Calculating Grow Light Efficiency, Coverage & Harvest Potential

In our Grow Light Calculator and our Grow Light Tests and Reviews, we provide calculations for Grow Light Efficiency, Coverage and Harvest Potential. In this article, I explain how our calculations work and the science behind them. It starts with understanding why we use Photosynthetic Photon Flux (PPF) rather than other metrics like wattage to evaluate lights. I discuss the difference between "Calculated PPF", "Total PPF" and “Usable PPF”, and how we measure or estimate Usable PPF. Usable PPF is the key metric that we use to calculate grow light efficiency, coverage and harvest potential. I discuss the formulas for each of these and how we developed them. Along the way, I discuss the myth of vegetative versus flowering coverage areas and explain our new benchmark harvest target to replace the old “gram per watt” rule.

Using PPF to Analyze Grow Lights

The grow light market and the internet are full of conflicting and confusing claims about the size or power of grow lights. Many growers still rely on wattage to estimate the size of grow lights. However, high-efficient LEDs now produce a lot more light per watt than older grow light technologies like High Pressure Sodium (HPS). Manufacturers have tried to take advantage of this by advertising their lights with an “HPS equivalency”. However, this is purely a marketing strategy and the numbers that they report usually have no grounding in science.

In order to accurately describe the amount of light that a fixture can deliver, we use the scientific measurement of Photosynthetic Photon Flux (PPF). PPF is a count of the photons that are in the Photosynthetically Active Radiation (PAR) wavelengths. These photons drive photosynthesis and are the source of all the energy for the plant. The quantity of PPF produced by a fixture determines how much area it can cover and its harvest potential.

To learn more about PAR, PPF and other grow light metrics see our Grow Light Metrics Primer.

Calculated, Total & Usable PPF

As we explain in our Grow Light Metrics Primer, there are three different types of PPF data. LED Manufacturer’s often publish “Calculated PPF”, which is determined based on the efficiency of the diodes used in the fixture. These values overstate the number of photons that the fixture can produce because they ignore thermal and other efficiency losses.

Some manufacturers publish measured PPF values, however they measure the “Total PPF”. Total PPF describes the number of photons that a fixture produces. It is measured with sophisticated sensors in a device called an integrating sphere which captures all the photons that a fixture produces. In the real world of indoor gardening, not all those photons will actually reach the canopy. Even in an ideal grow setting with an excellent grow light, 10-15% of the photons are lost in radiance and reflection.

The PPF value that is most relevant to us as indoor gardeners is the “Usable PPF”. Usable PPF describes the number of photons that reach the canopy of the plants. Usable PPF is lower than the Total PPF because some of the photons that are produced by the fixture do not reach the plant. Usable PPF is the best value to use for all other grow light calculations because it measures the photons that will be available to the plant for photosynthesis.

Measuring Usable PPF

Our grow light field testing is designed to measure the Usable PPF of grow light fixtures. We use analytical grade PAR sensors in a setting that simulates a grow tent. For the fixtures that we have tested we report this value as the “Measured Usable PPF”. To learn more about how we measure Usable PPF read our Grow Light Testing Protocol.

Estimating Usable PPF

For many grow light fixtures there are no Usable PPF data available. This creates problems for making accurate comparisons and calculations. Comparing the Calculated PPF from one fixture to the Usable PPF of a different fixture is not fair. Furthermore, we need Usable PPF in order to accurately estimate efficiency, coverage and harvest potential.

To resolve these problems, we created formulas to estimate Usable PPF based on Calculated or Total PPF. To create these formulas, we analyzed PPF data from over 60 different fixtures. We found distinct patterns in the data that suggest the relationship between the three types of PPF is broadly predictable. Our analysis reveals that Calculated PPF is typically about 18% higher than Total PPF as measured in an integrating sphere. Total PPF is typically about 15% higher than Usable PPF as measured in our field testing.

In our Grow Light Calculator, you can select which type of PPF data that you want to input. The default is set to “Calculated PPF” because this is the data that most manufacturers provide. If you click on the drop-down menu, you can select Total PPF or Usable PPF. When you select “Calculated PPF” or “Total PPF”, we use our formulas to estimate the Usable PPF. When you select “Usable PPF” in the drop-down menu we do not make any adjustments. The calculator will report the new PPF value as the “Estimated Usable PPF”. All other calculations for efficiency, coverage and harvest are then based on the Estimated Usable PPF.

We want to stress that these adjustments are not as precise as a measurement. However, it is better to adjust the other forms of data and estimate then it is to try and compare different forms of data directly. After creating our formulas, we have tested them and found them to be predictive. For most fixtures, our formulas estimate Usable PPF with an error of less than 5%. As we continue to do field testing, we will continue to refine these formulas.

Calculating Grow Light Efficiency

When comparing different light fixtures, it is critical to pay attention to their efficiency, not just their raw power or cost. Choosing efficient grow lights reduces both operating costs and the environmental impact associated with electricity use. However, most home growers on a budget also need to consider the upfront purchase price. Therefore, we use two separate calculations to measure grow light efficiency: photon efficiency and cost efficiency. Our Grow Light Calculator allows you to easily measure and compare these efficiencies for different grow light fixtures.

Photon efficiency

Photon efficiency is a calculation for how efficiently the fixture converts electricity into usable photons. To determine photon efficiency, we divide Usable PPF by Actual Power Draw in Watts. Manufacturers often publish Photon Efficiency ratings, but they are based on Total PPF or the Calculated PPF. For this reason, our Photon Efficiency ratings are lower than what the manufacturers generally publish.

Photon Efficiency: Usable PPF (µmol) / watts

  • Less than 1.25 µmol/watt is inefficient
  • 1.25-1.5 µmol/watt is average efficiency
  • 1.5-1.75 µmol/watt is high efficiency
  • 1.75-2.0 µmol/watt is elite efficiency
  • 2.0-3.0 µmol/watt is superior efficiency
  • Above 3.0 = Data Error: There is no current grow light technology that can reach 3.0 µmol/watt
Cost efficiency

Cost Efficiency is a calculation of how expensive the fixture is relative to the amount of light it produces. To calculate cost efficiency, we divide the purchase price by the Usable PPF.

Cost Efficiency: Purchase Price $ / Usable PPF (µmol)

  • Less than $0.30/µmol is cheap
  • $0.30-$0.60/µmol is an excellent price for a quality LED
  • $0.60-$0.90/µmol is a good price for a quality LED
  • $0.90-$1.20/µmol is an average price for a top end LED
  • Above $1.20/µmol is expensive even for a top end LED

Calculating Grow Light Coverage with PPF

In order to set up your grow space you need to determine how much space a given fixture can cover. Grow light manufacturers provide coverage estimates, but they are frequently exaggerated to make the lights more attractive. In our guide, “How Much Light (PPF) Do You Need for Indoor Cannabis”, we review the science of cannabis photosynthesis to determine the optimal amount of light. The research shows that the Optimal Usable PPF for indoor cannabis cultivation is 700 µmol (Usable PPF) per square meter of grow space. This converts to 65 µmol (Usable PPF) per square foot.

To estimate the amount of space a grow light fixture will cover, we divide Usable PPF by 65. This shows the optimal coverage area in square feet (Usable PPF / 65 = Sq. ft). For example, a fixture that produces a Usable PPF of 130 µmol can cover an area of about 2 square feet (130/65 = 2sq. ft).

Vegetative Coverage vs Flowering Coverage

Manufacturers often suggest that lights will cover a larger area during vegetative growth than during flowering. However, the optimal density of photons is based on photosynthesis which does not change between vegetative and flowering growth. Once the plants are mature, they can handle a full dose of light. The number of photons that cannabis plants can use for photosynthesis does not increase during the flowering period.

The only way to understand the manufacturers’ recommendations in this regard is to realize that optimal lighting is less important during vegetative growth. However, even during vegetative growth, plants will grow the best if they have optimal lighting. Our coverage estimates are based on the science of photosynthesis and reflect the optimal lighting for both vegetative and flowering growth.

To learn more about optimal lighting, see our article "How Much Light (PFF) Do You Need for Indoor Cannabis".

Estimating the Harvest Potential of a Grow Light

There are several factors that affect the size of an indoor cannabis harvest. Space, strain, climate, style of grow, media, nutrients, and general good gardening all play an important role. However, the amount of light (Usable PPF) that you use determines the maximum harvest potential.

The Old Gram Per Watt Rule

In the cannabis grow community, it is common to estimate harvest sizes using a “gram to watt” rule. The gram to watt rule states that good growers should be able to yield one gram per each watt that their lights use. This rule has emerged from experience growing cannabis with High Pressure Sodium (HPS) grow lights. As we show in our article, “How to Evaluate and Compare Grow Lights for Cannabis”, HPS and other High Intensity discharge lights like Ceramic Metal Halide (CMH), all convert electricity into Usable PPF at about the same rate. For this reason, wattage can be used as a proxy measurement for Usable PPF.

As long as growers are using HPS or CMH lights, the old gram per watt rule provides an adequate estimate for a benchmark harvest yield. However, LED grow lights now can get up to twice as much Usable PPF from the same amount of wattage. Therefore, the gram per watt rule no longer applies to High Efficiency LEDs. We need to estimate harvest potential and evaluate harvest success based on grams per Usable PPF rather than grams per watt.

The New Benchmark = 0.75g/µmol (Usable PPF)

We converted the old gram per watt rule to PPF and suggest that 0.75g per µmol (Usable PPF) be considered the new benchmark for evaluating the success of a grow. This conversion is based on HPS lights having an average photon efficiency of 1.3µmol (Usable PPF)/watt. If we consider a 600-watt HPS operating at an efficiency of 1.3µmol/watt it would generate 780µmol (Usable PPF). According to the gram per watt rule, we would expect good growers to harvest 600 grams. That is 600 grams per 780µmol (Usable PPF) = 0.77g/µmol.

Estimating Harvest Size With Usable PPF

The Benchmark harvest target of 0.75g/µmol (Usable PPF) is a good goal, but it is a high target. It should be considered the top end for what can be harvested with high yielding strains in a well-managed grow. To make a more realistic estimate we use a range from 0.55g/µmol up to 0.75g/µmol (Usable PPF). Many growers who follow our guides for growing in coco have topped 0.65g/µmol (Usable PPF) on their first grow.

We created this guide to share reliable grow light data and empower home growers with the best information about grow lights.

Grow Light Guide Articles
  • Grow Light Metrics Primer

    Learn about the science of horticultural lighting. We explain the key grow light metrics and terminology: PAR, PPF, PPFD, and more. Start here to make the most of our Grow Light Guide!

  • Grow Light Calculator

    Our grow light calculator provides an accurate way to analyze and compare different grow light fixtures. You can enter your own grow light data or select preloaded fixtures.

  • Grow Light Testing Protocol

    Field testing produces the most accurate measurement of Usable PPF, but it must be done correctly. In this article, we explain our testing protocol and the science behind it. Learn about test area size, the importance of reflective walls, and the scientific way to determine optimal hanging height.

  • How Much Light (PPF) Do You Need for Indoor Cannabis?

    We review the science of cannabis photosynthesis and explain the optimal quantity (PPF) and density (PPFD) of light for indoor cannabis. We include a Grow Space Calculator which shows the optimal light and harvest potential for any grow space.

Read All our Articles!

Author: Dr Coco

I am a university professor and have taught courses in horticulture. I am coco for cannabis and I hope you are coco for cannabis too :) Grower Love!