Quick answer: PPF measures the total photosynthetically active light a grow light emits each second, in µmol/s. PPFD measures how much reaches a given area of plants, in µmol/m²/s, so use it to judge canopy coverage. University of Missouri Extension explains this output-versus-delivery distinction.
This guide explains the numbers on a grow-light listing, how to turn a steady canopy reading into daily exposure, and how to estimate electricity use. The worked examples use assumed inputs; they are not crop recommendations. For general lighting tools, start at the LED lighting calculator hub.
What do PAR, PPF, and PPFD tell you?
PAR identifies the relevant wavelengths; PPF describes output; PPFD describes light delivered over an area. PAR covers 400–700 nm, according to the PAR reference. Keep these terms separate when comparing a fixture's headline rating with its coverage map.
| Term | Meaning and unit | What it helps you decide |
|---|---|---|
| PAR | Photosynthetically active radiation, the 400–700 nm band (PAR reference) | Which wavelength band is being counted |
| PPF | Total PAR output each second, in µmol/s (Missouri Extension) | How much plant-useful light the fixture emits |
| PPFD | PAR delivered over a surface, in µmol/m²/s (Iowa State Extension) | How much light reaches the growing area |
A PPF figure alone does not describe canopy coverage: the output measurement and the area-based measurement answer different questions. Missouri Extension distinguishes fixture output from the light reaching plants at a specified distance. Ask for the PPFD map alongside PPF so that the proposed growing area is part of the comparison.
How should you check coverage and mounting height?
Use a PPFD map at the intended height or measure at canopy level with a quantum, also called PAR, sensor. These sensors measure PPFD, as explained by Virginia Tech Extension. A central reading answers only the question about that location; include the edges in your assessment.
According to Missouri Extension, PPFD tends to increase as the light moves closer to plants. That is a reason to compare maps at matching heights, rather than selecting whichever listing displays the largest number. Judge the mounting height by delivered light over the intended area, rather than a universal distance rule.
Use this buying checklist:
- Look for a spectrum graph and PPF or PPFD data. Missouri Extension advises avoiding lights without them.
- Match the map's growing area and mounting height to your proposed arrangement.
- Compare readings across the canopy, including the perimeter, before deciding that the coverage suits your plants.
- Choose the target for the crop. According to Missouri Extension, lettuce needs 250–350 µmol/m²/s, and going above 350 µmol/m²/s wastes energy without a significant yield increase (that page's separate worked example uses about 510 µmol/m²/s for a full indoor system).
That lettuce range is a crop-specific reference, not a target to apply to every indoor plant.
Can lumens or watts replace PPF?
Neither provides the plant-light measurement you need. Lumens, lux, and foot-candles describe light for human vision, while watts describe power; Virginia Tech Extension recommends against using these units to measure light levels for plants.
For general illumination, the luminous efficacy calculator divides lumens by watts and reports lm/W. It is the site's closest efficacy comparison tool, but its input is lumens: entering PPF there would give a result with the wrong meaning. Use photon efficacy for a grow light's output-per-power comparison.
According to Missouri Extension, grow-light efficacy is expressed in µmol/J. Keep the units attached to every comparison; a lumen-per-watt result and a micromole-per-joule result are different measurements.
Worked photon-efficacy example
Assumed inputs: a fixture emits 600 µmol/s PPF and draws 240 W. Since a watt is a joule per second:
- Photon efficacy = PPF ÷ input power.
- Efficacy = 600 µmol/s ÷ 240 J/s.
- Efficacy = 600 ÷ 240 = 2.5 µmol/J.
This calculation describes the assumed fixture's output per unit of electrical energy. It does not calculate canopy PPFD; that still requires the area-based information described by Missouri Extension.
How do you calculate daily light integral?
For constant PPFD, multiply it by the daily lighting hours and 0.0036. According to the daily light integral reference, DLI counts PAR photons accumulated per square meter per day, in mol/m²/day:
DLI = PPFD × hours per day × 0.0036
The conversion factor is arithmetic: 3,600 seconds/hour ÷ 1,000,000 µmol/mol = 0.0036. It converts a per-second micromole reading into a daily mole total.
Worked exposure example
Assumed inputs: constant canopy PPFD of 200 µmol/m²/s, lights on for 12 hours/day, and no additional light contribution.
- Daily illuminated seconds = 12 × 3,600 = 43,200 seconds.
- Daily photons per area = 200 × 43,200 = 8,640,000 µmol/m².
- DLI = 8,640,000 ÷ 1,000,000 = 8.64 mol/m²/day.
- Using the compact formula: 200 × 12 = 2,400; 2,400 × 0.0036 = 8.64 mol/m²/day.
To solve for duration, rearrange the formula: hours = target DLI ÷ (PPFD × 0.0036). With an assumed target of 8.64 mol/m²/day and the same assumed PPFD, hourly delivery = 200 × 0.0036 = 0.72 mol/m²/hour; duration = 8.64 ÷ 0.72 = 12 hours.
These are exposure calculations, not a claim that this schedule suits every crop. The constant-input shortcut also does not represent a changing daylight contribution unless that contribution is accounted for; DLI is the accumulated daily total, as defined by the DLI reference.
What will the lighting schedule cost?
Multiply input power by operating time to find electricity use, then multiply by your electricity rate. The electricity cost calculator uses this same arithmetic, so you can change the wattage, quantity, hours, and rate to match your setup.
Assumed inputs: 1 fixture, 240 W, 12 hours/day, 30 days/month, and $0.15/kWh. This is an assumed rate, not a quoted local tariff.
- Total power = wattage × quantity = 240 × 1 = 240 W.
- Daily energy = total watts × hours ÷ 1,000 = 240 × 12 ÷ 1,000 = 2,880 ÷ 1,000 = 2.88 kWh.
- Monthly energy = daily energy × days = 2.88 × 30 = 86.4 kWh.
- Monthly cost = monthly energy × rate = 86.4 × $0.15 = $12.96.
The estimate covers the assumed lighting load only. Enter any additional equipment separately if you want a budget for the whole growing setup.
FAQ
Does moving a grow light change its PPF?
PPF describes the fixture's output independently of distance; PPFD describes light reaching plants at a given distance. According to Missouri Extension, moving the light closer tends to increase PPFD.
Is higher PPF always better?
Higher PPF means more total PAR output, but the delivered area-based measurement is PPFD, as Missouri Extension explains. Compare the canopy map with your crop's needs before choosing by the headline output alone.
Can I measure PPFD with a lux meter?
Use a quantum sensor for a direct PPFD reading. Virginia Tech Extension identifies quantum sensors as PPFD instruments and recommends against lux as the plant-light measurement.
What PPFD should I use for lettuce?
According to Missouri Extension, 250–350 µmol/m²/s is its lettuce reference range. That source says exceeding 350 µmol/m²/s wastes energy without a significant yield increase; its separate worked example for a full indoor system uses about 510 µmol/m²/s, so treat the range as a general guide.
How many hours should a grow light run?
For steady light, calculate hours = target DLI ÷ (PPFD × 0.0036), rearranging the formula in the DLI reference. Choose a crop-specific target before using the calculation; the worked schedule above is an assumption, not a universal recommendation.
Jack Shi
Founder & editor, LEDaskJack Shi builds and writes LEDask, an independent LED-lighting tools project operated by clooms. He designs the calculators, checks their formulas and reference values against published engineering data, and writes the guides across the site.
