Supplemental lighting decisions are usually made on fixture price per watt, and they usually disappoint. The variables that decide whether a greenhouse lighting project returns its capital are the target daily light integral (DLI), the photoperiod the crop needs, the delivered photon efficacy of the fixture at operating temperature, and the local electricity tariff. This white paper builds an explicit model connecting those four variables, then applies it to QUEENDOM’s Z-01 T8 Plant Tubes, Z-02 T8 Integrated, Z-03 T10 and Z-04 T12 fixtures so that a grower or specifier can compute payback before ordering anything.
1. Why DLI and photoperiod must be modelled together
DLI is the total quantity of photosynthetically active photons delivered to a square metre over one day, expressed in mol/m²/d. Photoperiod is the number of hours over which those photons are delivered. They are linked by a single identity:
DLI = PPFD × photoperiod (h) × 3600 / 1,000,000
where PPFD is the instantaneous photon flux density in µmol/m²/s. Two consequences follow immediately. First, raising PPFD and shortening the photoperiod can deliver the same DLI but a completely different crop response, because the plant integrates light over time. Second, extending the photoperiod to hit a DLI target reduces the required PPFD and therefore the installed wattage, which is the cheapest lever available to a grower.
The table below shows representative DLI targets and photoperiod constraints for common protected crops.
| Crop | Target DLI (mol/m²/d) | Photoperiod (h) | Required PPFD (µmol/m²/s) | Notes |
|---|---|---|---|---|
| Tomato, high-wire | 22–30 | 16–20 | 300–520 | Long-day tolerant, high DLI ceiling |
| Cucumber | 20–26 | 16–18 | 310–450 | Responds strongly to DLI above 20 |
| Lettuce, head | 14–17 | 14–16 | 240–340 | Bolting risk above 16 h for some cultivars |
| Leafy herbs, basil | 12–20 | 14–18 | 190–400 | Rapid response, short cycle |
| Strawberry, everbearing | 15–20 | 14–16 | 260–400 | Photoperiod-sensitive flowering |
| Young transplants | 10–14 | 14–16 | 170–280 | Morphology control critical |
| Cannabis-type, vegetative | 20–30 | 18 | 310–460 | Long-day vegetative regime |
| Microgreens | 9–14 | 12–16 | 210–320 | Short cycle, low DLI ceiling |
2. From target DLI to installed wattage
Converting a DLI target into a fixture count requires three intermediate quantities, and the most commonly mis-stated of them is efficacy.
Photon efficacy at operating temperature. A fixture rated at 2.7 µmol/J at 25 °C will typically deliver 2.4 to 2.5 µmol/J at a canopy-level ambient of 28 to 32 °C, because LED photon output falls as junction temperature rises. The model must use the hot efficacy, not the datasheet maximum.
Optical efficiency at canopy distance. For a tube fixture mounted on a truss and a canopy 0.6 m below it, wall losses, reflectance and spill outside the bed reduce delivered photons. A well-designed installation achieves 80 to 88 percent of emitted photons reaching the canopy plane.
Coverage efficiency. Photons delivered between beds are lost unless the layout is designed to overhang the canopy. Coverage efficiency of 85 to 92 percent is realistic for a well-laid-out tube installation.
The three combine into a delivered photon efficacy:
E_delivered = E_hot × η_optical × η_coverage
| Fixture | Rated efficacy (µmol/J) | Hot efficacy at 30 °C | Optical η | Coverage η | Delivered efficacy |
|---|---|---|---|---|---|
| Z-01 T8 Plant Tubes | 2.60 | 2.38 | 0.85 | 0.90 | 1.82 |
| Z-02 T8 Integrated | 2.55 | 2.33 | 0.83 | 0.90 | 1.74 |
| Z-03 T10 | 2.85 | 2.62 | 0.86 | 0.90 | 2.03 |
| Z-04 T12 | 3.05 | 2.80 | 0.87 | 0.91 | 2.22 |
3. Energy consumption and yield response
Energy consumption is the product of installed wattage, hours of operation and the number of days the installation runs. The model below uses a 1,000 m² bed area, a 16 hour photoperiod and a 180 day supplemental season.
| Fixture | Unit power (W) | Units for 300 µmol/m²/s | Installed load (kW) | Daily energy (kWh) | Season energy (MWh) |
|---|---|---|---|---|---|
| Z-01 T8 Plant Tubes | 36 | 340 | 12.2 | 196 | 35.3 |
| Z-02 T8 Integrated | 42 | 350 | 14.7 | 235 | 42.3 |
| Z-03 T10 | 50 | 265 | 13.3 | 212 | 38.2 |
| Z-04 T12 | 72 | 205 | 14.8 | 236 | 42.5 |
At an electricity price of 0.18 EUR/kWh, the season energy cost runs from approximately 6,350 EUR for Z-01 to 7,650 EUR for Z-04. The fixture that consumes least is not automatically the best choice, because the yield response differs: a higher delivered photon efficacy converts more of each kilowatt-hour into assimilated carbon, and the DLI response of most crops is not linear.
The curve below shows a representative yield response to DLI for a high-wire tomato crop. The response is steepest between 12 and 22 mol/m²/d and flattens above approximately 28 mol/m²/d, which is where incremental energy stops paying.
4. Payback calculation
Payback is computed from incremental margin divided by capital outlay, with energy cost and any yield premium treated as annual benefit.
Annual benefit = (Δ yield × price per unit) – (season energy × tariff) – maintenance
The table below applies the model to a 1,000 m² tomato bed over a 180 day season, with a 0.95 EUR/kg wholesale price and a 7 mol/m²/d DLI uplift.
| Metric | Z-01 T8 | Z-02 T8 Integrated | Z-03 T10 | Z-04 T12 |
|---|---|---|---|---|
| Delivered efficacy (µmol/J) | 1.82 | 1.74 | 2.03 | 2.22 |
| Installed load (kW) | 12.2 | 14.7 | 13.3 | 14.8 |
| Season energy (MWh) | 35.3 | 42.3 | 38.2 | 42.5 |
| Season energy cost (EUR) | 6,354 | 7,614 | 6,876 | 7,650 |
| Fixture capital (EUR) | 31,960 | 27,650 | 36,040 | 34,850 |
| Installed cost incl. labour (EUR) | 39,000 | 34,800 | 43,900 | 42,600 |
| Yield uplift (kg/m²/season) | 3.6 | 3.4 | 4.1 | 4.4 |
| Gross yield revenue (EUR) | 3,420 | 3,230 | 3,895 | 4,180 |
| Net annual benefit (EUR) | -2,934 | -4,384 | -2,981 | -3,470 |
| Simple payback (years) | 21.2 | 15.9 | 22.6 | 20.0 |
Two observations matter more than the arithmetic. First, at a wholesale tomato price near 0.95 EUR/kg, supplemental lighting in central Europe does not pay back on yield alone within a reasonable horizon; it pays back through consistency of supply, grade premium and contract security, which is how commercial growers actually justify it. Second, the sensitivity to electricity price is far larger than the sensitivity to fixture price, so tariff structure and operating hours deserve more attention than the purchase order.
The table below shows the payback sensitivity that follows from the two dominant variables.
| Electricity price (EUR/kWh) | Season energy cost @ Z-01 | Payback @ tomato 0.95 EUR/kg | Payback @ premium crop 3.50 EUR/kg |
|---|---|---|---|
| 0.08 | 2,824 | 12.8 years | 3.1 years |
| 0.12 | 4,236 | 16.6 years | 3.6 years |
| 0.18 | 6,354 | 21.2 years | 4.4 years |
| 0.25 | 8,825 | 27.6 years | 5.5 years |
| 0.32 | 11,296 | 34.2 years | 6.8 years |
5. Product mapping and layout guidance
The four tube families differ in the situations they suit, and the differences are mechanical as much as photometric.
| Model | Format | Typical power | Delivered efficacy | Best-fit application |
|---|---|---|---|---|
| Z-01 T8 Plant Tubes | T8 tube, external driver | 36 W | 1.82 µmol/J | Retrofit into existing T8 fluorescent rails, inter-lighting |
| Z-02 T8 Integrated | T8 tube, integrated driver | 42 W | 1.74 µmol/J | Direct mains replacement, low installation labour, seedling rooms |
| Z-03 T10 | T10 tube | 50 W | 2.03 µmol/J | Higher flux per position, multi-layer vertical benches |
| Z-04 T12 | T12 tube | 72 W | 2.22 µmol/J | Highest flux per position, tall greenhouse trusses, high-DLI crops |
Layout practice follows directly from the DLI identity. If a target DLI can be reached by extending the photoperiod within the crop’s tolerance, do that first, because it reduces required PPFD and therefore installed wattage. Only when the photoperiod ceiling binds should PPFD be raised by adding fixtures or moving to a higher-flux tube.
5.1 Photoperiod ceiling and required fixture density
The curves below show required fixture density for a 300 µmol/m²/s target as the photoperiod changes, for the four models at a mounting height of 0.6 m above canopy.
6. Common mistakes and how to avoid them
| Mistake | Consequence | Correction |
|---|---|---|
| Using datasheet efficacy instead of hot efficacy | 8–14 % short on delivered photons | Model at 30 °C canopy ambient |
| Ignoring bed-edge spill | Coverage efficiency falls below 80 % | Design overhang and reflectivity together |
| Extending photoperiod past crop tolerance | Bolting, photoperiod disorder | Respect the per-crop photoperiod ceiling |
| Quoting payback on yield alone | Unrealistic business case | Include grade premium and supply consistency |
| Mixed tube models on one circuit | Uneven DLI across the bed | Keep one model per control zone |
| No dimming provision | Full energy cost during low-tariff oversupply | Specify dimmable drivers from the outset |
7. Verification and test methods
- Fixture-level photometry — measure PPF and photon efficacy per IES LM-79 on a representative sample, at rated power and at thermal equilibrium, and record the case temperature at which the measurement was taken.
- Canopy-plane mapping — measure PPFD on a 1 m grid at canopy height across at least three bays, and report the mean, the minimum and the uniformity ratio U0.
- DLI accounting — integrate the measured PPFD over the actual operating schedule, including any dimming or daylight-dimming events, to obtain the true delivered DLI.
- Energy sub-metering — install a dedicated kWh meter on the lighting circuit so that measured consumption can be compared against the model at the end of the season.
- Yield and grade audit — record marketable yield and grade distribution per bay, with a control bay that receives no supplemental light, to quantify the incremental benefit rather than the absolute yield.
8. Conclusion and selection guidance
For retrofit work into existing T8 rails, Z-01 T8 Plant Tubes provide the lowest capital path and the widest availability of spares. For new seedling rooms and low-labour installations, Z-02 T8 Integrated removes the external driver from the bill of materials. Where bed area is scarce and vertical space is available, Z-03 T10 raises flux per position without redesigning the truss. For tall greenhouses targeting high DLI with the fewest mounting points, Z-04 T12 delivers the highest photon efficacy of the range at 2.22 µmol/J delivered. Whatever the model, size the installation on the DLI identity, model energy at the hot efficacy, and treat electricity tariff and photoperiod ceiling as the two variables that decide whether the project returns its capital.
9. Referenced standards
- IES LM-79 — Approved Method: Electrical and Photometric Measurements of Solid-State Lighting Products
- IES LM-80 — Approved Method: Measuring Luminous Flux and Color Maintenance of LED Light Sources
- IES TM-21 — Projecting Long Term Lumen, Photon and Radiant Flux Maintenance of LED Light Sources
- ANSI/ASABE S640 — Quantities and Units of Electromagnetic Radiation for Plants (Photosynthetic Organisms)
- DLC Horticultural Lighting Technical Requirements, Version 3 (design-ready reference)
- IEC 62471 — Photobiological safety of lamps and lamp systems
- EN 12464-1 — Light and lighting of work places
10. Contact us
QUEENDOM supplies Z-01, Z-02, Z-03 and Z-04 horticultural tubes from stock and can provide photometric files, PPF measurement summaries and a DLI-and-payback calculation for a specific bed area, crop and tariff. Contact our horticultural engineering group to request samples and a layout proposal.
Related products and applications
The grow tubes and fixtures used in the DLI model are available in the following families.
- Horticulture grow light (Z-01)
- Horticulture grow light (Z-02)
- Horticulture grow light (Z-03)
- Horticulture grow light (Z-04)
- Application overview: Lighting application solutions
- More technical papers: Lighting knowledge resources