Right, but one light can use that same amount of energy more efficiently in the conversion of electrical energy to radiant energy.
@Yodaweed How much studying of Thermodynamic black boxes did you do where the plant was in the closed loop?
The problem though is that the difference is negligible over the long haul.
Even a very good COB setup isn't double as efficient as a MH/HPS. It's closer to aorund 25% on average. That means that to replace a 600 watt MH you'd need to be running around 450 watts of COB lighting.
The formula for energy cost is simple. It's:
(Wattage x Hours Used)/1,000 x Electric Rate ($/kWh) = Total Cost
All you do is plug in your numbers. Let's say for arguments sake I'm running a 450 watt cobb against your 600 watt HPS. Here in Georgia, the average Kilowatt per hour cost is 11.1 cents. Here's how they stack up:
450 Cobb on 12 hours on 12 hours off flowering for 8 weeks:
(450 x 12) / 1000 x 11.1 = .4864 per day
8 weeks is 56 days x .4864 =
$27.24 total for the 8 weeks.
600 HPS on 12 hours on 12 hours off flowering for 8 weeks:
(600 x 12) / 1000 x 11.1 = .6486 per day
8 weeks is 56 days x .6486 =
$36.32 total for the 8 weeks.
A WHOPPING savings of $9.08
So when you consider that the typical 450 watt COBB light will cost you double what the typical 600 watt HPS, then there is absolutely no savings at all unless your power cost is so astronomical it's ridiculous. (The average across the U.S. is 12 cents.)
The other issue is the heat. As mentioned earlier, a watt is a watt when it comes to heat. A watt generates 3.412 btu no matter what. So then we look at heat output:
450 watt COB: 1535.4 BTU
600 watt HPS: 2047.2 BTU
So you're looking at 511.8 BTU increase in heat for about the same amount of light. It's not really that much of a savings at all.
They're simply not worth it if you already have a light that's working well for you. About the only time I would even remotely consider getting one would be if your ballast blew up on your HPS.
Even then, I'd probably just buy another ballast as they're only a fraction the cost of the new fixture for very little savings on anything heat or power wise.
The only time it really makes any sense at all is if you're doing huge grows with tons of lighting. I'm talking about 10 to 12 fixtures. Then the overall heat and energy savings would be well worth it. The heat especially. But just one or two lights doing your own thing?
Nope. Not worth it. At all.