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How To Calculate for LED Wattage Per Square Foot of Grow Space?


Calculate for LED Wattage Per Square Foot of Grow SpaceAs you probably know by now, there are various types of grow lights available in the market today. One of the most widely used is the LEDs. If you are still new to growing with this kind of light, then you have come to the right place.

In this article, we will discuss everything you need to know about LEDs. From the basic facts up to the calculation of how much wattage needed per square foot, we got it all covered here.

Factors to consider when choosing for the right LED grow light?

For first time buyers, here are some points you have to consider when choosing the right LED grow lights.

  1. Type of plant and its stages.

Of course, it is pretty much essential to consider the type of cultivar you intend to grow with. Plants thrive on varying levels of light intensity. Some like the peppers and tomatoes are favorable to use at high light conditions. Meanwhile, others like herbs and green leafy vegetables are best grown in low light.

Moreover, you also have to take into account the current stage of the plant’s development. In every phase, the amount of light required also differs. Thus, when buying LED bulbs you need to foresee the need to adjust from time to time as the plants grow.

  1. Type of LED grow light.

As mentioned earlier, the light requirements differ from one stage to another.

If you intend to grow plants from seedling up to its flowering stage, then a so-called full-cycle LED bulbs would be more appropriate for you to choose. It is convenient to use and it can also spare you some savings from buying different types of bulbs.

Our top picks for full-cycle LED bulbs:

Bloomspect 600W LED Grow Light

King Plus 1000W LED Grow Light Double Chip Full-Spectrum

Spider Farm SF-1000 LED Grow Light

There are also other models that offer settings that are targeted to cater to specific stages. The single-channel LED bulb provides a simple on and off feature. Meanwhile, the two-channel models feature separate modes. They come with customizable program settings for the vegetative and the flowering stages.

Our top picks for single-channel LED bulbs:

Higrow 36W LED Plant Grow Light Bulb 460nm Blue

Byingo LED Grow Light Bulb 50W 450-460nmAll-Deep Blue Spectrum

Higrow 36W All Deep Red 660nm LED Grow Light Bulb

G8LED 90W Red Flower Booster LED Grow Light

Our top picks for two-channel LED bulbs:

Growing Lamps LED Grow Light 200W

ZWD Upgraded 300W LED Grow Light with Two-Channels

Vivosun 1200W Cree Cob LED Grow Light

  1. Size of grow space.

Another major consideration when choosing an LED bulb is the dimensions of the growing area. Generally, an average LED grow light would draw about 32 watts to cover a square foot of grow space.

The bigger wattage in LED lighting doesn’t equate to bigger coverage. As opposed to what it usually represents in other types of light systems. These are metal halide (MH), high-pressure sodium (HPS), and high-intensity discharge (HID) lamps.

Furthermore, it is good to emphasize that wattage alone should not be the main determinant of area coverage. Nor does it dictate the level of effectiveness of LED lights. Rather, it’s used to provide an estimation of the correct fixture size in a given grow space.

The table below serves as your quick guide:

Size of Grow SpaceIdeal WattageApproximate Watt RangeOur Top Picks
1 x 1 or 1ft232 watts 30 to 40 wattsAzawa Grow Lights 30W
2 x 2 or 4ft2128 watts 120 to 140 wattsDerlights 120W LED Grow Light Bulb
3 x 3 or 9ft2288 watts 250 to 300 wattsViparspectra UL Certified 300W LED Grow Light
4 x 4 or 16ft2512 watts500 to 650 wattsParfactworks 500W LED Grow Light
5 x 5 or 25ft2800 watts700 to 900 wattsApollo Horticulture GL140X5LED 700W LED
6 x 6 or 36ft21,152 watts1,000 to 1,200 wattsSunraise 1000W LED Grow Light Full-Spectrum

Wattage is commonly used to represent the approximation of area coverage. But, Photosynthetic Photon Flux Density (PPFD) and the Daily Light Integral (DLI) are what counts as more accurate metrics. They measure the exact light intensity requirements over the size of the grow area. These will be discussed next.

Different terminologies to understand.

Before proceeding into the actual calculation of LED wattage, we must first discuss the different terms and metrics involved in LED lighting for horticultural purposes.

Photosynthetic Active Radiation (PAR)

Photosynthetic Active Radiation, or PAR, refers to the wavelength of light within the visible spectrum range of 400 to 700 nanometers. PAR itself is not a metric. Instead, it’s used to determine which type of light gets required to help photosynthesis in plants.

Photosynthetic Photon Flux (PPF)

Photosynthetic Photon Flux, or PPF, is the measurement of PAR light emitted by the light source per second. The PPF measurement is often expressed in micromoles per second or mol/s.

Take note though that PPF only pertains to the amount radiated by the light system. Thus, not the actual amount of light that will land on a specific surface area.

Photosynthetic Photon Flux Density (PPFD)

Photosynthetic Photon Flux Density, or PPFD, is the measurement of the amount of PAR light that actually reaches a specific surface area. It is often expressed in micromoles per square meter per second or mol/m2/s.

Technically, as PPFD increases, the rate of photosynthesis also escalates in effect. As a grower, you have to aim to achieve a suitable level of PPFD. This is to intensify the photosynthesis rate. Thereby, greater chances of getting more yield in the end.

Daily Light Integral (DLI)

Daily Light Integral, or DLI, refers to the cumulative measurement of the number of photons that reach the plant during the photoperiod. You can think of it as somewhat like the plant’s daily dose of light. DLI is expressed as moles per square meter per day or mol/m2/d.

It is important to identify your cultivar’s DLI so that you can program your lighting system. It helps deliver the number of lights based on its specific day-to-day requirements.

How to calculate LED wattage correctly?

Now, onto the very highlight of our article. The steps on how to calculate the correct LED wattage for your grow space.

Step 1: Determine the grow space size in meter squares.

The precise measurement of the grow space is fundamental in figuring out the correct wattage. To do this, you can measure the width of the area in meters, then multiply it by the depth in meters too.

Using a 4×4 grow space size as an example,

For reference, 1 ft = 0.30 m

Width = 4 ft or equal to 1.2 m

Depth = 4 ft or equal to 1.2 m

Width X Depth = Area (m2)

1.2 X 1.2 = 1.44 m2

Thus, a 4×4 grow room is equal to a 1.44 m2 area.

Step 2: Establish your desired PAR light intensity.

PAR represents the areas of the visible light spectrum. It’s where the plant utilizes for its photosynthesis. These blue, green, and red wavelengths range from 400 to 700 nanometers. To simplify, the higher the PAR output of your LEDs, the more light gets absorbed by your cultivars.

A PPF requires at least 1,100mol/m2/s or more to cover a 4×4 grow space or an area with 16 square footage. Besides, the most appropriate amount of PAR to use still depends upon the current phase of development of the cultivar.

Below are the optimal PAR outputs at different stages of plant growth:

StagesPAR Output
Seeding Stage200 to 400 mol/m2/s
Vegetative Stage400 to 600 mol/m2/s
Flowering Stage600 to 1,000 mol/m2/s

PAR outputs can actually reach as much as 1,500 mol/m2/s if given the right atmospheric conditions. The most optimum level that can help an increase in growth rate and abundant yield is at 750 mol/m2/s.

Step 3: Determine the PPFD per watt of your LED light system.

PPFD measures the number of photosynthetic active photons that actually falls on the 1 square meter of the target area per second. So, the higher the PPFD, the greater would be the rate of photosynthesis of the plants in effect.

The table below presents the associated efficiencies or PPFD per watt. They are on most LED grow light systems available in the market today.

Type of LED BulbPPFD per Watt
Budget Red/Blue0.9 to 1.2 PPFD per watt
High Power Red/Blue1.3 to 1.5 PPFD per watt
Full-Spectrum LED1.2 to 1.5 PPFD per watt

Step #4: Compute the wattage.

Now, you already have the area in meter square, your desired PAR level, and the PPFD per watt of your LED light system. You may finally be able to compute for the wattage.

So, going back to our example in Step #1,

Given:

Area = 1.44 m2

Desired PAR level = 750 mol/m2/s

PPFD per watt = 1.4 PPFD per watt (assuming you are using a Full-Spectrum LED)

To get the PPFD:

Area X desired PAR level = PPFD

1.44  X 750 = 1,080 PPFD

To get the wattage:

PPFD / PPFD per watt = Wattage

1,080 / 1.4 = 771 Watts

So, you would likely be needing about 771 watts to cover a 4×4 or a 16ft2 grow room size. You also have to take into consideration other elements. For instance, your target plant density and other lighting conditions. They may somehow affect your wattage requirements in actual.

What are the factors affecting the LED wattage?

Now, you already know how much wattage would likely need to cover your grow space. Let us move on to discussing the major considering factors to look into when determining the LED grow light wattage.

  1. The number of plants you intend to grow.

For every size of grow space, the number of suitable cultivars and its corresponding growth technique varies. Moreover, this also alters the number of lights you would need for your area.

The table below shows the techniques used. It comes along with the approximate quantities of plants it can cater to per square meter.

Grow TechniqueNumber of Plants
Low-Stress Training (LST)4 plants per square meter
Sea of Green (SOG)4 to 16 plants per square meter
Screen of Green (ScrOG)1 plant per square meter
Pruning1 plant per square meter

Generally, it is ideal to divide 1 square foot of area per plant. Most LED grow lights though can cover between 1 to 6 plants per bulb. If you aim to grow more, then try installing various evenly-spaced lights. This is to encompass all sides of the canopy.

  1. The number of LED units per plant.

LED lights come with specific wattages. For instance, a 600-watt LED would draw out only about 240 to 260 watt of electricity. This can achieve the same power as a 600-watt HID or HPS lights.

Read Also: Best LED Grow Lights in 2020: Top 10 Reviews & Buyer’s Guide

On that note, LEDs are always known to be far more efficient than other types. Its actual electricity consumption is estimated to be only about 40% than its rated power.

In terms of plant quantity, the table below shows how LED power increases. This is in relevance to various plant densities.

No. of Plants1 square foot/plant2 square feet/plant4 square feet/plant
130 to 40 watts60 to 80 watts120 to 140 watts
260 to 80 watts120 to 140 watts240 to 300 watts
4120 to 140 watts240 to 300 watts500 to 560 watts
6180 to 200 watts360 to 400 watts700 to 860 watts
8240 to 300 watts500 to 560 watts900 to 1,100 watts
10500 to 340 watts600 to 800 watts1,100 to 1,400 watts
12340 to 380 watts700 to 900 watts1,300 to 1,700 watts

So, how many plants can you grow then?

As mentioned above, it is best to divide 32 watts per square footage.

In this example, let us say you have 5 plants in a 6 square feet space.

5 X 6 = total footage is at 30 square feet

30 square feet X 32 watts = 960 watts LED grow lights

Here are some more samples for different plant quantities and area coverage:

CoverageActual LED WattsUnits of LED Grow Lights
2×2128 watts
  1. 120-140 watts
2×3192 watts
  1. 180-240 watts
2×4256 watts
  1. 240-280 watts OR (2) 120-140 watts
3×3288 watts
  1. 270-310 watts OR (2) 140 watts
4×4512 watts
  1. 420-480 watts OR (2) 240 watts
4×6768 watts
  1. 720-1,000 watts OR (6) 120-140 watts
5×5800 watts
  1. 750-1,000 watts OR (4) 180-240 watts

3. The height difference between LED light and the plant canopy.

The distance of the LED from the plant canopy has a direct impact on its light intensity. You have to always maintain the right and safe height to achieve the potential maximum yield.

Placing the bulbs too close to the cultivars runs the risk of burning, bleaching, and possible nutrient deficiencies. Meanwhile, hanging it too high up also reduces its efficiency to deliver the right amount of light to the plants.

The details below show the recommended LED grow light heights over the canopy.

LED WattageHeight/Distance
240-400 watts16 to 30 inches
450-550 watts20 to 30 inches
600-850 watts24 to 26 inches
900 watts and above26 to 42 inches

Final Thoughts

In conclusion, an LED lighting system draws up an average of 32 watts per square footage. There are a lot of varying factors that also come into play. They include the density of plants, the quantity of LED units, and even the height difference between the light over the canopy. Hence, it is very important to take everything into account. This will help you to arrive at the most favorable amount of light power for your cultivar’s development.

Indeed, determining the right wattage of LED grow lights can be quite overwhelming. Especially, for beginner growers. Nonetheless, once you understand how an LED lighting system works in relation to your plant’s growth, the process then becomes easier over time. Give yourself a little bit more practice and patience to learn through it all.

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