programs anchor eligibility to DLC’s Horticultural QPL, not to the ASHRAE or Title 24 thresholds themselves. Clearing the code minimum doesn’t guarantee rebate eligibility if the specific product isn’t on the current list.

Under 40 kW doesn’t mean under scrutiny

The exemption removes the stricter code requirement. It doesn’t change the underlying economics. A fixture running near the old 1.7 µmol/J floor still costs more to operate over its lifetime than one running near 3.0 µmol/J, exemption or not.

Browse the AGL directory for verified specs across current commercial fixtures.