The Impact of Lumen Maintenance Losses (a thought experiment)
It’s what you don’t know – you don’t know, that will hurt you.
Here’s a thought experiment that illustrates the impact of lumen maintenance losses.
Let’s say you have two shifts of employees working on your packing line. Each shift works 10 hours, with an hour break, for a total of 18 hours of work effort.
Your second shift of employees are less dedicated and leave work 15 seconds early on their first day, then 30 seconds early the second, 45 seconds the third day, and so on. By the 30th day, they leave 7 minutes early. You might not even notice. After all, their shift ends at midnight.
On the 180th day they leave 45 minutes early. Hmmm. You might notice that. Or you might notice that they didn’t finish packing the full harvest, which is starting to pile up on the shipping floor. Fast forward to the end of their first year. They’re leaving 90 minutes early – every day. Wait, what’s going on. You’re still paying them for a full day’s work, but they’re working less. In order to avoid produce piling up, you reduce your upstream production. Because you cut production, you shipped about 4% less produce than when you started.
It continues. At the end of year two; they are leaving 3 hours early – every day. They’re giving you just 6 hours of work effort every day! You have to slow production a bit more, and now you’re producing 8.5% less than when you started. Third year, 5 hours early, your slow everything else down again. Your production is off 12.7%. Fourth year, 17%. Fifth year, 21%. Sixth year, 25%. Seventh year – almost 30% less production.
Oh yes, I forgot to mention that when you hired them, you signed a seven year no-termination contract that paid them regardless of the number of hours they worked!
BUT YOU WOULD NEVER AGREE TO A CONTRACT LIKE THAT, RIGHT?
Are you sure? That’s what you do when you buy a lighting system that has an L70 rating of 50,000 hours, and plan to use it for all 50,000 hours. The light output of LEDs gradually degrades over time (all light sources degrade). And of course, less light output translates to longer growing cycles.
So to draw the analogy to our story above. When your L70 rated LED lighting system nears its useful life of 50,000 hours, it produces 30% less light and it takes you 30% longer to complete the growing cycle. And there is nothing you can do about it. The energy consumption doesn’t decrease at all. Just like continuing to pay your employees at the same rate, you are still spending the same amount on electricity to power the lights (as well as cooling the building). And the economic impact isn’t just the fixed cost of electricity – as you cut back your production, your variable costs (other than electricity) were also reduced (assuming you can layoff some employees). The real cost is the ‘opportunity cost’ associated with having less produce to sell. Depending on the produce and your net margins, that number can be quite large.
Lumen maintenance losses matter – a lot! But don’t all LEDs have roughly the same useful life ratings? Here it gets a little more complex. It is true that LED components from the best manufacturers are relatively close. However, the decisions the fixture (entire light system) manufacturer makes about the drive current and thermal management has a major impact on lumen losses. Put another way, two light system manufacturers can use the same LED component, from the same manufacturer, and have very different lumen maintenance losses.
When evaluating LED lighting, you should evaluate the differences in LM losses between different product suppliers as closely, and perhaps more closely, than claims of higher growth from a special ‘light recipe’ (spectrum). There is no question the right light spectrum will increase growth, but the improvements could be completely wiped out by high LM losses.
This is one reason our Association publishes the LED Performance and Quality Rankings. It ranks individual products by 30 different measures, including LM losses. You can get the First Edition for free here.