Grow Lights For Indoor Plants – How To Measure It, and Understanding Watts, PPF, PPFD and DLI
Watts, PPF, PPFD, PAR and DLI. What do they all mean, and why should you care? If you’ve spent even a few minutes shopping for growing lights, then chances are you’ve encountered at least some of this terminology, and if your foray into the world of grow lights was anything like ours – there’s a good chance you’re utterly confused right now.
In this article we’re going to explain what all of these terms mean and why they are important, we’ll show you how they are measured and related, as well as give you some worked examples that you can apply immediately the next time you go shopping for a grow light.
We’ve laid this article out in such a way that it follows the ‘flow’ of the energy from the socket in your wall, through the grow light, and ending on the leaves of your plants. If you’d like to learn more about some of the fundamentals of grow lights, then our article titled ‘Grow Lights – An Introduction For Indoor Gardeners and their Plants’ is a great place to start.
How Much Electricity Do Grow Lights Use?
Why Wattage Is Not As Important As You Think
The energy that we use for growing light indoors starts off as a watt.
A Watt is the global standard for measuring the rate of energy transfer. One watt is equal to one joule per second.
Wattage measures a rate of flow. If we were talking about water, we’d measure flow in litres (or gallons) per second. With electricity, we use Watts.
1000 watts = 1 kilowatt (kW). If you left an appliance that consumes 1kW of energy running for an hour, you’d have used 1 kWh. If you pull out one of your utility bills, you will likely notice that they are measuring your electricity consumption (and charging you) based on your kWh consumption. You will probably also see the price per kWh – and if you’re in the US this is likely to be in the $0.08 – 0.28 range.
To put this into the context of a grow light for plants, it’s useful to consider a simple example.
- Let’s say we have a grow light that consumes 20W. That is equivalent to 0.02kW.
- If we left that grow light on for say 14hours a day, then we’d be consuming 14 x 0.02 = 0.28KWH per day.
- Do that for an entire year, and we’d use 102.2 KWH.
- The average price of electricity in the US in 2019 was $0.1054, which means that the average cost of running a 20W grow light at 14hrs/day for an entire year in the US would be 102.2 x 0.1054 = $10.77.
You should use the electricity price listed on your utility bill to do this math yourself, but if you don’t have that handy the US Energy Information Administration has a handy list here.
Ok, so we now understand wattage and how the energy coming out of our wall socket is measured. All we really know at this point is how much it is going to cost us to operate this grow light. We don’t know anything yet about how well this grow light is actually going to work for growing plants.
Grow Lights And PPF
In order to know how effective a grow light is at growing plants, we need to understand what is called its ‘efficacy’. That is, how efficient is it at converting the Watts we’re putting in, with plant-available light that comes out. The next unit of measure we need to understand is PPF.
PPF (photosynthetic photon flux) is a measure of the number of photons that are emitted by a source of light. PPF is measured in umol/s. For grow lights, we only care about light that is useful to the plants – that is light falling within the wavelength range from 400nm to 700nm. Light within this range is called PAR, or ‘photosynthetic active radiation’. PAR is not a unit of measure like “watts” or “pounds”, rather, it is a qualitative assessment we use to define a certain type of light.
Watts go IN to a grow light. PPF is what comes OUT. To understand the efficacy of a grow light, we simply divide one by the other – i.e. PPF per Watt or PPF/W.
Grow Light Efficiency and PPF/W
Grow lights vary tremendously in terms of their PPF/W. There are many reason for this, the most significant of which is cost. Especially when it comes to LEDs, older models and less efficient models are typically much cheaper.
The chart below summarizes PPF/W for a number of grow lights that are sold on Amazon today. We’ve deliberately left off the product/brand names (mainly because we don’t want to get in trouble from any of these companies) – but take note of just how wide the range is. The most efficient grow lights we tested offer 10 times as much light output, for each Watt they consume. Wow! You want to know what’s even crazier? The worst grow light on this chart currently sells for $30 on Amazon and has over 7000 reviews. In our view, it is a total waste of time however.
Grow Lights and the Coefficient of Utilization
The other metric we need in order to translate Wattage into PPF available for the plant out is known as the co-efficient of utilization.
Coefficient of Utilization (CU) is a measure of how much light that exits from a fixture will fall on an area of a certain size. In other words, how much of the PPF that the light is producing actually makes its way towards the plant.
Imagine for a moment that we live in a 2D world had a light that was emitting light in all directions evenly. Let’s also say that the plants we are trying to grow occupy an area below that light, and are sitting in an area which is equivalent to 90 degrees. Since the light is being emitted evenly in 360 degrees, this would mean that our CU is 0.25. The other 75% of the light is essentially wastage, or spillage.
Combining PPF/W and CU into an example
Let’s put these pieces together with an example.
Say we have two grow lights – A and B. A uses 100 Watts and B uses 50 Watts. Well, if you believed what you read on Amazon, then the 100W light must be twice as good as the 50W light. Right? Wrong. Allow us to explain.
Units | Light A | Light B | |
Energy consumption | Watts / j/s | 100W | 50W |
PPF/W | umol/j | 0.25 | 0.75 |
CU | multiple (0 to 1) | 0.6 | 0.8 |
PPF | umol/s |
100 x 0.25 x 0.6 = 15 umol/s |
50 x 0.75 x 0.8 = 30 umol/s |
- Let’s assume that the 100W light has a PPF/W of 0.25 and a co-efficient of utilization of 0.6. The PPF of this light would be 15 umol/s
- Now lets also assume that the 50W light has a PPF/W of 0.75 and a co-efficient of utilisation of 0.8. This means that the PPF of this light would be 30 umol/s.
In this example not only will your electricity bill be 50% less with the 50W globe, but your plants would be 2x happier, due to the extra light you’re providing them!
How To Get The Most From Your Grow Light
The Importance of Distance For Grow Lights and Indoor Plants
So Watts tell us what we’re pulling out of the socket on the wall. PPF tells us how much useful light is being emitted. The next concept you need to get your head around is understanding how, and how much, of that light is actually making its way to your plant.
We need to introduce two more terms in order to complete this story; PPFD and DLI.
PPFD (or photosynthetic photon flux density) is a measure of the amount of PAR (measured as PPF) that actually arrives at the plant. Where PPF is measured in umol/s, PPFD is measured in umol/m2/s. The main variable that impacts PPFD is distance from the source of the light.
Light intensity degrades quickly as you move away from the light. In fact, a ‘point’ source of light follows what is known as an ‘inverse-square law’. What that means is that every time you double the distance from a light, the intensity reduces by a factor of 1/4th.
In the diagram above we have the same globe producing exactly the same amount of light at points d, 2d and 3d. The difference is that at ‘d’ that PPF is being spread over 1 square. At ‘2d’ it gets spread over 4 squares, and at ‘3d’ it is being spread over 9 squares. Spread a jar of peanut butter over a single piece of toast, it might be an inch thick and you’d just about choke on it. But spread that same jar over 1000 pieces of toast and chances are you might not even know it’s there. The same concept applies to light.
Why Grow Light Placement Is Critical – An Example
Lets go back to our example with Grow Light A and B. We started off thinking that 100W was twice as good as 50W. Once we understood the concept of light efficiency we realized that the 50W was actually twice as good as the 100W. Let’s now look at the importance of grow light placement and distance.
- Assume the 100W light is placed 15cm from the plant, and that it is illuminating an area of 0.0225m2. Our PPF value of 15 umol/s is equivalent to 15/0.0225 = 667 umol/m2/s of PPFD.
- Now let’s assume we place the 50W light a little further away, at say 30cm and that it is illuminating an area of 0.09m2 (4x larger) as a result. The PPFD would be 30 / 0.09 = 333umol/m2/s.
Whoa – the intensity of light delivered by the 100W globe is twice as much again! All because of its closer distance to the plant.
Units | Light A | Light B | |
Energy consumption | Watts / j/s | 100W | 50W |
PPF/W | umol/j | 0.25 | 0.75 |
CU | multiple (0 to 1) | 0.6 | 0.8 |
PPF | umol/s |
100 x 0.25 x 0.6 = 15 umol/s |
50 x 0.75 x 0.8 = 30 umol/s |
Distance | m | 0.15m | 0.3m |
Illuminated Area | m2 | 0.0225 m2 | 0.09 m2 |
PPFD | umol/m2/s | 667 | 333 |
If you’d like to learn more about light placement for plants, and the importance of distance, check out How To Figure Out The Best Distance To Place Your LED Grow Light From Your Plants.
How Long Should A Grow Light Be On For?
Daily Light Integral (DLI) of Grow Lights
Aside from distance, the other main variable you can adjust is of course time. That is; the number of hours per day that you are using the light for. In order to incorporate time or duration into our math, there’s one final metric to understand: DLI.
Daily Light Integral (DLI) is measure of the number of photosynthetically active photons that you accumulate on a surface over a 24 hour period. It is a function of only two things:
1) the intensity of a light (measured in PPFD) and
2) the duration that the light is active or turned on (measured in seconds).
Plants really don’t care how many Watts your grow light uses. They care only a little bit about how many PFF your grow light emits. Plants definitely DO care about how much PPFD you supply them with. But the most important metric of all (from a plant’s perspective) is DLI. Decorative indoor plants might have a DLI requirement of 2 mol/m2/day. Fruiting and flowering edible plants like tomatoes and peppers might need a DLI as high as 20-30 mol/m2/day.
DLI Example Calculation for Grow Lights
Let’s wrap up our 100W Grow Light vs 50W Grow Light case study here:
- Our 100W light placed at 0.15m from the plant is delivering 667umol/m2/s. Let’s assume we only have it turned on for 4 hours (per day) though. 4 hours = 14,400 seconds. DLI is therefore 667 x 114,400 = 9,600,000 umol/m2/day or 9.6 mol/m2/day
- The 50W grow light was delivering 333 umol/m2/s. But let’s assume it is on for 16 hours per day, or 57,600 seconds. DLI is therefore 333 / 57,600 / 1,000,000 = 19.2 mol/m2/day.
Units | Light A | Light B | |
Energy consumption | Watts / j/s | 100W | 50W |
PPF/W | umol/j | 0.25 | 0.75 |
CU | multiple (0 to 1) | 0.6 | 0.8 |
PPF | umol/s |
100 x 0.25 x 0.6 = 15 umol/s |
50 x 0.75 x 0.8 = 30 umol/s |
Distance | m | 0.15m | 0.3m |
Illuminated Area | m2 | 0.0225 m2 | 0.09 m2 |
PPFD | umol/m2/s | 667 | 333 |
Hours per day | hours | 4 | 16 |
DLI | mol/m2/day | 9.6 | 19.2 |
Once again, we’ve flipped the tables. By taking the final variable (time) into account, the 50W grow light is now back in the lead with a DLI that is 2x that of the 100W grow light.
If you’ve made it this far, well done! We know this has been some pretty dense reading. The good news is that you now understand all of the various metrics we use to measure and quantify grow lights. We hope this has made your next grow light purchase a little easier. Feel free to try out our free calculator below, and edit the values based on your own personal scenario.
If you’d like to learn more about the grow light that we are developing here at Urban Leaf, as well as receive a special promotion code when it launches, simply leave a comment below and we will add you to the list!