Grow Light Guide

Why Plants Need Light

Plants like all biological creatures need to generate energy to continue survive and grow. To generate energy plants use sunlight, carbon dioxide, and water to make energy via the chlorophyll molecule. The chemical reaction of the three generates, known as photosynthesis, oxygen(expelled by the leaves), and glucose that can be used by the plant for energy.

Light is a requirement for plants to make energy. The more light available the more potential for energy which means more growth potential assuming there is enough of the other needed components. There is of course a limit to the amount of light a plant can handle and too much at once will damage and kill the plant.

There is of course not a better light source than giant fusion reactor in the sky we call the sun. It emits the right amount of light, the right kinds of light, probably most importantly it is free. However there are benefits to growing with lights the number one being it is inside. That allows the grower to leverage the benefits of much more controlled environment and the the major reduction or complete elimination of pests. So how do you go about picking the best lighting for indoor grows?

When selecting lighting for plants it is important to understand the following things:


Lumens are the unit of measurement for amount of light emitted from a light source. The higher the lumens the more light from the light source. As an example the sun can deliver around 100,000 lux or 100,000 lumens/sq meter at midday. This is a lot of light!


Watts are the amount of energy consumed by a grow light. Energy charges at least in the US are measured in kilowatt hours(kwh) which is measuring the use electricity by using the unit of 1000 watts*hr. So for a 75 watt light it consumes .075 kilowatts of power. If you wanted to calculate the amount of energy consumption and cost of running that light you would also need the amount of time you would be using the light and the price of a kilowatt hour where you are at. Right now the US average is 12 cents/kwh. If you ran the light for 14 hours/day 365 days a year it will cost you approximately 365*14*.12*.075 = 45.99 USD. You can use this general formula to calculate energy cost if it isn’t explicitly stated. Hours * $/kilowatt hours * kilowatts = Total Energy costs.


Lux is lumens/sqm or the amount of light over a unit of area. Not normally seen on the packaging but is important to understand when talking about how big of area a light can support at a specific light intensity. Ex. 1000 lux = 1000 lumens/(square meter). I have seen lights that will tell you effective ranges for lights and at a specific distance. So from 3ft this grow light can cover a 1 square foot area as an example.

If you can find a lux rating at specific distances for a light that would helpful in deciding how much light cover you would have. You probably will not see this measurement. Do look for suggested hang heights and if they have a suggest effective area. Really the closer the better with grow lights provided they are not burning the plants. A grow light that is too close will damage or kill your plants so be wary of that.

Energy Cost

The cost of operating the grow light over a specific time. Normally there is an assumed amount of time used in the calculation on the package. See above if this is not on the package you can use the formula in the watts section to calculate it yourself

Lifespan and L70 Rating

The amount of time a light can be used for before it burns out or loses brightness. L70 rating is for LED lights, it is a measurement of how long an LED light will keep at least 70 percent of its output. Since LEDs just lose brightness over time this is important to note. Normally the lifespan of the light is based on this number. So for example the lifespan of 25000 hours means that the light will emit at least 70% of its normal brightness or more for 25000 hours of use. At the 25000 hour mark the light reaches that 70% brightness and decreases from there as time goes on.

Wave Length and Color Temperature

Light wavelength refers to the the distance between two consecutive crest or trough points on a light wave. They are generally measured in nanometers. You will not see these on your normal everyday lights, but generally on grow lights. Plants absorb light in the visible spectrum which is between 390 and 700 nanometers. Most grow lights will tell you which wavelengths they are providing by explicit mentioning them or giving you the color temperature rating.

Make sure that you have red and blue, and/or white emitting lights. Color temperature is what is used to relay the wavelengths of a white light whereas I have seen red and blue grow lights have their actual wavelength given. I would stick to the daylight bulbs which are between 4600k and 6500k for general purpose growing. Other visible light wavelengths are not as well absorbed by plants and thus don’t provide as good of light source for energy production. Blue is generally considered best for vegetative growth, and red for fruiting and flowering. The thing to remember with wavelengths is the lower the number, ex blue 450nm vs red 700 nm, the higher the energy of the light photons.

Grow Light Selection

Now that you know what the specifications of grow lights are and mean, it is time to go through some of your options. I will go through the 3 kinds I grow lights I have seen in hydroponic stores and across the internet the most.

LED Grow Lights

LED grow lights are my favorite choice. These lights work via a semiconductor that emit photons by electrons exiting electron holes. I like them for their efficiency and their lifespan. They normally are well designed for growing and have a good lumen level. For example an LED grow light I use has the following specs. 4019lx lumen output, 75 watt energy requirement although if you look at specs and read comments you find that it is really around 27 to 30 watts of power, a 50000 hour lifespan, and it has white, blue, and red lights for all important wave lengths. The suggested hang height is 12-18 inches.

One thing to note that for this light it mentioned 4019lx as the lumen output. That is important to note since lx is the unit for lux. This means this light will deliver 4019lumens/(square meter). So for you folks on the US customary system roughly a 3.3ft by 3.3ft area. LED lights are very efficient which means they don’t generate a lot of waste heat. One of the lights I use below.(affiliate link)

Fluorescent Grow Lights

Fluorescent lights work by heating mercury vapor in a glass tube to emit light. For Fluorescent grow lights it is best to select T5 bulbs. They are the most efficient of the fluorescent kind of bulbs. T8’s are also pretty good, they are not as efficient. T8’s are cheaper so would be better if you are on a tighter budget.

As mentioned above, I would stick with between 4600K-6500K for color temperature. Those are the daylight color temperature bulbs. That means you will get all of the proper wavelengths needed for plant growth. I have also see actual grow light fluorescent tubes. Fluorescent lights aren’t as light efficient as LED lights but T5 grow lights are still efficient light bulbs. For power output check out how much each bulb consumes so you can know energy requirements. I consider these lights a good middle ground between price and efficiency. I have placed an picture below that is also an affiliate link to a fluorescent grow light below so you can it out.

HID Grow Lights

HID grow lights or high intensity discharge lights are another type of grow light you can use. These lights heat up a gas in the light tube and are fairly large. There are two kinds HPS(high pressure sodium) and MH(metal halide). The buyers guide from is great at explaining these lights. I will summarize what they have to say on the topic for you. HPS lights are better for the flowering growth and MH are better for the vegetative growth. That means HPS lights are more red in light spectrum and MH lights are more blue in light spectrum.

There also two types of ballasts for these lights, digital and magnetic. Digital ballasts are capable of changing light output percentages. The article does mention that bulb and ballast wattage need to match and digital ballasts require digital bulbs and magnetic require magnetic. So keep that in mind if you decide to purchase on these lights. These lights require a reflector to get more light used and the wattage on them are pretty high. According to the buyers guide a 400 watt system covers a 3.5ft x 3.5ft area. That is roughly the same area of the LED light above but it uses 400 watts instead of 30 watts. That means it is using 13.3x the power for the same area. However a 400 watt hps light generates 50000 lumens. That is quite a bit of light!

HID lights generate a lot of heat! That is something to consider because you will need to the heat that they give off in your growing environment. A final note on HID lights is that they are by far the biggest. So if you have a cramped space these or not the lights for you. I have placed an picture below that is also an affiliate link to an HPS grow light below so you can it out.

Final Considerations for Choosing Grow Lights

As you can see you have a great deal of options to work with. When you are going to purchase a grow light. You should consider all of the variables described above and price. When you do your research weigh out what your needs are and how much money you have available. If you are just starting out I would suggest maybe starting with a fluorescent light system. You will get good efficiency and light quality at a lower price.

If you need heat for your indoor environment and have the space for an HID light, they might be your best option. If you want the best light my opinion is choose and LED light. They are the most efficient and I like they don’t generate a lot of heat. The LED lights I have used are lightweight and are easy to use in a vertical grow situation because you can easily hang them from the bottom of an upper shelf.

Regency Lighting
Maximum Yield
Westinghouse Lighting
Botany for Gardeners(affiliate link)
How Stuff Works

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