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Example Data Table
| Scenario | Area (m2) | Target (lux) | Fixture lumens | CU | MF | Estimated fixtures |
|---|---|---|---|---|---|---|
| Seed starting bench | 2.00 | 1500 | 3000 | 0.65 | 0.80 | 2 |
| Potting table | 3.50 | 750 | 2500 | 0.60 | 0.80 | 3 |
| Tool shed | 6.00 | 300 | 2000 | 0.55 | 0.75 | 2 |
Formula Used
This calculator estimates total light output needed using a practical illuminance method. It accounts for how well light is used in your setup and how output declines over time.
- Area (m2) = Length x Width (or use the area override)
- Required lumens = Area x Target lux / (CU x MF)
- Fixtures needed = ceil(Required lumens / Lumens per fixture)
- Achieved lux = Fixtures x Lumens x CU x MF / Area
- Suggested spacing ~= sqrt(Area / Fixtures) (grid estimate)
- kWh = (Total watts / 1000) x Hours x Days
How to Use This Calculator
- Select a task preset that matches your gardening work.
- Enter length and width, or check the area override box.
- Enter lumens and watts from your chosen fixture label.
- Adjust CU and MF if your surfaces are dark or dusty.
- Press Calculate to view results above the form.
- Use the download buttons to save a CSV or PDF report.
Illuminance targets for garden tasks
Task lighting is measured in lux on the work surface. For seed starting benches, 1200–2000 lux helps you spot thin stems, check moisture, and read labels quickly. Potting and transplanting usually perform well around 600–900 lux, while pruning, grafting, and tool sharpening benefit from 900–1500 lux to reveal cuts and edges.
Why CU and MF change the answer
The utilization factor (CU) describes how much emitted light reaches the task area. White walls and reflective benches may reach 0.65–0.80, while dark sheds or open greenhouse aisles can drop toward 0.35–0.55. The maintenance factor (MF) accounts for dirt, aging, and lens yellowing; 0.75–0.85 is practical for indoor garden zones that are cleaned periodically.
Selecting fixtures with usable lumens
Compare fixtures by rated lumens, not wattage. A 2500–3500 lumen fixture often suits small benches when mounted close to the task, while larger potting tables may need multiple units. If you prefer fewer fixtures, increase lumens per fixture, but keep glare controlled with diffusers and appropriate mounting height.
Spacing guidance and coverage balance
Even coverage matters more than a single bright hotspot. A simple spacing estimate uses the square root of area divided by fixture count. For example, 6.0 m2 with 3 fixtures suggests about 1.41 m between units. When benches are narrow, align fixtures along the long side and keep light centered above the work zone.
Energy and cost planning for routines
Running cost depends on total watts, hours, and your energy rate. Four 25 W fixtures used 2 hours per day, 5 days per week consume about 4.35 kWh per month. At a rate of 0.20 per kWh, that is roughly 0.87 monthly. Track seasonal use, and adjust hours when daylight increases. If the load shares a circuit with pumps or heaters, keep total below 80% of breaker rating and spread fixtures across outlets. LEDs at 100–150 lm/W usually cut energy by half compared with older lamps. For accurate budgeting, use your bill’s average rate including taxes and seasonal surcharges too.
FAQs
1) What is the difference between lux and lumens?
Lumens describe light output from a fixture. Lux describes light arriving on the work surface. Lux depends on distance, coverage, and efficiency factors, so the same lumens can produce different lux in different spaces.
2) What CU value should I start with?
Start with 0.60 for light-colored indoor areas, 0.50 for mixed surfaces, and 0.40 for dark sheds or open aisles. If your achieved lux looks low in practice, raise fixtures or add reflectors, not just more power.
3) How do I choose a maintenance factor?
Use 0.80 if you clean fixtures and surfaces occasionally. Use 0.70 for dusty potting areas or humid greenhouses where lenses fog. Higher MF gives fewer fixtures, so be conservative if conditions are harsh.
4) Why does the calculator round fixtures up?
Lighting plans must meet or exceed the target level across the area. Rounding up ensures coverage after losses and avoids a plan that looks good on paper but feels dim once installed.
5) Can I use this for grow lights?
This tool estimates human task lighting on a surface. Plant lighting uses different metrics and spectra. You can still plan safe working visibility around grow racks, but do not treat the result as a plant growth recommendation.
6) What mounting height works best?
Mount high enough to spread light evenly and reduce glare. For benches, 0.6–1.2 m above the task is common, depending on beam angle. If shadows appear, widen spacing slightly or add a second light row.