In the good ol' days of HID and Incandescent fixtures, you always knew the "size" of the fixture that you needed, and it was measured in "Watts". Rarely did anyone even think, or talk, about Lumens. You knew rather intuitively whether you needed a 400w Metal Halide lamp as a replacement bulb for your High Bay fixture. Or, if you were adding/replacing a WallPack to your building, you would estimate the size of the fixture by the mounting height of it. Above the door, then a 70-watter would do. Ten to 12 feet high? Then you'd most likely want a 150w fixture. 15-feet or higher? Back to the good ol' 400w HID WallPack!
When LED's first started to become a viable product, say around 2010, we used "40%" as our Rule-of-Thumb. If we had a warehouse full of 400w metal halide fixtures, our go to was going to be a 250w LED high bay. And as you can see, our mindset was still in "watts".
Then one day I was out with my Cree manufacturer, and he said something that blew my mind; "it's not about Watts anymore; it's about 'lumens-per-watt." Well, no duh! (Huh? What are "lumens-per-watt"...?). And he was 100% correct!
At first, we were happy with our 40% energy savings, and then it quickly became 50%. But as the efficacy (efficiency) of the LED's began to grow, you couldn't just make a standardized guess anymore. The efficacy of fixtures was changing so quickly, that our "Watts" model was no longer reliable. We had to start thinking differently, and the Cree guy was right; "lumens per watt" was the new standard.
So now we have to do a little more work to find our ideal solution. But lumens per watt (lm/w) is now the reliable formula. For example, a 400w Metal Halide lamp emits 14,400 lumens. If we have chosen a fixture that emits 160 lm/w, then we can now feel comfortable in using a 90w-100w LED fixture as a replacement! That's amazing! We've gone from 40% being the average amount of savings that LED's can provide, to 75% savings being the minimum! And as technology improves, that standard will continue to go down.
I heard a couple of years ago that they had developed a 300 lm/w LED chip in a lab. They haven't come out with it yet, but I have seen a 200 lm/w UFO high bay on the market. That would allow for a 72-watt LED fixture to replace a 400-watt HID. That's crazy! But it's real, and it's only going to get better.
There's going to be a point where the ROI on developing a more efficacious LED chip is minimal, but you can bet that lighting manufacturer's will continue to chase it. And then what will replace LED's? Hopefully I'll be retired by then...!