You have set up your growing area, picked your plants, and now it is time to set up your lights.  To get the best growing condition, you need to have the correct number of lights.  Figuring out just what that number is can be difficult.  There are several different factors to consider and inaccurate information to filter out.  

How many grow lights you need and how to set them up?  To calculate the correct number of lights, you must first determine your lighting needs based on the plants being grown and the growing area.  The placement and distance from the plant will vary based on the plant and the type of grow light.  

Grow lights output is calculated in watts. The correct number of watts needed will vary from set-up to set-up and from plant-to-plant.  Before you can determine what amount of wattage your set up needs, you will need specific information about your plants to determine your grow light needs.

How to Calculate How Many Grow Lights You Need

The number of grow lights needed is determined by a number of factors.  It is important to plan out your growing area and plants before trying to decide how many lights you will need.  To calculate the number of grow lights need you have to have three pieces of information:

  • Light requirements of the plants you are growing
  • Size of the growing area
  • Type of lighting you are using

Light Requirements of Your Plants

Plants have different light requirements, depending on how and what the plant grows.  This will determine how much light it will need from your set-up. 

  • Plants that flower or bear fruit, such as tomatoes, fruit trees, flowers and the like, typically have higher light requirements, usually around 40 watts per square foot. 
  • Plants that do not flower or fruit, such as herbs and lettuce, can do well with lower lighting. The prefer 25-30 watts per square foot.
  • If you are growing more than one plant type, you may need to set up your lights accordingly.  The high-performance plants can be placed together in one area with more watts per square feet, with the other plants having fewer lights.  

Having a pathway between the two sets can help make sure each type of plant is getting the correct light amount.

Each plant can only absorb a certain amount of light each day.  Because of this, it is important to know your plants’ lighting needs.  Plants have adapted to certain growing conditions for centuries.  Too little or too much light will decrease the yield. 

If you are unsure of your plant’s light requirements, it is important to check before installing lights. Check with the local garden center or consult a resource like a growing guide or website. 

Size of Your Growing Area

The growing area is calculated in square feet, which is the length times the width measured in feet.  

A common mistake many people make is measuring the whole roomand not just the area that the plants are in.  The size of your growing area is only the size that will have plants.  If you are putting 3 feet by 3 feet of plants in a larger room, your growing area is all you need to calculate.  Not the size of the whole room.   

It is also important to look at the height of the area to ensure you have enough space above the plants for hanging the lights when the time comes.

Calculate Your Wattage Needs

Once you know the wattage needs of your plants and the square feet of your growing area, it is a simple calculation to determine your overall wattage needs.  For example, with growing tomatoes, you will need 40 watts per square foot.

  • Length x Width of the growing area: 4 x 4 = 16 sq. ft.
  • Watts x Square Feet = Desired Wattage 40 (for tomatoes) x 16 = 640 watts

Type of Lighting

Grow lights come in a variety of types.  It can be difficult to decide which type of grow light to use.  Fluorescent lights can be the easiest to use if you have only one or two plants.  LED or HID can be used for more plants.  LED lights are often the easiest, cheapest to operate, and last the longest.  LED lights also generate less heat.

The three most common types are:

  • Fluorescent
  • High Intensity Discharge (HID) lights
  • LED (Light-Emitting Diode)

Fluorescent lightsare a popular choice for many home growers. They are often best used for germination and vegetative growth because they lack the needed spectrum for flowering. They produce less heat and are budget-friendly.  Some newer types can be all-purpose.  Fluorescent lights come in both compact (CFL) or in tubes.

  • Fluorescent Grow Tube Lights (T5) operate at low wattage, come in different colors:
    • White is used for flowering
    • Full-spectrum daylight is intended for seedlings and cuttings
    • Green grow tube lights are those which the plants cannot see.  This allows you to see, but not give the plants light if it is during their dark times.
  • Compact fluorescent lights (CFL)also come at low wattage and temperatures.  They are a good choice for where space is limited, like offices and residences.  The light bulb can be placed as close as 6 inches to the plant without burning it. They are also available in different color temperatures to maximize plant growth.

High-Intensity Discharge(HID) lights are good choices, but expensive to purchase and operate. These lights are extremely efficient ad create high levels of light.  Metal-halide (MH) and High-Pressure Sodium (HPS) are the two most common forms.  

  • MH lightscreate a blue light that assists in vegetative growth, but less flowering.  
  • Metal-halide grows lightshave a life expectancy of 6,000 to 20,000 hours.  They are especially good for leafy vegetables, such as lettuce, spinach, and herbs.  They are available in a variety of wattage.
  • HPS lights have a red to orange light that is perfect for producing flowers and fruit, but the plants will be more spiny and weaker.  They produce a high heat output.  It is important not to hang them too close the plants to prevent them from being burned.

MH and HPS lights often work the best when used together to promote leafy grow with the MH lights and then exchanging them for HPS to encourage the flowering of the plant.  

LED (light-emitting diode) lightsare one of the newer grow lights.  They have been becoming more popular due the low electricity needs, low heat output, and their color variety.  LED’s can be programmed to match 5700K color temperature of sunlight.  

  • LED lights can produce both blue and red light at the same time eliminating the need for two lighting sources.  
  • They can be more expensive than some other lights, but as the technologies keep advancing prices are falling. 
  • They do have some problems with consistency of output, inaccurate manufacture labeling, and the need to be careful in determining the current number.  However, they can be a good option.

Most indoor grow lights are used in a fixture with a reflector.  The reflector is used to aim the light down onto the plants.  The size and shape of the reflector will have an impact on both the footprint of the light and how many lights are needed.  

Calculate Lights Based on Square Footage

If you are using high-pressure sodium or metal-halide lights, once you know your growing space, the simplest way to figure you lighting needs is based on the manufacturer’s recommendations.  

  • 150 watts: 2 x 2 feet (4 square feet)
  • 250 watts: 2.5 x 2.5 feet (6.25 square feet)
  • 400 watts: 3 x 3 feet (9 square feet)
  • 600 watts: 4 x 4 feet (16 square feet)
  • 1000 watts: 5 x 5 feet (25 square feet)

This will help you figure out which size light will work for your growing area.  The most efficient (meaning you get the most watt per electricity used) light bulbs are 600 watts.  Second best are 1000-watt bulbs.  When your grow space can accommodate one of these size light bulbs,  they are the best choice.

For example,if you had a growing area of 200 square feet, you could divide that by 4 (the 150-watt light bulb) and get 50 light bulbs. Or you could divide the 200 by 25 (the 1000-watt light bulb) and only have to purchase 8 light bulbs.  

Even though each 1000-watt light bulb will cost more, the overall total purchase price and the electric usage will be less. Saving you money.

Calculate Lights Based on Wattage

LED, high-pressure sodium (HID) lights and metal-halide lights can all have the number of lights determined by using watt per square foot method.  For high-pressure sodium and mental-halide, the results will not always match the square footage calculations.

In the example above if you were growing tomatoes, you would need 40 watts per square feet.  If you round that off to 50 per square feet, that would be 10,000 watts to cover the area (200 (square feet) x 50 (watts) = 10,000.  This would mean ten 1000-watt light bulbs instead of the eight we calculated based on just growing space alone.  

Looking at the 25-30 watts that herbs and lettuce need, you can see the result would be less than eight light bulbs, (200 (square feet) x 30 (watts) = 6,000 (only six 1,000 light bulbs needed.)  

The growing area method is a quick and simple method, but if you have plants that really need a higher wattage, you will not have enough light, especially when they are producing.  If you are using low demand plants, you may be oversupplying light. As long as this doesn’t damage the plants, it is fine, other than you are spending more for the same yield.

Problems with Calculations with LED lights

Wattage per square footage is the most common way most people use to determine what size and number of LED light(s) they need. It is the simplest way, but not always the most accurate way.  

But it is a good compromise for a difficult problem.  The problem is that due to variation in lights and inaccurate manufacture specs, it is difficult to figure out how many lights are needed any other way.

There are three methods of conquering this problem:

  1. Using the Coverage Area Provided
    1. Most packages list the coverage area for the lights
    1. Doing simple math, you can use that information to determine the number of lights needed.
    1. However, many manufacturers exaggerate the area.
    1. The coverage often given only applies to plants needing low levels of lights.
    1. This information is not trustworthy.  It may be simple but should be tested for accuracy.
  2. Using the Wattage Method
    1. Take the plants wattage needs times your square feet and divide by the LED’s wattage. 
    1. This method works best for plants needing 40 or higher watts
    1. Two problems
      1. The label may give the theoretical and the actual wattage.  Make sure and use the actual wattage, not the theoretical.
      1. The numbers (especially from the Chinese companies) are often made up, but the correct number can be found usually if you search through the pages enough.
      1. The output may vary from bulb to bulb even with the same output.
  3. Using the Output Method
    1. This is the most difficult but most accurate method.
    1. This information may be hard to find.
    1. You want the output, not just dead center, but at the edges of the light.
    1. During vegetative state, you want 300 to 600 umol/m²/s.
    1. 600 to 1000 umol/m²/s  are needed during bloom/fruiting .

How Far from Plants Should the Grow Lights Be?

It is important to ensure that grow lights are hung at the correct level.  If they are too low the plants can be burned by the lights.  If they are too high, the plants will be thin and weak. The correct height varies depending on the light being used.  Some have to be hung far away and others very close.

The key is to find the place where the plants get enough power to grow, but they are still safe from being damaged by the lights.  

To ensure your plants are getting the right amount of light, you need to know the type of lighting you are using: LED, HID, or fluorescent.  Each has a different heat output, color spectrum, and needs to be hung at a different distance.

For example,LED lights typically need hung higher to prevent bleaching.  However, Fluorescent light usually needs to be hung lower than HID’s because the power coming from the fluorescent bulbs is weaker.

HID Lights

HID (High-Intensity Discharge) lights generate a large amount of heat.  They need to be placed higher to prevent damaging the plants.  HID lights need to be kept at least 12 inches above the plants. The higher the energy output (wattage), the higher the lights need to be placed.  

  • 400 w HID Grow Light 12-19 inches away from the plant
  • 600 w HID Grow Light 14-25 inches away from the plant
  • 1000 w HID Grow Light 12-31 inches away from the plant

It is important to make sure your growing area is high enough when using these lights.

LED Lights 

LEDs do not generate as much heat, but their light spectrum can lead to bleaching when hung too close.  Light bleaching will interfere with your plants’ ability to take in light and convert it to energy for growth.  So, LED lights will need to be hung even higher than HID lights.

LED lights vary in the distance based on shape, size, and wattage.  It is important to purchase LED grow light from trusted brands so that you can check the manufacture’s recommendations for height placement and know it is trustworthy. 

A starting point for LED lights is:

  • 240-400-watt LED Lights 16-30 inches away from the plants.
  • 450-550-watt LED Lights 20-30 inches away from the plants.
  • 600-800-watt LED Lights 28-42 inches away from the plants.
  • 900-1000-watt LED Lights 36-46 inches away from the plants.

It is important to watch the plants carefully when using LED lights.  Some trial and error will occur.  You may have to raise or lower them based on plant conditions.

Fluorescent Grow Lights

Fluorescent lights have a balance of energy and spectrum output.  They can provide enough energy for about any plant, but typically have to be hung close to the plants.  As with LED lights, hanging fluorescent lights can be tricky as they are not all the same.

Compact fluorescent light (CFL) bulbs tend to generate more heat over an extended period of time than longer tube fluorescent (T5) lights due to their compact size.  It will be important to experiment and watch your plants closely to get the right height.

  • Start off about 5 inches above the plants.
  • Use the hand test or thermometer to check the temperature
  • If the temperatures are too high raise the lights by one inch.
  • Re-check and move again as needed
  • Remember fluorescent does not provide enough output to go higher than 12 inches

Grow lights may need to be adjusted during a plant’s growing cycle.  For example, flowering and fruiting plants may need the lights dropped slightly during flowering or when producing fruit.

A lux meter can be used once you have your lights to test the overall brightness levels.  This can be another guide to use when determining the height of the grow lights. As you adjust them, the meter can test how much light/brightness is hitting your planting surface.  

The lux meter will not measure heat, so that will still also need to be accounted for.

Stage of GrowthLux Reading
Vegetative15,000 Lux to 70,000 Lux
Flowering35,000 Lux to 35,000 Lux
Clones and Seedlings5,000 Lux to 7,000 Lux

One simple test that can test for heat is the hand test.  Place your hand under the grow light at the same height as the plants.  If you cannot leave your hand for 30 seconds or more without it getting too hot, the light source is too close to the plants and will burn them.  

The hand test can be helpful to double-check the recommended heights to prevent overheating and burning.

How far apart to set up grow lights 

As with height, the distance between grow lights depends on several factors, the type of light, the wattage, and the height above the plants.  The reflectors also will impact how and where lights are hung, as they focus and intensify the light shining on the planting areas.

When looking at how far apart lights need to be hung, it is important to look at the light’s footprint.  This is the amount of space that the light can be expected to shine enough light of enough intensity to create plant growth.

Lights need to be hung in an arrangement so that their footprints have minimal overlap.  The reason for the decreased overlap is two-fold.

  1. The area of overlap may get too much light and result in the plants receiving too much light and/or heat.
  2. The overlap will increase electricity costs with little to no increase in plant growth.

Reflectors will impact hanging height and footprint size. This makes it important to choose the right reflector for your planting needs.  Growing tents made of reflective material will help bounce back the light into the growing area.  

Footprint information for High Powered Sodium (HID) lights and LED lights, two of the most commonly used grow lights.

1000 DE HPS5×5
1000 HPS4×4
600 HPS3×3
400 HPS2×4
250 HPS2×2
200 LED2×2
400 LED3×4
600 LED4×4

Fluorescent light bulbs are not included in the comparison as they typically are only used for small growing areas due to how close they must hang to the plants.  They are often also used only for germination and seedlings.  It would not be cost-effective to use fluorescent grow lights over a large area.

The lux meter can be used after lights are hung to look for “hot spots” or areas where the lights are more intense. Moving the lights or adjusting the reflector can decrease these spots.  

Careful observation of the plants can show places where lights are hung too close or too far apart.  If the lights are too far apart, the plants on the borders will be thinner, weaker, and not produce as much fruit/flowers.  

If the lights are too close together, the plants may become “burned,” show bleaching or the soil may dry out much faster.

To ensure the best yield and plant health, the lighting arrangement will be based on your specific growing needs. Setting up grow lights will vary based on several factors, including: 

  • Plants
  • growing space
  • type of lights
  • reflectors used

 It is important to consider all factors when setting up your growing space and growing lights. 

Length of Light Time

The amount of time the grow light is left on can vary with the plant and  the lifecycle of the plant.  Some plants require only a few hours of light.  Others do well even when exposed to light 24 hours a day.  

One thing to remember is that artificial light does not fuel the plant as well as the sun so it will need more time under grow lights than it would sunlight.

Some plants would do well if the light was left on 24 hours a day.  While they will do okay in 24-hour light.  It is still best to give them at least some darkness.  Some examples of these plants are:

  • Spinach 
  • Lettuce 
  • Orchids 
  • Cacti 
  • Coleus 
  • Dieffenbachia

Some plants need at least eight hours of darkness in a 24-hour period to thrive.  A timer that turns the light on and off at sunrise and sunset can help create a natural light-dark cycle that the plant would have if growing outside.

Especially dark loving plants need at least 12 hours of darkness per day.  Again, a timer can help make sure the light is being turned on and off as needed to create this.  Plants that need increased darkness are:

  • Poinsettias 
  • Kalanchoe 
  • Spider Plants 
  • Dracaena 
  • Tomatoes 
  • Creeping 
  • Fig Peppers 
  • Maidenhair Fern 
  • Parlor Palm

It is often thought that seeds need light 24 hours to germinate.  However, this is not true.  Just like a regular plant, seeds and seedlings can benefit from a regular light-dark cycle that includes at least 8 hours of darkness.  

During periods of darkness, it is important to make sure no other lights bleed through into your growing area.  This means you should not enter it or use overheard lights in the area.  Green lights can be used during these periods if you need to enter the growing area. Green lights allow you to see but are not seen by the plants, so they are not disturbed.

In Conclusion

While there are many factors to consider, once you’ve taken into account your specific situation including what plants you are growing, how large your growing space is and what kind of lights you are using, you can be sure you are using the right number of lights in the right way – and your plants will show it!