The answer used to be, "if you’re running HID’s, just get the same wattage in an LED." That was bad advice then and it’s even worse now.
It takes more than finding an LED grow light with the same wattage as your HID (HPS/MH) to give your plants the light they need to thrive. In fact, wattage is probably the last thing you think about when converting your grow room's lighting. Thankfully there’s a way to find an equivalent LED to replace your HID grow light.
All you have to do to find an equivalent LED grow light to replace your HID is find an LED with the same PAR (PPFD) reading.
Now if you're not sure what PAR is or how to find it, don't panic. We'll be walking you through what it is and how to find it so you can definitively find the best LED to replace your HPS/MH setup.
But first, we think it's best to clear the air and clear up a big misconception: you can use wattage and/or lumens to determine what sort of LED can replace an HPS/MH grow light.
Wattage: How Much Energy Your Light Uses
When growers are thinking about switching from HID's to LED's, some make the mistake of matching wattages. This might seem like the most logical thing, but this idea is both flawed and ineffective. In some cases, using this strategy can actually cause problems for your plants. That's because wattage actually refers to the amount of energy a light uses while it's running. It doesn't factor in how bright the light actually is (lumens), or how much of that light can be used by your plant (PAR).
So forget wattages for now, because we need to focus on a grow light's performance. That starts with lumens.
Lumens: How Bright Your Light Is
Lumens refer to the total amount of visible light that a grow light emits. The more lumens a light source offers, the brighter the light will be. This may also seem like a logical route to take, but it's a little misleading because an LED can be deceptively powerful.Here’s an example of what we mean:
| High Intensity Discharge Lamp Wattage | Visually Effective Lumens Emitted | LED Equivalent Input Wattage | Visually Effective Lumens Emitted |
|---|---|---|---|
| 400 Watt MH | 24, 511 lumens | 194w | 24, 453 lumens |
| 400 Watt HPS | 19, 530 lumens | 155w | 19, 537 lumens |
At less than half the watts of an HID, an LED can give you almost identical lumen output. That means if you wanted the same brightness out of a 400w MH, you'll most likely find an equivalent LED grow light at a much lower wattage.
See why we told you watts alone aren't a good indicator of a grow light's effectiveness on your plants?
But just because a light is bright doesn't mean it has all the wavelengths your plants need. Lumens aren't great at letting you know how much of that brightness a plant can use. That's why it's important to know the PAR of a grow light to help you figure out what LED can replace your HID.
PAR & PPFD: How Much Light Your Plants Can Use
PAR (Photosynthetic Active Radiation) is a measurement of the amount of light that can be used for photosynthesis, no matter if it's from the sun or a grow light. Without photosynthesis, plants can't grow. If you want your plants to grow at the same rate from one grow light to another, PAR is what you'll want to focus on.In general, PAR is the measure of a grow light's effectiveness on your plants. It lets you know the concentration of useable light hitting your plants at any given point along their canopy. That's why PAR is exactly what you'll need to know when upgrading your grow tent or grow room's lighting.
How to Find an LED That's Equal to Your HID Grow Light
Alright, let's get down to business: let's find the correct LED grow light to replace your HPS/MH grow light system.It's a simple job that starts with one tool: a light meter. If you don't have one, we sugguest purchasing a meter just like the one in our How to Measure PAR article (don't worry, you can find them pretty cheap).
Once you've got your meter, here's what you do:
- Turn on your HID light and let it completely warm up.
- Center the probe of the meter in the middle of your growing area. Be sure your light's as high above the meter as it is above your plants. *Note: This measurement works best with no plants in the growing area.
- Record the reading you get from your HID. This is your PPDF (Photosynthetic Photon Flux Density) reading.
For example, say you need a light that gives you around 600-800 PPDF. You have two choices:
- The California Light Works SolarXtreme 500 (power draw of 400w) that has 500 PPFD at 24" above your plants, and 800 PPFD at 18" above your plants.
- The KIND K3 Series2 XL600 LED Grow Light (power draw of 320w) that has 663 PPFD at 30" above your plants.
Conclusion
The great thing about using PAR to find the right LED to replace your HID grow light is that it's accurate. Finding the PAR of your HID will let you know right away what to look for in an LED. From there you just find an LED grow light with the same PPDF as your HID at the optimal height.Part of the reason why LED's will save you money on energy costs over HID's is because of the amount of energy used to operate an LED. As you see here, most times the perfect LED with the right PAR to replace your HID will take less power to run.
When it's time to upgrade your HID's to LED's, do yourself a favor: forget about wattages, don't worry about lumens. Look for the PAR like we've shown you here, and you're on your way to growing big yields while saving money on energy.