The efficiency (or “luminous efficacy”) of lighting systems can be expressed in a basic equation: output (lumens) divided by total units of power consumption (Watts). High-efficiency lighting is generally accepted to output at least 90 lumens per Watt (lm/W), with some bulbs achieving 150 lm/W or even higher. Increasingly, greenhouse grow lights are also rated in terms of PAR (photosynthetically active radiation) output, measured in µmol/s (the number of photons per second falling on 1m² of plant matter).
Mmol/s is increasingly seen as superior across the board for horticultural lights, as it is a measure of light usable by plants. Lm/W is a measure of what is visible to the human eye. Thus, it is advisable to get to know how to calculate PAR output measurement. A guide on the concept of PAR can be found here.

HPS lights emit light by sending a pulse of high-voltage energy through a pressurized quartz tube filled with vaporized sodium, along with other elements such as xenon and mercury.
As the gases heat, they emit light. Sodium produces an intensely yellow-orange light; this can be mediated by xenon and mercury, both of which emit light in the blue end of the visible spectrum. The final result is whiter light.

LED (light-emitting diode) lights are semiconductors, which allow electrical energy to pass with little resistance in one direction and overwhelming resistance in the other, across a “p-n junction”.
Currently, the highest-performing HPS bulbs produce around 150 lm/W. For example, GE Lucalox 600W bulbs produce 90,000 initial lumens (“initial lumens” refers to mean output over the first 100 hours), while Lumatek’s high-par 600W HPS lamps produce 92,000. Although previously seldom the case, newer HPS lights are now beginning to include PAR output ratings. For example, Lumatek’s 600W bulbs have a PAR output of 1,030 µmol/s.
For powerful led grow light, lm/W is generally considered less meaningful than the PAR output. This is due to the fact that red-blue systems generally have much lower overall lm/W ratio – as they only provide the most necessary bands for photosynthesis, therefore expenditure of “unnecessary” light energy can be avoided.

However, newer cob led grow light use multiple bands to create a more complex and full spectrum of light. This is thought to be beneficial to plant growth, and modern LEDS often incorporate white along with reds and purples of the visible spectrum. These new full spectrum light may or may not list lm/W rating. However, it is generally accepted that the most reputable vendors include both lm/W and the PAR output.
For example, Kind LED, based in California, advertises the PAR values at different spots in a grow room for all of its products. Under a single light, the PAR value can change considerably from the space directly below the light, to the perimeters of the light’s reach. With Kind LED’s LumiGrow light, PAR is measured at a total value of 1856 µmol/s, with the highest figure being read directly in the centre of the square metre measured.

Theoretically, LEP’s luminous efficacy should exceed a HPS source by 15-20%, as no power is wasted on heating electrodes. In practice, light source efficacy of LEP (also known as “HEP” (high-efficiency plasma) or even simply “plasma”) grow lights range from 80 – 100 lm/W.