LED Lifespan: Lumen Depreciation and Replacement

LED lumen depreciation is the maintenance term hiding inside the efficiency band: e_eff = e x m x tau x delta, so a bar rated 2.5 umol/J at hour zero is not a 2.5 umol/J bar at hour 10,000. When the maintenance derate has shed 10%, the same fixture lands at roughly 1.92 umol/J — out of the green (above 2.0) band and into yellow — while the wall meter and the kWh bill read identically. LED lumen depreciation shows up in delivered PAR = N x w x e x m x tau x delta / A, never in the power draw, which is why the replacement trigger lives in the calculator’s efficiency band rather than in how dim the bulb looks.

How Lighting Cost Calculator — LED vs HPS & Electricity Blueprint Helps You Master This

1. Compute the required photon flux: PPFD × grow area — both in the units the canopy actually receives and the area it covers.
2. Split that flux across one de-rated fixture: N = PPFD × A ÷ (efficiency × wattage × maintenance × thermal × dimming), rounded up to whole fixtures.
3. Charge the photoperiod: kWh/day = N × watts × hours ÷ 1000 × (1 + ballast/100) — the ballast overhead sits on top, invisible at the bulb.
4. Price it: cost per day = kWh × rate, then cost per month (×30) and cost per 90-day cycle fold the recurring line into the production budget.
5. Verify the optics: delivered PAR = N × w × e × m × τ × δ ÷ A against the target, plus the effective µmol/J after the three derates.
6. Run the comparison and the band: LED cost at your efficiency vs HPS at 1.7 µmol/J, then green/yellow/red on e_eff with the $50/day bill red flag.

The Formula Behind the Result

• Fixtures needed — required photon flux over one de-rated fixture → N = frac{PPFD cdot A}{e cdot w cdot m cdot tau cdot delta}
• Energy per day — fixtures × watts × photoperiod, ballast overhead → E = N cdot w cdot frac{H}{1000} cdot left(1 + frac{B}{100}right)
• Cost per day — energy × electricity rate → C = E cdot r
• Cost per month and per 90-day cycle → C_{30} = 30 cdot C, qquad C_{90} = 90 cdot C
• Delivered PAR — the full bank spread over the canopy → PAR = frac{N cdot w cdot e cdot m cdot tau cdot delta}{A}
• System efficiency after maintenance, thermal and dimming → e_{eff} = e cdot m cdot tau cdot delta
• LED vs HPS savings — your bank at e, alternative at 1.7 µmol/J → S = C_{HPS} - C_{LED}, qquad e_{HPS} = 1.7
• Efficiency band and the $50/day bill flag → text{green } e_{eff} > 2.0, text{yellow } 1.5{}^{circ}2.0, text{red } < 1.5 , text{or} , C > 50

Target Ranges: What the Numbers Should Be

• Fixture count is the archived grow-light watts-needed equation resolved into whole units: the old engine computed required watts = target DLI ÷ (photoperiod × 0.0036) × area ÷ efficacy — this arm works off PPFD directly and rounds up.
• Efficiency figures come from the archived efficacy table — LED 2.0–2.7, CMH 1.5, HPS 1.3 µmol/J — and the light-type select feeds those typicals into the efficiency field when you switch.
• The ballast line is the running-cost overhead: an HPS magnetic ballast banking 10–25% loss is exactly what LED drivers largely eliminated, which is why kwh × (1 + ballast/100) belongs in every LED-vs-HPS bill.
• Delivery is never the rating: PAR actual divides the de-rated bank over the real footprint, matching the archived coverage-factor discipline where PPFD = PPF × coverage ÷ area.
• The HPS benchmark sits at 1.7 µmol/J — a modern double-ended HPS rather than the archived 1.3 legacy figure — so the savings arm is honest against equipment the room can actually buy today.

Use the free pH & Alkalinity Architect — Buffer Blueprint to run the numbers on your own setup.

Use the free Energy Load & Solar Microgrid Sizer — Load, Solar & Cost Blueprint to run the numbers on your own setup.

Use the free Mist Culture Dashboard Finalizer 32 Tools Hub + Quizzes + 365 Posts to run the numbers on your own setup.

Compare every free calculator side-by-side on the MistCulture tools hub — each one keeps your workspace in sync.

Related guides

Worked Example: A 600 W Bar From Hour Zero to ~10,000 Hours

Start with the guide’s reference: a 600 W LED bar at 2.5 umol/J with maintenance 0.90, thermal 0.95 and no dimming. Effective efficiency at hour zero is 2.5 x 0.90 x 0.95 = 2.14 umol/J — comfortably in the green band above 2.0. Run 12 h/day and that bar logs about 4,400 hours per year.

By about 10,000 hours — just over two years of 12-hour runs — a 10% loss of output takes the maintenance derate from 0.90 toward 0.81, and e_eff drops to 2.5 x 0.81 x 0.95 = 1.92 umol/J, crossing into the yellow 1.5-2.0 band. Delivered PAR across the canopy sheds the same ~10% (PAR = N x w x e x m x tau x delta / A) even though the meter still reads a full 600 W and the power bill never blinks.

Because C = E x r cannot see the depreciation, the loss only surfaces when you re-run the fixture count at the real e_eff — a canopy that needed 2 bars at 2.14 umol/J needs more photon flux once the fixture is really 1.92. That re-run is the replacement trigger the green/yellow/red band is built to flag.

Frequently Asked Questions

References & Science

• HortScience — hydroponic nutrient studies
• Purdue Extension — hydroponics guide
• Cornell CALS — hydroponic research manual

Frequently asked questions

References

• Michigan State University Floriculture — Daily Light Integral reference tables
• Bugbee, B. — Toward an optimal spectral quality for plant growth and development (USU)
• USDA NIFA — controlled environment horticulture lighting guidelines

Related resources

• All Lighting energy tools
• More Growing resources guides
• Open Light Coverage Architect
• Open Par Dli Lab