November 04, 2020


By Shane Torpey


Should you upgrade from HPS and MH to CMH or LED?

Spectrum comparison

Traditionally growers used Metal Halide (MH) bulbs during the VEG cycle due to the high percentage (23%) of blue in the MH spectrum.

Blue regulates internodal distance (the distance between branches). If the proportion of blue exceeds about 6% than the plants will have short internodal distances and develop short and dense growth in the VEG stage. This is ideal for indoor growers who are generally limited for space. The downside of MH bulbs for growing is their relative inefficiency compared to High Pressure Sodium (HPS) bulbs.

HPS bulbs are 30%+ more efficient than MH bulbs but they only have around 3% blue in the spectrum. This means they have the power to generate high yields and a large bud mass in the flowering stage. However, due to the low percentage of blue in the spectrum the plants will also stretch excessively.

A recent addition to the market is Ceramic Metal Halide (CMH) bulbs. They are more expensive per watt but they have a good spectrum (about 11% blue) and a higher efficiency than HPS in most cases. This makes them ideal for growing from seed to flower with the one bulb. You will get more growth during the veg stage due to the higher efficiency and less stretching in the flower stage due to the high percentage of blue in the spectrum. The one downside of these fixtures is that HPS ballasts cannot be used with these bulbs (you have to get special ballasts for CMH bulbs) and the bulbs are more expensive than MH or HPS.

Even more recently the ceramic HPS bulb has been released to the market. Some (but not all) of these bulbs can even be run on HPS/MH ballasts making the switchover much cheaper.

Gone are the days of the 'Blurple' LED grow light (blue and red diodes only). Most LED grow lights now emit a neutral white spectrum which is very similar in output to CMH and is optimised for ideal plant development and high yield.

HPS vs CMH vs LED Spectrum comparison  

Cost comparison

There is a wide range of fixture cost for HID grow lights. Generally for a 600W MH or HPS system with digital ballast and a quality reflector you will need to spend about $200 to $300. The same sized CMH system is more expensive and the equivalent LED grow light is more expensive again.

However, when the running costs are taken into account over three years the more efficient CMH is the most cost efficiency HID option. It is an easy switch to ceramic HPS if you have a digital ballast, just replace your MH and HPS bulbs with a ceramic HPS one. Alternatively you could upgrade your ballast and bulbs to CMH and you will get payback within a few years compared to running MH and HPS.

The initial higher investment in the LED light pays off very quickly. The total cost of the LED system compared to the best HID system is 30% less over three years.

HPS Vs CMH Vs LED cost comparison 
The break even point to recover the additional cost in LED is now only 6 months due to the increase in LED efficiency and reduction in cost for LED fixtures. It is now difficult to justify the description of HID lights as cheap if you are taking into account running costs.
HID Vs LED cost comparison graph 

Heat output

The other main consideration is heat output of the lighting system. HID will emit about 75% of its consumed power in heat and HID bulbs emit a lot of radiated hear in the for of infra red as the bulbs glow at above 320 DegC or 600 DegF. The LED systems emit about 40% of its consumed power as heat into the grow tent and emit almost radiated heat as the led surface temperatures are low. LEDs will generally run at less than 50 DegC or 120 DegF.


Comparison of LED and HPS heat output 
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