Use this tool to plan grow-light lamp swaps before output drops too far. Results are estimates; verify with your PAR meter and crop stage.
| Lamp type | Typical rated life (hours) | Suggested swap threshold | Why it matters |
|---|---|---|---|
| LED Grow Light | 40,000–60,000 | 80–90% | Slow decline; keep uniform canopy output. |
| HPS | 18,000–24,000 | 70–85% | Output and spectrum shift can reduce yield. |
| MH / CMH | 12,000–20,000 | 65–80% | Useful for veg; aging can weaken growth rate. |
| T5 Fluorescent | 15,000–20,000 | 70–80% | Uniform seedlings need consistent intensity. |
- WeeklyHours = DailyHours × DaysPerWeek
- AvgDailyHours = WeeklyHours ÷ 7
- TargetHours = RatedLife × (Threshold% ÷ 100)
- DaysToReplace = TargetHours ÷ AvgDailyHours
- ReplaceDate = StartDate + DaysToReplace
Many lamps decline nonlinearly, but a linear planning model helps.
- Output% = 100 − (HoursUsed ÷ RatedLife) × DropAtRatedLife%
- Bound output between 0% and 100% for safety.
- Enter your lamp type and the manufacturer’s rated life.
- Set a replacement threshold to match crop sensitivity.
- Add your photoperiod and weekly schedule for accurate runtime.
- Pick the start date when the lamp first went into service.
- Optional: add costs and energy inputs for budgeting.
- Press Submit to see the next replacement date and totals.
Why grow-light output declines over time
Grow lamps lose usable intensity as emitters age, optics yellow, and dust builds on lenses or reflectors. In indoor gardens, heat cycles and high humidity accelerate degradation. HID bulbs may shift spectrum as salts change, while some LEDs slowly lose photon flux from junction stress. Even a 10–20% decline can reduce canopy PPFD enough to slow vegetative growth, stretch internodes, and lengthen flowering.
Choosing a replacement threshold
A practical threshold is 80–90% of rated life for flowering crops that need tight light uniformity. For hardy greens and seedlings, 70–85% often works if you monitor height and leaf color. If your target is consistent harvest timing, lean higher. If you run supplemental lighting only part of the day, you can accept a lower threshold while maintaining adequate DLI.
Scheduling based on operating hours
This calculator converts your photoperiod into weekly runtime and then estimates an average daily runtime. It multiplies rated life by the selected threshold to create target replacement hours. Dividing target hours by average daily hours gives days-to-replace, which is added to the start date to produce a swap date. A simple linear output model estimates current output and expected output at replacement using your chosen drop-at-rated-life value.
Cost planning and inventory control
Use lamp count, unit cost, and labor cost to estimate the budget per replacement event. The annual replacement estimate scales daily runtime across 365 days and helps set reorder points. Staggering swaps by zone spreads labor and avoids sudden uniformity changes across the room. Keeping one spare lamp per fixture row minimizes downtime during peak growth weeks, and logging serial numbers simplifies warranty claims.
Field checks to validate the plan
Treat dates as planning anchors, not guarantees. Record baseline PPFD or PAR at canopy level when lamps are new, then recheck monthly and after cleanings. Maintain airflow, keep drivers cool, and verify timers to prevent over-hours. If measured output falls faster than expected, tighten the threshold or shorten the schedule.
What does “rated life” mean for grow lamps?
Rated life is the manufacturer’s expected operating hours under standard conditions. For LEDs it often relates to lumen maintenance (such as L70), while HID bulbs usually describe typical service hours before significant output or spectrum shift.
What replacement threshold should I use for flowering?
For flowering rooms, 80–90% of rated life is a common planning range. Use the higher end when uniformity is critical, plants are light-sensitive, or you want consistent finishing times across benches.
Should I replace all lamps at the same time?
Replacing everything at once maximizes uniformity but can spike labor and inventory needs. Many gardens stagger swaps by zone or week to spread cost, reduce downtime, and avoid abrupt changes across the canopy.
How accurate is the output percentage shown?
It is a simplified linear estimate based on your chosen “drop at rated life” value. Real lamps decline nonlinearly and depend on heat, dust, and drive current, so validate with a PAR meter when possible.
My lamp is already used. How do I set the start date?
Set the start date to when the lamp actually went into service. If you do not know the date, estimate it from logs or hours. A more recent start date will push the replacement date later.
Does cleaning fixtures change the replacement date?
Cleaning can recover lost intensity from dust and haze, improving short-term output. It does not reset aging hours, so keep the schedule, then confirm output again after cleaning to decide whether to replace sooner.