Greenhouse Supplemental Light Runtime Calculator

A bright reading at midday does not show how much light a crop receives over the whole day. Short days, cloud cover and greenhouse shading can leave a daily shortage even when the growing space looks bright. Supplemental lighting supplies the missing light, so the useful starting point is the daily total at the crop rather than the brightness of the room or the wattage printed on a lamp.

The daily light integral (DLI) counts photosynthetic photons reaching a square metre over a day, expressed in mol/m²/day. It combines light intensity with exposure time. A crop target describes the daily amount being sought, not a lamp setting. Targets depend on the crop, its growth stage and production goals, so one value should not be treated as a recommendation for every planting.

Fixture specifications and canopy measurements describe different parts of the light supply.

Photosynthetic photon flux (PPF)

The total photosynthetic photon output of one fixture, measured in µmol/s. It does not say how much light reaches each square metre of plants.

Photosynthetic photon flux density (PPFD)

The photon arrival rate per square metre at crop height, measured in µmol/m²/s. This intensity, maintained for a known time, determines the lamp contribution to DLI.

For sunlight, the relevant transmission is the fraction that reaches the crop after the covering, structure and shade have taken their share. For lamps, some emitted photons may miss the growing area or be intercepted before reaching it. These are separate losses. A glazing specification alone does not establish natural light at the plants, and dividing a fixture's total output by floor area does not establish uniform lighting.

More lamp intensity can supply a given daily photon total in fewer hours. However, equal daily totals delivered over different durations are not a guarantee of equal crop response. The photoperiod, meaning the daily light period between dark periods, also matters. Lamps may supplement weak daylight without extending the day, whereas lighting after sunset can change that period. A suitable dark interval must come from crop-specific guidance rather than from the photon total alone.

Electrical power and plant light are also different quantities. Watts describe electrical draw, while lumens and lux describe light weighted for human vision. None can be substituted directly for PPF or PPFD. A useful runtime estimate therefore needs both a photon measurement or specification and a separate electrical draw, followed by checks across the actual crop canopy.

How to Use This Tool:

Use light values for the same growing zone and seasonal conditions. The initial target of 18 mol/m²/day and 16-hour lamp window are editable examples, not crop recommendations.

1. Enter Crop target DLI, Outdoor daily light and Greenhouse transmission. Use a crop- and growth-stage-specific target and measured or mapped outdoor DLI.
   For directly measured natural DLI inside the greenhouse, enter that value in Outdoor daily light and set Greenhouse transmission to 100%. The reading must exclude lamp light. For a room without sunlight, enter 0 for outdoor light.
2. Set the Allowed daily lamp window in hours or minutes. This is the maximum lamp-on time within your crop's permitted light period, including any overlap with daylight. It is not automatically the time between sunset and sunrise.
3. Choose Lighting data to match the information you have. With Fixture photon output (PPF), enter the growing area and photon output per fixture. In Advanced, adjust Photon capture for losses and leave Existing fixture count at 0 to size the minimum count, or enter a positive count to test that existing array. The default 100% capture is an ideal upper bound.

   With Measured canopy intensity (PPFD), enter the lamp-only reading averaged across the crop zone at canopy height, plus the number of installed fixtures. Measure without sunlight so that natural light is not counted twice.

4. Enter Electrical draw per fixture. Add an Electricity price and Billing period in Advanced to estimate operating cost. Leave the price at 0 for an unpriced plan.
   A blank or invalid Electricity price makes costs unavailable without removing the lighting or energy results. Enter a number from 0 to 100 currency units/kWh to clear the pricing warning.
5. Check the updated runtime and Unmet daily light in the Lighting plan. A full lamp window does not necessarily meet the target. When Check lighting inputs appears, correct the named missing or out-of-range value before relying on a result.

The lighting arithmetic runs in your browser. Shared page addresses can carry the entered planning settings, including the electricity price.

Interpreting Results:

The headline shows the Scheduled lamp runtime, which never exceeds the allowed window. Compare it with Uncapped runtime needed and Unmet daily light. When the window is too short, fixture mode gives the minimum total fixture count for that window. Measured mode instead gives PPFD needed within window; it does not calculate a replacement fixture count.

Total daily light achieved is a calculated total, not a sensor reading or a crop-performance prediction. Verify lamp-only PPFD with a photosynthetically active radiation (PAR) meter across the growing zone, and check natural DLI for the conditions being planned. An acceptable average can still hide underlit parts of the canopy.

When sunlight already meets or exceeds the target, supplemental runtime and operating energy are zero. Excess sunlight remains in the achieved total; it is not removed or turned into a shading recommendation. An explicitly entered fixture count can still show a nonzero Connected load even when the lamps are scheduled off.

A zero tariff produces zero cost figures but means the plan is unpriced. It does not mean electricity use is zero. Period totals assume the same daylight, lamp output and runtime every day, rather than forecasting the weather over the billing period.

Technical Details:

A daily photon budget adds natural light at the crop to the light supplied by lamps. The lamp contribution assumes a constant average PPFD while the fixtures are on. The resulting duration is then limited by the allowed window, and any undelivered daily light remains a shortfall.

Daily photon budget

Sunlight and supplemental demand

Let D_t be the crop target and D_o the outdoor DLI, both in mol/m²/day. The transmission fraction τ is the entered percentage divided by 100. Natural canopy DLI is D_s, and the supplemental deficit is G.

The deficit cannot be negative. A transmission of 60% uses a fraction of 0.60, not 60 and not the fraction of light blocked.

Canopy intensity and whole-fixture sizing

For fixture estimates, A is the growing area in m², F is PPF per fixture in µmol/s, c is Photon capture divided by 100, and N is the selected fixture count. Their captured output per area gives average lamp PPFD, P. With an allowed window H in hours/day, the deficit determines the minimum PPFD and minimum whole-fixture count.

The ceiling brackets mean round upward to a whole fixture, subject to the numerical equality rule below. Any positive deficit requires at least one fixture when sizing automatically; zero deficit gives a minimum of zero. A positive Existing fixture count replaces the automatic count rather than being added to it. Runtime is recalculated from that selected count, even when it is below the minimum.

In measured mode, P is the entered Lamp-only canopy intensity directly. Area, fixture PPF and photon capture are not applied again. The installed count is used for electrical load, not to multiply the measured PPFD. The entered count affects electrical load and energy, but does not change the light budget or runtime. A changed physical arrangement needs a new canopy measurement.

Required time, scheduled time and delivered light

One hour contains 3,600 seconds and one mole contains 1,000,000 micromoles. The factor 0.0036 converts constant PPFD multiplied by daily operating hours into DLI. For a positive deficit, required runtime h_r is the time needed without a cap, while scheduled runtime h_s is the shorter of that requirement and H.

Here D_l is light added by lamps, D_total is total daily light achieved, and U is unmet daily light, all in mol/m²/day. When G is zero, required and scheduled runtime are explicitly zero, including an automatically sized plan with no fixtures.

The uncapped requirement can exceed 24 hours and is then physically impossible to supply in one day at that intensity. The scheduled duration remains within H. These hours are a daily total, not clock-time start and stop instructions; no automatic daylight sensing, dimming or dark-period scheduling is calculated.

Lighting energy and tariff cost

Let w be electrical draw per fixture in watts, B the billing period in days and r the price per kWh in the user's tariff currency. Connected load L is in kW. Daily energy E_d and period energy E_b use the scheduled time, not the uncapped requirement.

C_d and C_b are daily and period costs. Every counted fixture is assumed to draw the same entered power for the same scheduled duration. Costs use one flat tariff without currency conversion, taxes, demand charges, cooling or installation costs.

Electrical draw does not determine photon output in these equations. Entering zero watts leaves the photon results unchanged and produces zero lighting energy; it does not verify that a real lighting system consumes no power. When a target is fully met, adding identical fixtures at unchanged capture can shorten runtime without reducing daily energy, because the larger array draws proportionally more power while on.

Unit conversion and rounding

Square feet are converted to square metres by multiplying by 0.09290304. Minutes are divided by 60 to obtain hours, and kilowatts are multiplied by 1,000 to obtain watts. Changing an input unit converts the existing number, retaining 14 significant digits in that converted entry. Calculations otherwise use values before display rounding.

For whole-fixture sizing, let R be the unrounded fixture ratio and k its nearest whole number. A ratio is treated as exactly k only when it satisfies this machine-roundoff test, with ε equal to 2⁻⁵², approximately 2.22 × 10⁻¹⁶.

Outside that tolerance, the ratio rounds upward. The positive-deficit minimum of one still applies. For example, without sunlight, a 10 m² area, 1,000 µmol/s fixtures, 100% capture and a 10-hour window need exactly two fixtures for 7.2 mol/m²/day. A target of 7.200001 requires three. This equality tolerance is not a lighting-design safety margin.

The headline runtime shows two decimal places in hours, and other numeric readings show up to two decimal places. Thus 15.79 h is about 15 hours 47 minutes, not 15 hours 79 minutes. Very small positive values may display as zero, and displayed rounded values are not used to recalculate energy.

Result boundaries

The short-window warning uses the unrounded unmet DLI. Its tiny tolerance prevents numerical roundoff from being presented as a practical light shortage; it is not a crop-tolerance threshold.

Accepted input ranges

These inclusive limits define accepted entries, not recommended growing conditions. Fixture counts and billing days must be whole numbers. Only the numeric fields relevant to the chosen lighting-data mode affect that mode's calculation.

The lamp-window range is 6 to 1,440 minutes when minutes are selected; calculated runtime may be zero or shorter than the minimum allowed window entry. A permitted 24-hour entry is not a recommendation for continuous lighting. The average-light calculation does not design fixture spacing, predict lamp aging or changing shade patterns, establish crop yield, or assess electrical installation requirements.

Worked Examples:

References:

• Supplemental Lighting Run Time Worksheet, University of New Hampshire Extension.
• Calculating and Using Daily Light Integral (DLI): An Introductory Guide, Virginia Cooperative Extension, SPES-720NP.
• Growing Seedlings Under Lights, University of New Hampshire Extension.