How to calculate the light required for cannabis

Posted on Aug 27 , 2020


Knowing how much marijuana needs to be healthy, thriving and productive seems to be one of the foundations of success.

Light is an important condition for photosynthesis and many metabolic processes in plants. In this case, the ratio of the average consumption and the effective luminous flux required for the normal development of a factory is 3-4 000 lumens per square foot (30x30cm). However, since most growers do not have an illuminance meter, the intensity of the light emitted by the lamp itself does not provide enough information (light efficiency is more important), so it is easier to focus on the wattage of some lamps.

Lighting options for hemp

There is no single lighting capacity, it is individual. The grower determines the plant's demand for light and provides the required light. The calculation should take into account the stage of the cannabis life cycle and the growth conditions and characteristics:

  • The size of the bush or the area occupied by several cultivated bushes.

  • The type of lamp used.

  • The light is located above the top of the plant, and it represents the distance from the top to the light.

  • Based on the expected harvest volume of the selected lamp, the yield of a specific variety (considering the selected lighting power), the number of bushes (for the planned harvest volume).

  • The relationship between light power and cannabis lamp type.

Modern growers can use three main types of artificial light sources:

Cannabis light

  • HPS lamp (sodium gas discharge).

  • ESL (Energy Saving).

  • LED (diode).

They are suitable for growing alone and in groups, each type has its own characteristics, advantages, and disadvantages. All of these should be taken into consideration.

HPS lights may be attributed to the leader in popularity. Moreover, these are powerful light sources characterized by a broad spectrum of rays. But especially HPS is rich in warm light (even "hot" light) red rays in all aspects. Cannabis badly needs the red spectrum to enhance root development and maturation. At the same time, the obvious disadvantage of this kind of lamp is the intense heating of air and leaves. Therefore, it is impossible to use a box device with a continuously operating system of efficient operation of continuous air supply and exhaust for cooling, which is an additional (sometimes considerable) cost. On average, an HPS 150-250 W lamp is sufficient to meet the needs of adequate lighting for a medium-sized bushing. For a planting box with an area of 1 m2, a lamp of 400-600 W is required, and a power of 1000 W is sufficient for the box area of 1.2-1.3 m2.

ECL lamps or fluorescent lamps are smaller and more energy efficient. At the same time, the emitted radiation is rich in the "cold" blue and green "spring" spectrum, which is particularly necessary for plants to have a set of active green qualities during the growing season. Such a light source is not very hot, but it does not generate too much light energy. Therefore, you will have to choose a more powerful option or combine 2-3 or more options together. For a medium-sized bush, 250-400 W lights will be needed, but how to illuminate a 1 m2 growth box will get at least two ECL lights, each of which is 400 W, and preferably 4x250 W (out For uniformity).

LED lights do not heat at all and can be placed very close to plants. They are very economical, durable, and very compact, but are severely limited in terms of the emission spectrum and luminous efficiency. It is best to choose special Phyto-LED lamps with extended-spectrum and brightness, but they are much more expensive. For 1 bushing, a 115-180 W lamp may be sufficient, and a 360-530 W lamp is sufficient.

The relationship between the lighting power and the distance to the lamp

Calculate the light of cannabis

The efficiency of the luminous flux consumed by the device depends to a large extent not only on the type and power of the lamp used but also on the distance between the lamp and the device. The closer the light source, the stronger the luminous flux. However, due to the risk of light burns on the leaves, even temperature-safe LED lights should not be placed too close.

HPS and ESL will preheat (especially HPS), so it is important to place the light source on the top of the lampshade within a safe distance to maintain lighting efficiency:

HPS 600 W should be located 60 cm above the bush (this distance must be considered when designing and equipping planting boxes and selecting varieties based on size).

ESL 400 W is suspended 30-40 cm above the top of the bushing.

It is sufficient to place the 360 W LED lamp just 20 cm above the bushing.

The height of the lamp should be adjusted as the bushing grows to maintain the best distance.

The relationship between optical power and bushing size

When choosing a lamp for a planting box and installing a backlight, the bushing size inherent to one or the other variety must be considered. There are short cannabis (up to 40 cm), short (70-80 cm), medium (up to 1.5 m), and tall (up to 1.5 m, up to 2 m, usually indoors without LST and ScroG methods). The selection and installation of the bulbs must be such that their power and quantity are sufficient to cover the growth area so that all branches receive enough light.

When calculating the amount of light required to grow hemp, you can start with a value of at least 20 watts (30x30 cm) per square foot. The minimum required effective value of the luminous intensity of this box is 2000 lumens. In order to realize this kind of power indicator, HPS lamp 50-70 W, ESL 100-150 W, and LED 45-70 W are suitable. If the size of the box is larger, more light is needed. For large bushes, you can install additional diode lights (they do not heat) on the wall of the box as a backlight.

You can replace something with the lowest bush so that all the tops are about the same height.

Dependence of lighting power on cultivated land

Calculate the light of cannabis

Usually, in order not to stretch the growth box upwards, growers use green screening (ScroG) and low intensity training (LST) methods to stretch shrubs horizontally (especially medium and high varieties). The level of "green" areas that require lighting increases. For effective lighting of at least 1 square meter, you need:

HPS lamps 400-600 W, but better than two 150-250 W lamps, and require effective ventilation to cool the intense heat (HPS lamps up to 400°C and DRI lamps up to 700°C).2 ECL lamps, 250-400 watts each. Since it is heated to 60-70°C, it also needs cooling.2 LED lights 115-180 W or 1530 W.

In order to increase the efficiency of the light output from the light source to the plants, the reflector can be constructed, for example, using foil. In some cases, this will increase the level of lighting without the need to use more powerful, hotter lights.

Dependence on production indicators

Production index

For growers, it will be helpful to know how to calculate the light of the planting box to provide the required amount of mature crop conditions.

The grower's standard is the relative yield per 1 watt of energy to 1 gram of sprout, but for that matter, this is the standard-not always achievable. When determining the optical power required to obtain a specific crop volume, you can base on this approximation (taking into account all previously stated factors):

Therefore, a good, bright 100 W lamp with a wide emission spectrum will not bring much gain (30-40 grams per square meter).

  • 150 W-60-70 grams per square.

  • 200 watts-100 grams.

  • 250 watts-250 grams

  • 400 watts-400 grams

  • 600 watts-600 grams

  • 1000 watts-1000 grams

However, in order to obtain a more realistic index, it is worth dividing the yield by about 1.5 or creating and maintaining ideal conditions (space, temperature, humidity, soil quality, watering, fertilization, lighting patterns, etc.) and choosing the correct variety….


Only consider all the points about lighting (not only consider the amount of light, but also the required spectral ratio, and consider the stage of plant development), as well as all other conditions (microclimate, soil, watering, disease prevention, etc.), which can improve cannabis Productivity and harvest quality.

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