First, LEDs do produce heat, just not nearly as much due to efficiencies. Hence, a 300w LED can be cooled with a heat sink or a small computer fan while a 300w HID needs a squirrel cage. LEDs only produce light in few wavelengths while an HID produces a ton light across a wider spectra including yellow and orange light which equals heat. An HID also literally burns inside the bulb. So 300w of HID = tons more heat than 300w of LED. That much I'm sure of. We need khyberkitsune in here to straighten this out...