Methodology
This page explains how Plant Light Planner produces ranges — sources, uncertainty, and known limits. To put the method to work, open a lighting tool below.
Lighting tools
Each tool applies the assumptions on this page to a specific job. Pick the closest match, or see all tools.
- Electricity cost — Estimate daily and monthly electricity use and cost for grow lights.
- Daily schedule — Estimate how many hours to run your light for the plants you grow.
- Hang height — Compare manufacturer or meter readings at several mounting heights.
- Seedlings & herbs — Plan grow-light intensity and duration for seedlings and indoor herbs.
- Window light — See whether a window spot is likely bright enough, and optionally when to add a grow light.
- Shelf coverage — Plan fixture quantity, spacing, and coverage for multi-shelf setups.
Simple explanation
Plant Light Planner estimates daily light and related schedules for indoor plants. Window tools turn a room description into a broad range. Meter or manufacturer readings narrow that range. Plant categories use published daily-light ranges, not species-level prescriptions.
Why results use ranges
Rooms, seasons, glass, furniture, and fixture placement all change light at the leaves. A single midpoint would imply false precision. Ranges keep the recommendation honest.
Estimate mode versus measured mode
- Estimate mode uses window direction, distance, shade, and season for a useful starting range.
- Measured mode uses a leaf-height reading (or a chart value at that height) for a tighter plan.
Plain-language glossary
- Window direction — which way the glass faces (not where to place a sun icon indoors).
- Brightness at the leaves — how strong the light is where the plant sits; meters and charts often label this PPFD.
- Daily light total — brightness × hours; often discussed as DLI in technical references.
When estimates may be inaccurate
- Heavy films, deep overhangs, or nearby buildings not captured in the form
- Readings taken at the fixture instead of at leaf height
- Fixtures dimmed, aged, or angled differently than assumed
- Plant-health limits that light alone cannot fix
Technical reference
Formulas convert intensity and hours into daily totals, then compare them to category ranges. Tool-specific assumptions are listed with each calculator. Exact sources and review dates appear below.
Sources and review dates
Actual wall-draw wattage for electricity estimates
Publisher: U.S. Department of Energy / user-supplied fixture rating
Reviewed: 2026-07-26
Source: https://www.energy.gov/energysaver/estimating-appliance-and-home-electronic-energy-use
Use measured wall watts when available; nameplate values can differ from draw.
Utility electricity rate
Publisher: User-supplied local rate (EIA residential averages for context)
Reviewed: 2026-07-26
Source: https://www.eia.gov/electricity/monthly/epm_table_grapher.php?t=epmt_5_6_a
Monthly and annual cost scale linearly with the entered $/kWh rate.
Daily Light Integral from PPFD and photoperiod
Publisher: University of New Hampshire Extension
Reviewed: 2026-07-26
Source: https://extension.unh.edu/resource/supplemental-lighting-run-time-worksheet
Uses DLI = PPFD × hours × 0.0036 (µmol/m²/s × h → mol/m²/day).
Indoor plant light category DLI planning ranges
Publisher: University of New Hampshire Extension (synthesized planning bands)
Reviewed: 2026-07-26
Source: https://extension.unh.edu/blog/2021/02/optimizing-plant-growth-indoor-lighting-qa
Category bands are planning ranges for consumer tools, not species-specific optima.
User-estimated natural-light DLI contribution
Publisher: Plant Light Planner (user input)
Reviewed: 2026-07-26
Source: https://extension.unh.edu/resource/supplemental-lighting-run-time-worksheet
Natural contribution is user-supplied or coarsely estimated; meter readings are preferred.
Manufacturer or meter PPFD map preferred over inverse-square
Publisher: Horticultural lighting practice / UNH Extension
Reviewed: 2026-07-26
Source: https://extension.unh.edu/resource/growing-seedlings-under-lights-fact-sheet
Bar and panel fixtures are not point sources; use published maps or meter readings.
Transplant / seedling DLI example guidance
Publisher: University of New Hampshire Extension
Reviewed: 2026-07-26
Source: https://extension.unh.edu/resource/growing-seedlings-under-lights-fact-sheet
Example guidance near ~12 mol/m²/day for transplants is treated as a 10–14 range.
Indoor window orientation DLI estimate bands
Publisher: Plant Light Planner planning heuristic
Reviewed: 2026-07-26
Source: https://extension.unh.edu/blog/2021/02/optimizing-plant-growth-indoor-lighting-qa
Orientation/season/distance/obstruction bands are intentionally wide. Measured PPFD mode narrows the range.
Shelf fixture count and spacing heuristic
Publisher: Plant Light Planner geometry heuristic
Reviewed: 2026-07-26
Source: https://extension.unh.edu/resource/growing-seedlings-under-lights-fact-sheet
Fixtures-per-shelf from bar length vs shelf width; verify with a PPFD map.
Bar-light edge falloff on deep shelves
Publisher: Plant Light Planner coverage heuristic
Reviewed: 2026-07-26
Source: https://extension.unh.edu/resource/growing-seedlings-under-lights-fact-sheet
Deep shelves relative to fixture length often show edge falloff.
Limitations and corrections
These tools support planning decisions. They do not guarantee plant performance.
Last methodology review: 2026-07-23.
Ready to plan?
Use a lighting tool to turn these ranges into a placement or schedule.