This blog post is our third to one of our most popular ones, “How Many Watts does it take to replace 1000W Metal Halide”. Many of the concepts are the same but there are some differences between 250W and 1000W Metal Halide. One of the most common questions we get asked is
“How many LED watts does it take to replace a 250 watt metal halide bulb?” “I just talked to another LED company and they said to use their 75W LED Fixture. How much is your 75W LED Fixture?”
This approach is fraught with error. The amount of watts a LED fixture consumes does not guide replacing an existing Metal Halide or HPS light. Let us explain.
Converting from 250W Metal Halide to LED?
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How many LED Watts does it take to replace a 250 watt metal halide?
The best advice we can give anyone is never buy a LED product based on watts. Buy it based on lumens and then figure out how little watts you can use to make those lumens.
What is a lumen?
In simple terms, it’s a measurement of light that defines how much light a certain fixture or bulb produces. It is the light you are replacing. Fixtures produce lumens and consume watts. And all the fixtures lumens combined provide a foot candle measurement. When we do a photometric calculation, we use the data associated with that fixture or bulb. We then insert it into photometric software, and calculate the foot candles for the area. In real terms, a light meter will give you a foot candle reading at the place you are standing. Move close to a light source, the foot candles will rise. Move away, it will fall. The foot candle reading between 2 fixtures is the collective lumen output. These 2 fixtures distributed to that point where you are standing.
Why does this matter? A lot more than you may think. Because it’s what matters when replacing old systems with new lighting technology.
To replace an existing fixture (or entire system), you need the lumen output per fixture. Let’s say your metal halide fixtures produce 18,000 lumens per 250 watt bulb. So instead of thinking, “I need 250 Watt LED to replace 250 Watt Metal Halide,” you need to be thinking
“I need an LED that can replace 18,000 lumens.”
Take this example of 18,000 lumens for a 250 Watt metal halide. That means its outputting 72 lumens per watt. In the world of lighting, this isn’t that efficient, with LEDs approaching 180-200 lumens per watt. But, there are some other factors going on here that you need to be aware.
Understanding Initial lumens and L70
18,000 lumens seems bright. Metal Halide Bulbs have high initial output because they suffer from rapid lumen depreciation. It is not uncommon for a metal halide bulb to lost 50% of it’s lumens at half life. Full life for a metal halide bulb is 15,000 hours. That means at 7500 hours of use, it has lost half (or 9,000) of its lumens.
L70 is a term the number of hours before a bulb is performing at 70% of initial lumen output. For metal halide, that calculates to 5,000 hours of use.
You will notice L70 times for LED is in terms of 50,000 and 100,000 hours, not 5,000 hours like Metal Halide bulbs.
Loss of Light due to Reflection
LED light is directional, that means all its light shines where pointed. It does not need a reflector to collect it and deliver it to where its needed. Conventional light sources are omni-directional. They need reflectors to gather the light and focus it where needed. Any reflection of a lumen that is over 1 bounce in the reflector is a loss of that lumen. You can lose up to 30% of the effective lumens in this reflective process.
If a Metal Halide bulb has 18,000 initial lumens, the loss accounted to reflected lumens drops the lumen output to around 12,600 lumens.
Quality of Lumens – CRI
Color Rendering Index, or CRI, is the best way to compare the quality of light sources. The higher the quality of light, the less quantity you need. It is common for customers to report that 10,000 lumens of LED appears brighter than 18,000 lumens of other light sources, like HPS.
The basic truth is you need less quantity when you have higher quality.
Photopic vs Scotopic Lumens – How we perceive light versus how a light meter perceive light.
Photopic lumens are lumens detected by a device, like a camera or a light meter. It picks up all sources of light that we cannot see, like UV and IR. Scotopic lumens are lumens detected by the human eye. Its how we as people perceive light.
LED produces light within these spectrum’s, which means the light perceived by LED is light that we use. It is rare to see a LED light produce IR or UV spectrum’s. These wavelengths are invisible to people, so they have no value to us from a vision perspective.
It is common to stand under one LED and one traditional light sources. First read the fc measurement with the light meter of both light sources. Then ask individuals standing near by what light source they think looks brighter. And have the one that looks brighter contradict what the light meter tells us.
We perceive light sources and brightness different, in some cases, than light meters.
Converting from 250W Metal Halide to LED
The basic takeaway from all of this is you need far less LED lumens than metal halide or HPS lumens.
- Metal Halide bulbs are very bright out of the box, but not so much even after only 6 months of use.
- LED light is directional. The light goes where needed. No loss due to reflection.
- LED light is high quality light. You need less quantity when you have higher quality.
So how much LED Lumens do you need?
Only a photometric can tell you exactly how much you need. But over almost 10+ years of selling LED lighting, our general rule of thumb is:
- Indoors: 10,000 to 15,000 lumens (depends on application and mounting height). You would find these lights in a warehouse, factory, gymnasium, auditorium or swimming pool.
- Outdoors: 9,000 to 15,000 lumens (depends on application and mounting height)
By searching for a LED to a 250 Watt Metal Halide you can find fixtures that output at over 100+ lumens / watt. If you choose a reputable company with high quality product, you can reach higher than a 150 lumens / watt output. This allows you to cut your wattage usage per fixture by 2-3 and sometimes 4 times. This will save a lot of energy when you replace many fixtures.
It is possible to replace a metal halide fixture, that of any wattage (30 on up to 1000+), with LED technology. It can be as simple as retrofitting your existing fixtures or more work intensive to replace.
It may seem difficult and expensive but the benefits far surpass the cons when upgrading to LED. An investment in LED will provide payback in many ways: less energy usage and the higher quality of light produced.