How warm do LED grow lights get and how do they compare in temperature to HIDs (HPS)? Do LED grow lights run cooler than HIDs? A question often asked by new growers or growers transitioning from HIDs to LEDs. Let’s get to the bottom of this commonly asked question that many get wrong.

The short answer to the question “do LED lights run cooler than HIDs” is both yes and no.

Yes, as in most decent LED lights are more efficient than HIDs. Meaning they need less electricity to produce the same amount of light.

No, as in watt for watt, both lamp types produce the same amount of heat.

Ok, to some theory. When electricity is drawn and used by an appliance, it will generate heat as a bi-product. Same with all types of grow lights, both LED and HIDs. The ratio of drawn power to heat output is constant, no matter the device. In other words, a LED grow light that draws 1000W will generate the same amount of heat as a HID/HPS lamp drawing 1000W.

The term for conversion from watts to heat is BTU, British Thermal Unit.

1 watt will always equal a generation of 3.412142 BTU/hr, rounded down to 3.4 BTU/h.

BTU can then be translated to heating of air. Heating one pound of air one degree Fahrenheit requires 0.24 BTU.

The weight of one cubic foot of air is roughly 0.08 lbs.

Let’s put these numbers into a real world example.

The volume of air in a 3x3x6’ grow tent equals 54 cubic foot. Lamp, pots, plants, and other items inside the 3×3’ grow tent will take up some of the volume so let’s round down the 54 cbf to 45 cbf.

45 cbf of air = 3.6 lbs of air

A 100W grow light will produce 100*3.4 = 340 BTU. 

Calculation : 

0.24 BTU per lb of air * 340 BTU / 3.6 lbs of total air = 22.6F temperature increase

This is not the actual temperature increase in practice though. The example above doesn’t take into account that the tent is cooled both by the ambient room temperature outside the tent, incoming cool air, exhausted warm air, and cooling by air movement (fans).

The example above would, however, be true if the tent was completely insulated and with no in/outgoing air.

In practice, speaking from experience, a ~200W LED grow light will increase the temperature in a 3×3’ tent by about 10F (7C) with two small fans and an ambient temperature of 72F (22C).

So, no matter what lamp type is used, whether it’s HID, HPS, CH, CMH, CFL, LED, COB, QB, or any type of lamp that runs on electricity, the temperature increase is determined by the watts drawn and nothing else.

LEDs are typically more efficient than HIDs.

LEDs give more light per watt and also more light projected to where it’s needed. HIDs, including HPS, run at around 0.7-1.0 umol/J efficacy in the lower wattages (<300W) but become more and more efficient as the wattage increases. The most efficient HIDs, the double ended 1000W HIDs for instance, run at around 1.5-1.8 umol/J efficacy.

Very low end LED light fixtures, the cheap kind you see on Ebay and Amazon, typically run at 0.7-1.0 umol/J, comparable to low wattage HIDs. Decent LED fixtures run at around 1.5 umol/J and high end LED lamps (high end COBs and QBs) achieve 2.0-2.5 umol/J.

LED lamps consists of diodes. These diodes are soldered to an aluminium board. This means that the light these diodes emit will be projected at maximum a 180 degree angle. Some lamps use lenses to narrow down the angle further. 

HIDs on the other hand, they use bulbs that emit light in all directions, 360 degrees. Reflectors fixed on the sides of HIDs reflect *most* of the light down in the “right” direction, but some still goes wasted.

Unless growing in a large space, LEDs typically project more of their light to the correct area; the area with growing plants.

This means that LEDs need to put out less light in total to still deliver the same amount of light to a certain area, a canopy, for instance. This behaviour in combination with being more efficient (having higher efficacy) means that LEDs (usually) need to consume less electricity (watts) to deliver the same quantity of light to the plants compared to HIDs. This means that a HID drawing 600W could potentially be replaced by a LED lamp(s) drawing 300-400W without the plants receiving less light. If a 600W HID is replaced by a 300W LED, the heat generated will be cut in half.

So, once again, watt for watt, HIDs and LEDs generate exactly the same amount of heat but LED lamps typically need fewer watts to emit the same amount of light that is useful for plants.