HPS vs LED Grow Lights

Looking back in time

LED grow lights have been improving steadily for years. Early models from years ago made some ridiculous claims about their performance. For example, that a 90 watt LED unit was equal in performance to a 400 watt HPS.

There's only one way to put this - the claims being made 10 years ago were very misleading. Growers that trustingly believed the hype and tried them out ended up being sorely disappointed. The reasons for that were two-fold:

Firstly, most of the early units only included 2 colours of LED - red and blue (sometimes called "blurple"). This was based on the knowledge that photosynthesis is mostly performed by chlorophyll and chlorophyll is mostly activated at just those 2 colour points. It seemed to make sense that for photosynthesis (which drives plant growth), the only requirement was to hit those 2 colour points, and that no other colours were needed. The result of the very limited colour spectrum resulted in plants with an odd structure. They were thin, stringy, and generally looked unhealthy.

Secondly, the PAR light output per watt was comparatively lower than HPS. Yield is roughly proportional to the amount of PAR light that a plant is given. Because of the lower PAR output per watt, the LED grow lights of the day produced less yield per watt of electricity than the benchmark HPS.

How things have progressed

These days, we know much more about the real light spectrum preference of plants - they thrive under full spectrum light and will grow optimally and normally. The electronic component LEDs themselves have advanced tremendously. White LEDs are now more available and affordable. The advances in technology have meant that the efficiency of all colours of the component LEDs has risen considerably.

The full spectrum output of LED grow lights means that plants grown under them look far more natural in terms of growth pattern, and much more healthy. The increase in efficiency of the component LEDs has resulted in much greater yield per watt.

What about advancements in HPS?

While LED grow light performance has been steadily on the increase, there have been a couple of worthwhile advancements in HPS technology over the last couple of decades. First was the introduction of digital ballasts which had a positive impact on the efficiency of an HPS Grow Light system. Second was the innovation of 400 volt technology which improved both efficiency and the spectrum slightly. The HPS lamps themselves (especially the 400 volt types) have likewise improved with premium models delivering results never experienced from HPS before.

So, back to the HPS vs LED Grow Lights Question - Which is Best?

From the information that we have been given by experts in the industry, the improvements in both HPS and LED grow light technology are slowing down as they approach the pinnacle of what can be achieved.

With fewer and smaller improvements likely to be coming along in the future, we can now make some meaningful comparisons regarding what is possible with the 2 technologies.

With that in mind, let's talk about spectrums, the amount of PAR light output per watt, and footprint coverage.

Comparison of Light Spectrums:

Different brands of HPS lamps have very comparable spectrums. Yes, they do vary a bit, and yes, those variations do make difference. However, they're not night and day. A typical HPS light spectrum, such as from a Gavita Pro 400V grow light, is as follows:

As you can see, the spectrum is very heavily biased towards the orange-red part of the spectrum. The orange and red part of the spectrum is what produces yields and, it is because of this, that HPS became the go-to type of grow light.

However, there is some yellow and green which helps a lot with plant health. Unfortunately, there isn't a lot of mid-green or (more importantly) blue which helps to keep plants short, and improves crop quality considerably.

Now let's look at the spectrum from a typical, well designed, full spectrum LED grow light like the Gavita Pro 1700e:

The spectrum from the included red LEDs gives a generous helping of the productive red part of the spectrum. The included white LEDs provide plants with plenty of yellow, green and cyan which is great for plant health. There's also plenty of blue which will not only keep plants nice and short, but most importantly makes a huge difference in the final quality of the crop.

With different colour LEDs being available it means that there are more possible variations in the final spectrum than there is with an HPS. However, the manufacturers of the very best LED grow lights have pretty much all settled on a spectrum that is fairly similar to the one above.

Everything considered, the full spectrum from a quality LED grow light will bring out the best in plants.

Comparison of Efficiencies

Most people are familiar with the term "Watts". It is a measure of power usage. The way we measure the efficiency of a grow light is by how many micromoles of PAR light that it produces for each watt (actually, it's Joules, but we won't go into that here) of electricity you put into it.

HPS grow lights have come on a lot in the last 20 years or so. Back then, a basic HPS grow light comprising a standard lamp and a magnetic ballast would produce as little as 1.5 micromoles per watt. However, these days, the very best of modern lamps and ballasts can achieve as much as 2.1 micromoles per watt.

By comparison, the best LED grow lights are now kicking out around 2.7 micromoles per watt. This is a big advancement over the first LED grow lights which produced nowhere near that amount.

So, it's 2.7 micromoles per watt for LED, and 2.1 for HPS. The battle of efficiency goes to LED as well.

Coverage

The light from a grow light needs to be spread out over an appropriate size area. The objective is to achieve a reasonably even intensity of light from directly underneath it right out to the edges of the grow space.

The element in an HPS is small. In fact, it is so small that it can almost be considered to be a "point-source". For a point light source, the light has to travel further to get to the edges of a grow area than it does to directly under the reflector. Light intensity goes down with distance following an inverse square law. What might seem like a small increase in distance can actually make quite a large reduction in light intensity.

This means that the job of the reflector to spread that light reasonably evenly over an area is a tricky one. It would seem intuitive that a large reflector would be better at spreading the light out evenly than a small one. Actually, this is not really the case. Some of the best reflectors are are small ones such as those produced by Gavita and Dimlux. All things considered, a modern, quality reflector actually manages to do a pretty good job.

For an LED grow light, in the case of a compact unit, the spread is achieved by how the LEDs themselves are arranged and pointed, and also often by a lens. In a well-designed, quality unit, this achieves a fairly uniform intensity about as good as a decent HPS reflector.

Because multiple LEDs are used in a grow light, there are alternative arrangements which are possible. They do not have to be all clumped together in a small unit. There is one particular arrangement that ensures the most even footprint possible. It looks a "grid" which is almost the same size as the grow space that it is designed to illuminate. For example the 645w Gavita 1700e ML consists of 8 spread out bars containing the LEDs. The unit is about 112cm square and it is designed to cover an area of 120cm square. Most peoples' first impressions of this arrangement is that it looks a bit big and clumsy. However, it achieves an almost perfectly even footprint which is simply not possible with an HPS.

Other Differences between HPS and LED grow lights

Heat: HPS lamps get very hot. In fact, at full temperature they are at several hundred degrees centigrade. Care needs to be taken so that nothing accidently touches them when they are on. LED grow lights still produce heat but the heat is spread out, most usually in a heatsink. There is no single point that runs particularly hot. This means there is far lower chance of accidental injuries and they are much less of a fire hazard. If you run a HPS grow light then we highly recommend you invest in a Mabo or Fireblitz fire extinguisher to safeguard you, your grow, and your property.

Also, because the best LED grow lights are more efficient than HPS, less heat is produced overall for a given wattage. This makes it easier to keep a grow-space within the correct temperature range, particularly during the warmer months.

Cost of purchasing and running: To get comparable (or slightly larger) yields from an LED grow light we recommend you use about the same number of watts as an HPS that you would use for that area. For example a 600w HPS grow light is generally considered ideal for a 120cm square area. So, for that size area we recommend about 600 watts of LED lighting. That is why the Gavita model mentioned above which is intended for a 120cm square area is rated at 645 watts. Because of this, LED grow lights are not really that much cheaper to run.

Quality LED grow lights are generally more expensive than a similar wattage HPS. However, HPS lamps deteriorate much more quickly and need to be replaced regularly while LED grow lights should last an absolute minimum of 5 years, if not much longer. Let's say that in that time you would change out a £20 HPS lamp 20 times, that comes to £400. This means that, over a bit of time, an LED grow light can turn out to be the cheaper option.

Conclusions

Here is a summary of the Pros and Cons of HPS grow lights:

  • Pro - Can be cheap to purchase
  • Con - They are now less efficient than a quality LED grow light
  • Con - The lamps run extremely hot (risk of injuries and fire)
  • Con - Perfectly uniform footprints are not possible
  • Con - Spectrum is good for yields but not quality

Here is a summary of the Pros and Cons of LED grow lights:

  • Pro - Spectrums are possible which produce high yields, healthy plants, and great quality crops
  • Pro - Do not have any intensely hot parts
  • Pro - Last for many years with little or no maintenance
  • Pro - High quality models are now more efficient than HPS
  • Pro - Grid type models produce close to a perfectly even footprint
  • Con - High initial outlay for quality models