Wattage vs Actual Light Output: Understanding Efficiency

What wattage means on a listing

The wattage figure in a grow light title is the power the light draws from the wall, stated in watts. It is the number most listings lead with, and it is useful for two purposes: comparing the stated power of two lights and matching a light to the space you plan to light.

The figure to read is the actual draw, the watts the light consumes when it runs. Some listings add an 'equivalent wattage' phrase that compares the light to an older type of bulb or fixture. Equivalent wattage is a seller's comparison, not a measurement of this light's draw. When a title says 'equivalent', look for the actual draw figure that appears beside it.

A listing may also state a wattage as a total for a pack or a fixture, such as '2 x 9W' for two bars. The total draw is the sum of the individual draws, so a pack of two strips draws more than a single strip of the same rating.

Why wattage is not actual light output

Watts tell you what a light consumes, not what your plants receive. Plants need light for photosynthesis, so the light a fixture emits matters more than the power it draws. Part of the electricity becomes light and part becomes heat, and the proportion depends on the fixture design, the driver, and the LEDs.

The term 'actual light output' appears on listings in different ways. Some listings state a figure in umol/s, which is the PPF term. Others state an efficacy figure in umol/J, the PPF divided by watts. Both are maker-stated numbers that belong on the listing and on the maker's page, and they are the closest terms a listing offers for light output.

A high wattage figure does not guarantee that a light is efficient. The same watts can come from a board with many diodes, a bar with fewer diodes, or a panel with a single driver. The way to compare is to read the stated details on each listing and the maker's page for each model.

PPF and efficacy as listing terms

PPF stands for photosynthetic photon flux. It is the term for the total number of photons a light emits per second in the photosynthetic range. The unit is umol/s. A listing that states PPF is telling you the total photon output of the fixture before any light is lost to distance or reflection.

Efficacy, written as umol/J, is PPF divided by input watts. It tells you how many photons a light produces for each joule of electricity it consumes. A higher efficacy figure means more photons per watt, and it is the term that lets you compare two lights of different wattage on an equal footing.

Both terms are maker-stated. They appear on some listings, in the maker's specification sheet, and in the manual. When a term is not stated, the listing simply does not give that figure, and the maker's page is the next place to look.

How to compare lights by stated details

This site's guides compare lights on stated details: wattage, light type, coverage wording, dimming, timers, and the maker name. PPF and efficacy are part of the stated details when a listing includes them. The maker's page and manual are where the figures belong.

To compare two lights, put the listings side by side. Read the actual draw from each title, note the light type from the first words (board, bar, panel, strip, clip-on, bulb), and check whether a PPF or efficacy figure appears in the bullet list. If it does, the maker page explains how it was measured. If it does not, the listing simply leaves it out.

The maker's manual is the document to follow for mounting, distance, and power connections. Local electrical rules come first when you install any fixture.

Where the numbers live

The listing for a grow light shows the title, the bullet list, and the image set. The title usually carries the wattage and the light type. The bullet list is where PPF, efficacy, coverage, dimming, and timer details appear when the seller states them.

The maker's product page repeats those details and adds the manual, which is the authoritative source for setup and care. The spec sheet on a maker page collects a model's stated figures. Maker pages, such as those for Vivosun and GE, organize products by category. Verification programs, such as the DesignLights Consortium's Qualified Products List for horticultural lighting, publish performance and efficiency requirements and verify products against them.

For any figure you want to confirm, the maker's manual and page are the places to read it. A listing can change, and the maker is the source that stays current.

What watts and PPF cannot tell you

Watts cannot tell you how a light spreads across a shelf or a tent. Coverage wording on a listing covers that part. PPF cannot tell you how much of that light reaches the plant, because distance and reflection change the numbers at the canopy. That is the job of PPFD and a PAR map, which a maker may publish for a model.

Read the list of what a listing states and leave the rest. To understand how efficiency compares across brands, read the maker pages for the models on your shortlist, then use the buying guides on this site to narrow your choice.

What to pick for your job

Questions

Is a higher wattage light always better for plants?

No. Wattage is the power draw. What matters for a plant is how much photosynthetically active light reaches it, which is stated as PPF and efficacy when the maker publishes those terms. A lower wattage light with a higher efficacy figure can produce as many photons as a higher wattage light with lower efficacy.

What is PPF on a grow light listing?

PPF is photosynthetic photon flux, the total number of photons a light emits per second in the photosynthetic range, in umol/s. It is a maker-stated figure. Read it on the listing or the maker's page.

What does efficacy (umol/J) mean?

Efficacy is PPF divided by the input watts. It expresses how many photons a light produces per joule of electricity. Compare efficacy figures across listings when both makers state them.

What is equivalent wattage?

Equivalent wattage is a seller's phrase that compares a light to an older type of bulb or fixture. It is not the actual draw. The actual draw is the wattage the light consumes when it runs, and it is the figure to read.

Where is the actual light output of a light stated?

On the listing, the maker's product page, and the manual. The listing states the terms when the seller includes them; the maker page is where the specification sheet and manual live.

Why do two lights with the same wattage look so different?

Because wattage is only the power draw. The number of diodes, the layout, the spectrum, and the driver all affect how that power becomes light. Compare the stated details and read the maker's page for the model.