A Beginner’s Guide to Calculating Garden Lighting Needs

By Eric Hopper
Published: August 20, 2017 | Last updated: November 1, 2019 02:08:57
Key Takeaways

See how easy it is to calculate the lighting system needs for an indoor garden.

There are several crucial things to consider when you set up your first indoor garden. Ventilation, nutrition, and the growing medium are all crucial, but the most important factor affecting an indoor garden’s overall performance is the lighting system.


The bulk of the energy that fuels plant growth comes from the lighting system. There are many different types of lighting technologies available to growers, including high-intensity discharge and LED lighting systems.

Before you go out and buy your lighting system, there are a few important things to consider first. The amount of light energy needed for the garden space, which type of reflector you want to use, and how to set up multiple reflector cross-patterns are all things you should know before going shopping.


Calculating Light Energy Needs for HID Systems

There are a few different ways to calculate the amount of light energy needed to properly illuminate a particular space. There are fancy devices that can help determine micro-moles and also estimate the amount of PAR (photosynthetically active radiation), but for most new gardeners, this information is not necessary. If you have free access to expensive light measurement devices, by all means use them, but don’t feel like you need to purchase a micro-mole light meter to determine your garden’s best lighting set-up.

Most gardeners can determine their lighting needs by calculating the desired wattage per square foot. Taking into consideration the crop’s particular light intensity needs, you can determine how many watts per square foot will be required.

From there, you can purchase a lighting system that will fit your individual needs. When growing lettuce or other crops that have lower light requirements, use a calculation of 20-30W per square foot of garden space.


For example, a 1,000W lighting system would provide coverage for a 35-50 square foot area. If you want to grow vegetables like tomatoes, you should calculate 40W per square foot of garden space. In this instance, a 1,000W lighting system would cover 25 square feet of garden space.

Keep in mind the calculated area is the actual garden space (green space), not necessarily the physical size of the room itself.


If you want to maximize yields for certain crops, you can push the wattage per square foot ratio up to 60-70W per square foot. However, for most fast-growing annuals, the point of diminishing returns falls between 40-50W per square foot, so pushing the light levels far beyond that is inefficient.

Light Reflectors

Believe it or not, there was a time when most indoor growers didn’t use light reflectors. Luckily, now there are tons of lighting reflector options. Choosing a lighting reflector is an important decision. Reflectors come in small, medium and large sizes, and can be equipped with ventilation ports for air-cooling. Some reflectors are even water-cooled.

You can generally choose a reflector size based on the wattage of your lighting system, so a 400W light works well with a smaller reflector, a 600W light works well with a medium reflector, and a 1,000W light works well with a large reflector.

Yes, there are always exceptions, but for novice growers, this is a good guideline to follow. Choosing between an air-cooled reflector and a non-air-cooled reflector will come down to your ability to control heat.

If you have a small space that accumulates heat rapidly, you should consider an air-cooled reflector. Also, gardens with multiple lighting systems will generally benefit from air-cooling reflectors since the excess heat from all the lights can build up in the growroom if not removed.

Reflector Lighting Cross-Patterns

A reflector lighting cross-pattern is when two or more reflectors’ light footprints overlap each other. To get the most efficient set-up possible, lighting cross-patterns must be taken into consideration. When the light from multiple reflectors is overlapped, the light energy is combined.

When the light on the outskirts of a reflector’s footprint, which normally would not be powerful enough to promote large yields, is combined with light from the outskirts of another reflector, the overlapping portion becomes powerful enough to promote big fruits and flowers.

By strategically placing the reflectors, you can create cross-patterns that will increase productivity and maximize efficiency. That being said, every growroom is different and so are, to some degree, each grower’s intentions.

Some growers will place reflectors close to one another to create higher watts per square foot in a given area. This allows them to further concentrate the light which, in turn, equates to larger fruits or flowers.

Some growers will place reflectors much farther apart, trying to maintain a consistent watt-per-square-foot ratio throughout the garden. This type of set-up creates an energy-efficient garden that will produce an abundance of medium-sized fruits or flowers.

Calculating Light Energy Needs for LEDs

When calculating light energy needs for LED lights, growers must first understand the way LED lights are manufactured and marketed. To extend the longevity of LED lighting fixtures, most manufacturers produce LED lights that do not operate at full-wattage capacity—an LED fixture may contain 5W LED bulbs, but when the unit is operating, the bulbs only operate on 3W each.

An example of this is an LED light system advertised as a 1,000W unit with an actual power draw of 400W. This is confusing because a grower looking to set up a light system may believe the unit can cover the same square footage as a 1,000W HPS or MH.

However, to get comparable results to that of HID lighting, growers using LEDs should use around 40W of actual light output wattage per square foot.

For example, an LED unit with a full-capacity rating of 1,000W but an actual power draw of 400W would be sufficient for a 10 square foot area (40W per square foot = 10 square feet).

Even though LEDs produce higher ratios of PAR, to get comparable growth results to that of HIDs, most plants still require about the same amount of wattage per square foot. The biggest savings with LEDs are lower heat loads and extended longevity.


The lighting system is the heart of any indoor garden and provides plants with the energy needed for photosynthesis. Novice indoor gardeners who grasp the concept of setting up a lighting system and make the calculations will already be on the right path to creating an efficient, productive garden.

Seeing a return on investment is important to any grower. An indoor garden with the right reflector, an adequate amount of light energy per square foot, and, when applicable, a comprehensive cross-pattern set-up, will provide a larger and faster return on investment.


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Written by Eric Hopper | Writer, Consultant, Product Tester

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Eric Hopper’s past experiences within the indoor gardening industry include being a hydroponic retail store manager and owner. Currently, he works as a writer, consultant and product tester for various indoor horticulture companies. His inquisitive nature keeps him busy seeking new technologies and methods that could help maximize a garden’s performance.

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