What Is umol/s Output and How Much Do You Need?

What umol/s means on a grow light listing

When you read a grow light listing, you may see a figure written as umol/s or µmol/s. This is the unit for photosynthetic photon flux (PPF). PPF is the total amount of light, in the range plants use for photosynthesis, that the light emits every second. The unit is micromoles per second, where a mole is a count of photons and a micromole is one millionth of that count.

PPF is a property of the light itself, not of the space it lights. A light with a higher PPF emits more usable light in total. It does not tell you how bright that light is at any one spot, because the same total output can be spread over a small or a large area. That is why PPF is only one part of a listing: you also need coverage area and, often, a figure like PPFD (photosynthetic photon flux density) to understand the light at plant level.

This site reads the PPF figure only when the listing or the maker's page states it. Many listings do not state PPF at all, and some state it in a way that is hard to compare, such as for a pack of lights rather than one light. When a figure is missing, the article tells you to read it on the listing, in the maker's manual, or on the maker's page.

• umol/s is the unit for photosynthetic photon flux (PPF).
• PPF is the total usable light emitted per second.
• PPF does not describe how the light is distributed over an area.
• Read PPF on the listing or the maker's page when it is stated.

How much output do you need?

There is no single umol/s number that fits every grower. The output you need depends on the size of the area you light, the type of plants, and the stage of growth. A small shelf of houseplants needs far less total output than a 4x4 tent full of vegetables, even if both use LED lights.

A useful way to think about it is to compare the light's PPF to the area it covers. The same PPF spread over a larger area gives each plant less light. So a light with a modest PPF can be right for a windowsill, while a tent grower needs a higher PPF to cover the whole floor.

The University of Minnesota Extension page groups indoor light levels into low, medium, and high, and gives a PPF range for each. Low light is 50 to 150 umol m-2s-1, medium light is 150 to 250 umol m-2s-1, and high light is 250 to 450 umol m-2s-1. These are PPFD values, which measure light at the plant surface, not the total PPF of the light. Still, they show the range of light levels that different plants need.

For a windowsill of herbs or a desk plant, you are likely in the low to medium range. For a seed starting rack or a grow tent, you need higher output. The exact figure for your light is on the listing or the maker's page. This article cannot give you a target number, because the catalogue does not state PPF targets for specific plants or stages.

PPF versus PPFD: what is the difference?

PPF and PPFD are easy to confuse because they sound alike. PPF, in umol/s, is the total light emitted by the fixture. PPFD, in umol m-2s-1, is the light that arrives at a square meter of plant surface. PPFD depends on the PPF, the distance from the light, and the reflector or lens design.

A listing may state both, or only one. When you see a PPFD figure, it is usually measured at a specific distance, such as 12 inches or 24 inches. That distance matters, because PPFD drops as you move the light away from the plants. The maker's manual or page should tell you the distance used for the measurement.

For choosing a light, PPF tells you the total output, and PPFD tells you how that output lands on the plants. Both are useful, but neither replaces the other. If a listing gives only PPF, you can still compare lights by total output, then check the coverage area to see how the light spreads that output.

Where to read the PPF figure

The PPF figure is not always in the listing title. It may be in the bullet list, the description, or the maker's specification sheet. When the catalogue does not state it, the listing will say that the detail is not stated by the seller. In that case, the maker's page for the model is the place to look.

Some makers publish PPF for each model, along with a PPFD map that shows light levels across the coverage area. The DesignLights Consortium page explains that horticultural lighting products can be listed on a Qualified Products List, which verifies performance claims. That list is a resource for comparing products, but this site does not read it as a source.

When you read a PPF figure, check what it applies to. A listing for a pack of 4 strips may state a total PPF for the pack, not for one strip. The same is true for wattage: the catalogue reads the draw of 1 light, not a pack total. Read the PPF the same way, and compare like for like.

Matching output to your growing job

Your job determines the output you need. A single potted plant on a desk can be served by a clip-on light with a modest PPF. A shelf of houseplants may need a strip or a panel with a higher total output. A seed starting rack in a basement needs enough output to cover the whole shelf area, and a grow tent needs a light that fills the tent floor.

The guides on this site group lights by type and by job. For a desk plant, look at clip-on lights. For a shelf, look at strips. For a tent, look at panels, bars, or quantum boards. Each guide compares lights on stated details, including wattage, coverage area, and controls, and tells you what the listing does not state.

When you compare two lights, use PPF alongside coverage area. A light with a higher PPF but a much larger coverage area may give each plant less light than a lower-PPF light with a smaller footprint. The maker's PPFD map, if published, shows this directly. If no map is available, the coverage area stated in the title is your guide.

What PPF cannot tell you

PPF is a total output figure, so it cannot tell you how evenly the light spreads. Two lights with the same PPF can light a 2x2 area very differently, one with a hot spot in the center and dim edges, another with a uniform spread. Evenness is described by a PPFD map or by the coverage area wording.

PPF also does not tell you the spectrum. A light can have a high PPF but a spectrum that is not ideal for your plants. Spectrum is a separate listing term, often described as full spectrum or by Kelvin values. The article on full spectrum versus targeted wavelengths explains that term.

Finally, PPF does not tell you efficiency. Efficiency is PPF divided by the power draw, often written as umol/J. That is a separate topic, covered in the article on efficacy. For choosing a light, PPF and coverage area are the first numbers to read, then efficiency if you want to compare energy use.

What to pick for your job

Questions

What does umol/s stand for?

umol/s stands for micromoles per second. It measures photosynthetic photon flux (PPF), the total amount of light a grow light emits each second that plants can use for photosynthesis.

Is a higher umol/s always better?

Not always. A higher umol/s means more total light, but the right amount depends on your growing area and plants. Too much light can scorch leaves, and too little can make plants leggy. Match the output to your space and plant needs.

How is umol/s different from PPFD?

umol/s (PPF) is the total light emitted by the fixture. PPFD, in umol m-2s-1, is the light that reaches a square meter of plant surface. PPFD depends on PPF, distance, and reflector design.

Where can I find the umol/s figure for a light?

Look on the listing's bullet list or description, or on the maker's page for the model. If the listing does not state it, the maker's manual or specification sheet is the place to check.

Do I need a high umol/s for a windowsill herb garden?

A windowsill herb garden is a small area, so you likely need a lower total output than a tent grower. A clip-on or bulb light with a modest PPF can be enough. Read the PPF on the listing to compare.

Can I compare two lights by umol/s alone?

You can compare total output, but also check coverage area. The same PPF spread over a larger area gives each plant less light. Use PPF with coverage area, and look for a PPFD map if the maker publishes one.