How Many Watts Per Plant? A Useful Guide for Indoor Gardening

The needs for all plants can be boiled down to these three things: soil, water, and light. The greatest challenge to meet these needs for indoor plants is giving them sufficient light. Outdoor plants not only have sunlight to rely on, they are also following the suns timetable year round which can be difficult to replicate indoors.


Insufficient lighting leads to dingy plants with poor growth. The solution for this issue is the use of grow lights for indoor plants. This leads to the next logical question for those who would like to garden indoors, how many watts per plant should I use?

What Does Wattage Depend On?

What Does Wattage Depend On

The first things to consider is the type of plant you want to grow and the environment that's it's used to. Certain species require higher wattages than others to mimic its natural habitat. The next thing you can think about is how many plants you are going to grow, their heights, the size of the space the will take up, and the stage that they are in on their growth cycle.

The larger the area the plants are in the more wattage will be required as a general rule. These guidelines break down all plants into two overarching categories: plants that require a lot of light which are considered “high lights” and plants that require less light which are considered “low lights.”

Examples of high light plants are tomatoes, peppers, and cannabis. Low light plants include leafy greens, such as lettuce, and herbal plants.

Calculating Wattage

Calculating Wattage

Once you've determined whether your plants are high lights or low lights, start measuring their growing area that they occupy in square feet rather than the square feet of the room they are in. High light plants require 40 watts per square foot and low light plants require 25-30 watts per square foot.

Here is an example of how to determine wattage for lettuce, a low light plant, growing in an area measured at 5ft x 5ft.

Total Area in Square Feet

Length x Width = Square Feet

5 x 5 = 25 sq ft

Watts x Square Feet = Desired Wattage

25-30 watts x 25 sq ft = 625-750 Desired Wattage

Whatever number you calculate, round it to the available wattage for bulbs. For example, there is no such thing as a 38.45 watt bulb but there is a 40 watt bulb.

For a general overview of calculating wattage, watch this summary from Steve's Hydroponics:

Alternatives to Calculating Wattage

Another way to calculate wattage is to use the simple rule of 100 watts per plant. If you're comfortable with conversions, calculate with lumens and then convert to watts based on the plant's state in their life cycle.

If your plant is in the vegetative state of their cycle it will require 2500 lumens per square foot and a plant in its flowering state will require 10,000 lumens per square foot. Here's a link to a website who can do the conversion for you.

If you're wondering how many plants you can grow for each light you use you can watch this helpful YouTube video from No Fail Hydroponics here:

Potential Issues

You may have done your calculations correctly to the dot and you see that's rather than thriving, your plants are in fact wilting and slowly dying. What is the issue? Your lights that provide a necessary part of your plants growth cycle may also be killing them.

However, what is killing them is not too much or too little wattage. In fact, it is the heat that they produce. To solve this issue try to distance your plants from their light source.

Conserving Your Light

Indoor Plants

Now that you've determined the wattage you need for your plants and ensured that your plants are not being baked by the light source being too close, consider conservation. To maximize the light reaching the plants, invest in reflectors.

As the name implies, they reflect back light to the plants that would have been lost without them. This creates an even more efficient system.


When determining watts per plant determine the type of plant you have first. Then calculate the plants area and use any of these general equations given to get an accurate answer to exactly how many watts you should use.

Try to prevent potential heat issues from your lights by distancing them from your plants. Because you want to get the most out of your lights, consider possibly adding reflectors to your setup.

If you have any questions let us know in the comments section. If you run into any other issues read up on the links provided in the article.

About the Author Laura Bennett

Hello, I’m Laura Bennett. I love nature especially when it comes to flowers and different kinds of plants. I started a very small garden behind my house and I named it Humid Garden. So, I created this blog to provide aspiring and inspiring thoughts about gardening for gardeners and anyone who has the intention of keeping a garden.

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