Grow Lights For Indoor Plants - How To Measure It, and Understanding Watts, PPF, PPFD and DLI

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. If you’d also like to have a FREE copy of our grow light calculators, check out below.

How Much Electricity Do Grow Lights Use?

Why Grow Light Wattage Is Not As Important As You Think

The energy that we use for grow 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 indoor 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 grow light 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.

What is the Meaning of PPF in Grow Lights?

In order to know how effective a grow light is at growing plants, we need to understand what is 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.

What is Grow Light Efficiency and PPF/W?

Indoor grow lights vary tremendously in terms of their PPF/W. There are many reasons for this, the most significant of which is cost. Especially when it comes to LED grow lights, 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 indoor 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 Explained

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 indoor 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.