How Many Watts Per Square Foot for Led Grow Lights You Need?

So, you’ve got everything set for a rocking indoor garden…

The grow tent is up, you have the right seeds and soil, the air is flowing fine and everything seems good to go.

But you’ve not decided yet on which lights you should buy and hang on the pants. To be more specific, it’s the perfect wattage that you’re still looking for.

Well, let us tell you- it’s one thing to have the plants with a great growing system, and it’s another to have the perfect light source with perfect wattage. And that’s what can either flood you with great yields or mess the whole grow system up.

If you agree with us till now, we promise that at the end of this article, you’ll be an expert on this very concern. Because we’ve researched out the most Exclusive guide ever on selecting the right watts of lights for any grow system.

And when we say it’s ‘Exclusive’, we mean it.

Before heading into the core of this article, we’d like to have a couple of questions answered first. These will help you to decide whether you’re on the right track of finding proper watt demands of your grow space.

Question 1: Is ‘Wattage’ The Right Indicator of Light Power?

I’m sure you’ve heard it many of times that ‘wattage’ isn’t the actual to you should care about the LEDs. Well, that’s a partially true fact. But I will rather mark it as a baseline, from where all the calculations regarding grow light begins.

Now why ‘Wattage’ isn’t the direct indicator of lights that your plants receive? Here is the catch-

Wattage is, however, the measure of electric power that a light uses. But when a LED powers up and emits light, it’s not converting the whole energy into lights only. There is heat as a bi-product, and a good amount of electric energy is drained away for that. On top of that, much of light’s intensity drops down because of the distance between the LEDs and plants.

But, as more energy gives us more light, we can assume ‘wattage’ as an indicator. Higher wattage brings more light for your grow plants.

Question 2: What’s Your Plant Style?

First thing first, how many plants do you want to grow? Do you want to grow a bunch? Or a specific number like 1, 5, 10 or 99?

In typical sense of hydroponics or indoor grow, there are some convenient styles. See in which you fit within-

  1. SOG(Sea of Green): A lot of little plants, planted densely.
  2. SCROG(Screen of Green): Growing plants larger, but in a single sphere.
  3. Bush: Multiple single plants, planted with a gentle distance.

Obviously, this is the answer that will help you pick up the right LED powers from a bunch of options out there.

Wattage Requirement by Plant Per Square Foot(Generalalized)

Before anything else, let me tell you- finding the right wattage for your grow space isn’t a simple task. What I mean is, you have to find the answers of ‘deciding factors’ before you find the wattage.

We will talk about the deciding factors later. For now, let’s learn what’s the ‘listed wattage’ and the ‘actual wattage’.

‘Listed Wattage’ Vs. ‘Actual Wattage’

LED lights in the market have a certain wattage printed on their specs chart. Say, it’s three watts for a particular model. But if you run them at 3W electric power, it will burn out!

So, it’s best to run them on a 60% power of what’s listed. We’re calling it the ‘Actual Wattage’ for instance. And from this point, everything we’ll talk about is the actual wattage, not the listed one.

That being said, let’s go for a generalize or ideal wattage chart for different density of plants-

Total Plants Required Light Watts(Actual)
1 sq ft. for each plant 2 sq ft. for each plant 4 sq ft. for each plant
1 30-40 watts 60-80 watts 120-140 watts
2 60-80 watts 120-140 watts 240-300 watts
4 120-140 watts 240-300 watts 500-650 watts
6 180-200 watts 360-400 watts 700-860 watts
8 240-300 watts 500-650 watts 900-1100 watts
10 300-340 watts 600-800 watts 1100-1400 watts

Having a look at this chart, you might be thinking of a common formula, a ‘Rule of Thumb’ that helps you to calculate the wattage in any given case.

Yes, there is a ‘Rule of Thumb’ and here it goes-

“30W for Each Square Foot”- The Rule of Thumb

From thousands of cases with hundreds of parameters, growers had agreed that a minimum wattage of 30W/per square foot is what you need to grow well.

Example: You have 4 plants of 4 square feet space for each. So you need a total footage of 16 sq. ft. From the rule of thumb, you need a minimum of (16 sq. ft. X 30) 380 watts light energy.

Let’s keep calculating in the same way and expand the list a bit-

Coverage Area LED Units
2 by 2 feet One 120-140 watts Unit
2 by 3 feet One 180-240 watts Unit
2 by 4 feet One 240-280 watts Unit Or Two 120-140 watts Units
4 by 4 feet One 420-480 watts Unit Or Two 180-240 watts Units
4 by 6 feet One 720-1000 watts Unit Or Six 120-140 watts Unit Or Four 180-240 watts Units

Again, this is the ideal and most general scenario. Based on many factors like plant types, growing stage, light type, light positions an so on, it will deviate.

Our Recommended LED Grow Lights

LED Grow Lights That We Recommend

The Deciding Factors

Remember, we’ve said that the aforementioned charts are the most ‘ generalized cases’ of growing? Well, at this point, we’ll break down why.

Each of the grow systems has its own parameters that separate its environmental requirements from others. Therefore, when you’re up for finding a light source for your system, you’ve to keep these parameters in mind.

We’re calling it deciding factors and here they are-

Deciding Factor 1: What Type of Light You’re Using?

See, the watts of a light is just the power drawn from the electric board. With the same watts, different plants will give you different kind of yields for the same plants.

As an example, Fluorescent grow lights gives less yield than LEDs and HPSs of the same wattage. With a fluorescent, you’ll get 0.25 grams of buds(in case of weeds) for 1 watt, where it’s 0.5-2.0 grams for LED and HPS lights.

For you to be sure, here are the all possible lighting options for a usual indoor grow-

  1. Fluorescent Grow Lights
    – Compact Fluorescent lights(CFL).
    – T5 Grow Lights
  2. High-Intensity Discharge(HID) Grow Lights
    – Metal Halide(MH)
    – High-Pressure Sodiums(HPS)
    – Ceramic Metal Halide/Light Emitting Ceramic(CMH/LEC)
  3. LED Grow Lights
    (There are too many types of them to list them up at once).

Learn More about Types of Grow Lights and Their Pros/Cons

Surely, each of the light types has their own pros and cons. As an example, using LED grow lights will lower your cost 38% of what a HID grow light will do. But that doesn’t make an HID light useless.

Each of them has their perks that’s beneficial for given growing scenario.

Deciding Factor 2: What Color Your Lights are Of?

Once again, the power(watts, let’s say) isn’t the only thing to care about your grow lights. Color or color spectrum is something equally important as long as the yield is concerned.

Let’s say you’re using a WHITE, a RED and a Blue LED light of same power on three identical grow spaces of same plants. Will the results be the same?

No, they won’t.

In a research, it had been found that plants grown under red LED, are poorly grown with leaf deformities and so on. To get the best result, a decent amount(15-30 µmol/m2/s) of blue light is needed to be added with red lights(660nm wavelength). In some cases though, white LEDs work better than anything else.

Eventually, with a mix of White, Red and Blue lights, “Pinkhouses” is becoming the future or urban growing.

As long as indoor non-commercial grow is concerned, here are the two best options-

  1. White LEDs
    Being a perfect combination of HPS and MH Lights, white LEDs claims to be the closest one to natural sunlight.Best White Led Grow Light: Miracle LED 100W Spectrum Grow Lite
  2. Multi-Colored/Full Spectrum LEDs
    A problem with white LEDs is, they contain too much yellow and green spectrum that are useless for plants. A solution of this problem is the Multi-colored LEDs, which are just the combination of spectrum that your plants need.Best Full Spectrum Led Grow Light:  Advanced Platinum Series P300 300W 12-Band

Deciding Factor 3: The efficiency of the LED grow lights used

Your grow light source emits a certain amount of light power. But how much of it is actually consumed by the plants?

A big question, isn’t it?

Well, that’s where the question of efficiency pops up. Upon a number of factors, the efficiency may rise or fall. Let’s have a look at them-

The Height The of Light

You can put the light at any possible height, and any possible position you want. And guess what, with ever inches of these changes, the efficiency or amount of lights received by the plants will change.

Usually, the calculation works like these-

Seedling Height> Vegetative Height> Flowering Height>Harvesting Height

As we’ve talked earlier, each of these stages has their own wattage requirement. So, with changing the height of the lights, you can be able to serve them with more intense light without buying new ones at all.

Warning: If lights are put too close to the plants, the heat emitted from lights can be a threatening issue for them.

The Light-Plant Alignment

If you’re using a single CFL bulb or number of them as the light source, there are some areas on the floor where the light intensity will be the most. Therefore, plants or parts of plants that are in that zone will get more lights. Light distribution at rest of the areas, will be uneven.

If you can’t help changing the light-plant alignment, it’s best to choose the best ‘Average Wattage’ for the entire space.

Loss of Light

A very usual fact is that you can’t feed every single photon from your light source to the plants. So, there will be losses.

But as a smart grower, your duty is to prevent it as much as possible. Using some of these products can give you a hand-

  1. Light Reflectors.
  2. Light Luminaires(explained later).

Deciding Factor 4: The Light Intensity

We’ve said before that the light that emits from a LED/HID, isn’t entirely fed to the plants. A part of it goes to plant leaves and there is a measuring unit of that. It’s called the Intensity, and it’s measured in µmol/m2/s.

Let’s say, we’re growing lettuce plants and did multiple experiments to observe the yield for various intensity.

Here are what the final results will look like-

Image Source: valoya.com

Note: Learn the from PPFD(µmol/m2/s) to Watts(Wm2) conversion technique.

Cutting the Cost of Lighting

All right, now this is the part where we’ve provide you with some idea that may help you to work with the same wattage and get more yield. In other words, it will cut the cost of growing.

Have a look-

Use Light Reflectors

The first and most effective cost-saving idea any grower will give you is- to use a good reflector. Not using a good reflector is simply sacrificing your money and yield.

There are a handful of variations when it comes to grow light reflectors. We’ve made a generalized list for you-

Types of Reflectors Our Favorite Sizes of Reflectors Our Favorite
Open Hydro Crunch XXL Square Sun System Grow Lights
Air-cooled Giantex 6″ Air Cooled Reflector Parabolic Hydro Crunch Parabolic Reflector
Adjustable Yescom Adjustable Reflector   Rectangular Giantex 6″ Air Cooled Reflector
Double Ended Hydroplane Doule Ended Large Koval XL Grow Light Reflector
Compact Hydrofarm Compact Reflector

Check Out the Full list of Best Grow Light Reflectors

Make The Right Choice of Luminairs

Reflectors are the tools that prevent lights from scattering around. Another tool that does this job equally good is the luminaire. In fact, using them together is one of the finest ideas for any grower.

The right choice of luminaires can help you to gain more efficiency from the same old light source. Typically, there are three sorts of luminaires used-

  1. Tube LED grow light.
  2. Bar LED grow light.
  3. Box LED grow light.

If you’re certain about the exact amount of light your plants need, you can choose one from these three. Usually, the application of these luminaires ranges from tissue culture laboratories(10-100 µmol/m2/s) to high-intensity growth chambers(2000 µmol/m2/s).

We’ve taken a few scenarios in consideration and suggested the type of settings you should use-

  • Tubes: Low in intensity, often used as a retrofit for T8 fluoroscent tubes. Mostly, small grow spaces, tissue culture and vertical farming are the most appropriate uses of these.
  • Bars: The intensity varies a lot in case of bars. They hang high on medium grow spaces, multi-tier cultivations, where you’ve to cover a wide area of plants.
  • Boxes: Boxes are of ultra high-intensity lighting, typically used in large numbers to cover greenhouses.

Frequently asked questions (FAQ)

How many lumens per plant led lights need?

If plants are placed at 1 plant/sq. Ft. rate, each of the plants need 30w of light energy from LED lights. If the density increases to 2 plants/sq. Ft. or 3/plants/sq. Ft., it will increase at same manner.

How many plants can I grow with a 300w led light?

It depends on how densely plants are placed, and how high the LED light is from the plant canopies. If pants are placed at 1 plant/sq. Ft. rate, you can cover up to 8 plants with a 300w LED light.

How many lights for 12 plants do I need?

If you chose to buy a 120w light for a grow space of 1 plant/sq. Ft. rate, you will need 3 lights to cover it up. If you chose to buy 180w lights, you will need two of them to serve 12 plants.

how many 600w lights for 10 plants do I need?

600w taken as the ‘actual’ wattage served by the light, you don’t need more than one 600w light for 10 plants, if plants are planted at 1 plant/sq. Ft. density. Even for some crops, a 600w light source can be more than what they need.

How much does it cost to run a 600 watt light?

If you run the lamp 14 hours a day, the monthly cost of a 600 HID light in America will be $31 per month.

Final Thoughts

Thanks man, you’ve just got done with a 2500-words article, with the solution of your problem of course. Hopefully, figuring out the right wattage for any grow space will be none of a problem for you.

If you’ve any question in mind or any ideas you want to be listed it the article, please leave a comment below. We promise that we’ll get back to it.

Happy growing!

smsaleh

I'm Saleh, a hobbyist DIYer, and a blogger. Whatforme.com is my little place on the web to express what I've learned first-hand, specially about home improvement.

3 Comments
  1. I delight in, lead to I discovered just what I used to be having a look for. You’ve ended my four day lengthy hunt! God Bless you man. Have a nice day. Bye

  2. Very good job sir, i am M.E students of energy analysis and i want to know about requirement various types of lighting intensity watt per sft and i really need ur help on priority ..so i am sharing my no 865204716

    If u know plz help or txt me on my no

    Glad to get ur your help

Leave a reply