How many lights – and how much power – do I need for my greenhouse?

How exactly do you know how many lights you might need for your own greenhouse, though? And how much power are they likely to need?

In this blog, I’ll show how some simple calculations can help you answer both of those questions.

Establishing factors

Before we get our calculations underway, we need two supporting pieces of information. These are:

•    The size of the greenhouse that you are lighting, and;
•    The intensity of light that you want to provide to your crop.

Let’s begin by looking at the issue of size. commercial greenhouses can be very large structures, occupying hectares of space. With that in mind, breaking the overall area down into smaller and more manageable sections helps to keep things simple – particularly when it comes to issues of light intensity.

Within a horticultural lighting design, we usually specify an individual section as being the width of a bay multiplied by the distance between each of the trellis posts. For example:

•    Each bay has a width of 8m.
•    The distance between each of the trellis posts is 5m.
•    Multiplying these distances together gives us a total section area of 40m².

We can now turn our attention to the issue of light intensity. The light intensity required differs by geographical location, type of greenhouse from crop to crop, of course, and by growing phase too. For the purpose of our calculations, however, let’s assume that the crop is tomatoes, grown on high wires and the required light intensity is around 250µMol/m²/s (micromoles per square meter per second).

By multiplying our section measurements with our desired level of light intensity, we can now calculate our requirements for the area as a whole:

•    Our section area is 40m².
•    Our desired light intensity is 250µMol/m²/s.
•    40 x 250 = 10,000, giving us a total requirement of 10,000µmol.

The fundamentals of this calculation remain the same no matter how we change the core parameters. For example:

•    A 60m² section and a desired light intensity of 150µMol/m²/s would equate to a total requirement of 9,000µmol.
•    A 20m² section with a desired light intensity of 300µMol/m²/s would demand a total requirement of 6,000µmol.

The importance of output

We now know how much light we need to provide to our small subsection. With this, we can begin to deduce how many lights are required. To do so, we simply need to divide our total light intensity requirement by the output of each of the luminaires that will be used.

When evaluating the technical specifications of a grow light, one of the key figures to take into account is its PPF. PPF – or, Photosynthetic Photon Flux – measures the number of “useful” photons produced by a luminaire. In essence, the higher the PPF, the more useful light a luminaire generates. By knowing the PPF of the luminaires that we’re planning to use, we can also calculate how many will be needed in total.

For this example, let’s use Midstream’s own Hyperion Pro Max grow light as our basis. The Hyperion Pro Max has a PPF output of up to 4,200 µmol/s. To make our calculations a little more straightforward, lets assume the output of the Pro Max for this project is 3,330 umol/s. Now we just need to divide our total light intensity requirement by the output of an individual luminaire. Thus:

• Our total requirement is 10,000µmol.
• Our chosen luminaire generates 3,330 µmol/s.
• 10,000 ÷ 3,330 = 3, which means we need three luminaires to provide us with our desired light intensity of 250µMol/m²/s.
  
  

Final calculations

Through the process above, we have determined that it will take three luminaires to light a 40m² area effectively. We can now take that result and scale it up across the entire greenhouse.

The easiest way to achieve this is to divide the total square meterage of the greenhouse by that of the area that we sectioned off in the first step. This tells us how many “sections” are in our greenhouse, and thus the total number of luminaires required. 

For this calculation, let’s imagine that the greenhouse that we want to light has a total area of one hectare, or 10,000m². So:

•    Our total area is 10,000m².
•    Our initial section was 40m².
•    10,000 ÷ 40 = 250 sections.
•    Three luminaires are required to light each section.
•    250 x 3 = 750, meaning that we need 750 luminaires to light a one hectare greenhouse at a lighting intensity of 250µMol/m²/s.

This information is very useful when it comes to calculating the approximate costs of a grow lighting solution. There are other advantages to knowing how many luminaires your greenhouse requires, however, and one of the principle benefits lies in being able to anticipate power consumption. 

To understand the total amount of energy that your lighting system will use, we need to make one final calculation. Here, we take the power absorption of the chosen luminaire, and multiply that by the total number of luminaires that will be used. Again, let’s use the Hyperion Pro Max as our basis, which gives us the calculation below:

•  Power absorption will vary by spectrum and light output but let’s assume that in this case, the 3,330 umol/s Hyperion Pro Max’s power absorption is 1000W.
•  We will use 750 Hyperion Pro Max luminaires across the greenhouse.
• 750 x 1,000 = 750,000, meaning that our lighting system will use 750,000W (or 750kW).
  
  Naturally, it is highly uncommon for growers to need to make any of these calculations on their own. Any professional lighting provider should offer figures, costs, and recommendations as part of their proposal, which Midstream does as part of its complimentary design service. 
  
  Nonetheless, the exercise outlined above can be a useful process to follow – not just in terms of anticipating the potential costs of a new or upgraded lighting system, but in qualifying those presented back to you as well.
  
  

In summary

To work out how many lights and how much power your greenhouse will need:

1.   Calculate the size of one small area by multiplying the width of a bay by the distance between trellis posts (i.e. 8 x 5 = 40).
   
   
2. Calculate the total amount of light required by multiplying the size of the area by the desired level of light intensity (i.e. 40 x 250 = 10,000).
   
   
3.  Calculate the number of luminaires needed to light each section by dividing the total amount of light required by the output of one of the chosen luminaires (i.e. 10,000 ÷ 3,330 = 3).
   
   
4. Calculate the number of individual sections by dividing your total floorspace by that of the area outlined in Step 1 (i.e. 10,000 ÷ 40 = 250)
   
   
5. Calculate the number of luminaires required across the greenhouse by multiplying the section luminaire total by the number of individual sections (i.e. 3 x 250 = 750).
   
   
6. Calculate the energy required by multiplying the total number of luminaires required by the power absorption of one of the chosen luminaires (i.e. 750 x 1000 = 750kW).