Greenhouse Supplemental Lighting with LEDs: When It Pays Off

Q: Should I size a supplemental lighting system on wattage or on DLI?

On the daily light integral. Michigan State University Extension's decision thresholds are expressed in mol per square metre per day rather than in watts per square metre: adding light is of large value for young plants below 12 mol/m2/day, moderate between 12 and 15, and small above 15. A wattage calculation cannot tell you which side of that line a greenhouse sits on, because it does not include how much sunlight already reaches the crop.

Q: Do LED fixtures replace the heat that HPS lamps supplied?

Not in the same form. High-intensity discharge lamps emit a large amount of radiation that warms the crop and the growing medium directly, which is an advantage in cold climates. In a Michigan study run in late January and early February, MSU found that high-pressure sodium lamps at 680 foot-candles, about 90 micromoles per square metre per second, raised the average plug media temperature by 4.3 degrees Fahrenheit compared with unlit plugs. A LED installation moves less of that radiant heat into the canopy, so the heating and cooling balance of the greenhouse changes when the lighting type changes.

Q: Is supplemental lighting useful in summer?

Usually not in a greenhouse at northern latitudes. Michigan State University Extension notes that supplemental lighting is typically used from October through March on cloudy days and at night, and that it is generally not used on ornamentals in northern latitudes from April through September. The reason is that as days lengthen and the sun climbs, the natural daily light integral rises past the point where added light changes the outcome.

XineLam Technical Desk · Published 18 September 2026

Commercial greenhouse interior with LED supplemental lighting fixtures suspended above benches of young potted plants, daylight coming through the glass roof — Figure 1: Illustration of a greenhouse running supplemental lighting over benches of young plants. The decision to switch the fixtures on is a lighting-quantity question, not a seasonal habit.

The short answer is that greenhouse supplemental lighting pays off when the natural daily light integral is low enough to be the limiting factor. Michigan State University Extension gives decision thresholds for young plants: adding light is of large value below 12 mol/m²/day, moderate between 12 and 15, and small above 15. Most of the confusion around greenhouse lighting comes from skipping that measurement and going straight to a fixture specification, but the daily light integral is what decides whether the electricity buys anything.

Why a greenhouse needs supplementing at all

A greenhouse solves temperature, not light. Glass transmits most of the visible spectrum, but the total quantity of light arriving per day still tracks the season, and in the northern United States and Canada the winter months deliver a fraction of what a summer day does. MSU Extension points out that many young plants, particularly bedding plants and herbaceous perennials, are commercially produced from January through March, with some growers starting in December, and that during this period the days are short and the daily total is low.

That timing is the whole problem. The production window for plugs and cuttings coincides with the lowest natural light of the year, and the crop is therefore grown in precisely the weeks when light, not temperature or nutrition, limits how fast it develops.

The threshold that decides whether it pays

Rather than classifying weather as good or bad, MSU Extension's guidance puts numbers on it. Working from daily light integral maps produced by Jim Faust at Clemson University, and assuming that 50 to 60 percent of sunlight reaches the plants as a typical value, the guidance assigns a value to supplemental lighting for young plants: large when the daily light integral is below 12 mol/m²/day, moderate between 12 and 15, and small above 15.

Those three bands are a decision rule, not a target. A greenhouse sitting at 8 mol/m²/day in January is a clear case for adding light; one sitting at 17 is spending electricity on a factor that is no longer limiting.

Table 1: Whether to add light, by measured greenhouse daily light integral. Thresholds and values are from Michigan State University Extension guidance for young plants; the "practical reading" column is the operational interpretation of those bands.
Greenhouse DLI (mol/m²/day)	Value of adding light (young plants)	Practical reading
Below 12	Large	The clearest case for supplemental lighting; light is the limiting factor and added light changes rooting and transplant timing
12 to 15	Moderate	Marginal; whether it is worth running depends on crop value and the electricity price
Above 15	Small	Light is usually no longer the limiting factor; adding hours or wattage buys little

When in the year it is worth running

The seasonal window follows from the same logic. MSU Extension describes supplemental lighting as typically used from October through March on cloudy days and at night, and notes that it is generally not used on ornamentals grown in greenhouses at northern latitudes from April through September. As the days lengthen and the solar angle rises, the natural daily light integral climbs on its own.

Within the window, timing matters as much as duration. MSU Extension states that to increase the daily light integral both light intensity and the duration of use are important, and that lamps are most efficiently used during periods of low light, meaning cloudy weather and at night. Running fixtures at midday on a bright day adds cost without adding much to the daily total, because the crop was not short of light at that moment.

LED or high-intensity discharge

Folding four-panel LED grow light shown from below with densely packed rows of warm white diodes and interspersed deep-red diodes, a folding aluminium frame and a small control box at one corner, on a white background — Figure 2: Photograph: XineLam. A folding four-panel LED grow light. Fixture format decides how evenly light is distributed over a bench; the choice of light source decides how much electricity that distribution costs.

The comparison has moved since the earliest greenhouse LED guidance. MSU Extension's 2010 guidance for young plants named stationary high-pressure sodium lamps as the most efficient option at the time at converting electricity into photosynthetic light, and noted that they emit a large amount of radiation that can raise media and plant temperature, which is a benefit in cold climates. By 2017 the same institution was publishing a direct evaluation of whether the results obtained from producing high-quality seedlings under LEDs justify the higher capital outlay of an LED installation.

Oklahoma State University Extension's comparison of lamp types sets out the dimensions that separate the two families, and they are not all about efficiency. Discharge lamps remain inexpensive per unit and radiate heat into the crop; LED fixtures cost more to buy per unit but draw less energy and last longer.

Table 2: Dimension comparison of LED and high-intensity discharge light sources. All values are from the Oklahoma State University Extension fact sheet HLA-6450 lamp-type comparison; rated life is defined there as the point at which output falls to about 70 percent of its initial value.
Dimension	LED	HID (incl. high-pressure sodium)
Rated life to 70% of initial output	50,000 hours	24,000 hours
Typical wattage range	12 to 215 W	35 to 2,000 W
Energy consumption	Lowest	Highest
Cost to operate	Lowest	High
Conversion efficiency	Very high	Medium
Relative purchase price per unit	Highest	High
Spectrum	Narrow and broad formats available	Broad

Running a supplemental system efficiently

Measure or estimate the daily light integral first. The decision thresholds are expressed in mol/m²/day, so a light sensor or a DLI model for your location is the starting point, not a fixture catalogue.
Concentrate the hours where they are worth most. MSU Extension identifies periods of low light, cloudy weather and nighttime, as the most efficient times to run lamps, because that is when they add to a total that is otherwise short.
Set the target from the crop, not the hardware. Young plants are the strongest case, because plant density is high and the lighting cost per plant is correspondingly low.
Re-check the decision each season. A threshold that justified running fixtures in January does not automatically hold in April, when the natural daily total has already risen.
Treat heat as part of the calculation. If the previous installation supplied radiant heat to the canopy, replacing it changes the greenhouse's heating balance as well as its electricity bill.

Three ways the calculation goes wrong

Buying fixtures before measuring light. Without a daily light integral figure, there is no way to know whether the greenhouse is at 8 or at 17 mol/m²/day, and those two greenhouses should be making opposite purchasing decisions.

Running all day because the lamps are already installed. The most efficient hours are the low-light ones. Adding light at midday on a bright day raises the energy bill without moving the daily total much.

Comparing fixtures on wattage alone. Two fixtures with the same input power can differ in how much of that power becomes photosynthetic light, and a discharge lamp's radiant heat is a benefit in one climate and a cooling load in another.

Frequently asked questions

How many hours should greenhouse supplemental lights run?

There is no single hour count, because hours are only one of two levers. MSU Extension states that to raise the daily light integral both light intensity and duration matter, and that lamps are most efficiently used during periods of low light, meaning cloudy weather and the night. The practical method is to set a target daily light integral for the crop and then add hours and intensity until the measured or estimated figure is reached.

Should I size a supplemental lighting system on wattage or on DLI?

On the daily light integral. The MSU Extension thresholds are expressed in mol per square metre per day rather than watts per square metre: large value below 12, moderate between 12 and 15, small above 15. A wattage calculation cannot tell you which side of that line a greenhouse is on, because it does not include how much sunlight already reaches the crop.

Do LED fixtures replace the heat that HPS lamps supplied?

Not in the same form. Discharge lamps emit a large amount of radiation that warms the crop and growing medium directly, which is an advantage in cold climates. In a Michigan study run in late January and early February, high-pressure sodium lamps at 680 foot-candles, about 90 µmol·m^-2·s^-1, raised the average plug media temperature by 4.3 degrees Fahrenheit compared with unlit plugs. An LED installation moves less of that radiant heat into the canopy, so the heating and cooling balance changes with the lighting type.

Is supplemental lighting useful in summer?

Usually not in a greenhouse at northern latitudes. MSU Extension notes that supplemental lighting is typically used from October through March on cloudy days and at night, and that it is generally not used on ornamentals at northern latitudes from April through September. As days lengthen and the sun climbs, the natural daily light integral rises past the point where added light changes the outcome.

About the publisher

XineLam manufactures LED grow lights and LED lighting modules, with 17 years of experience in LED lighting and 300+ patents in China and internationally. The fixture shown in this article is an example of the folding multi-panel format used for higher-output installations; this guide is written as a technical explainer and the threshold values above come from the cited extension sources, not from product testing.