Understanding HID (MH/HPS) Grow Lights - Lighting Buyers Guide

Before we get started, for those interested in our staff's recommendations of best HPS and MH grow lamps, scroll to the bottom for quick links for 1000w, 600w and 400w options.

The difference between HPS, MH and HID Grow Lighting

HID, what is it? Firstly, HID stands for High Intensity Discharge. In easy terms, it's a type of light bulb. A very large light bulb (some about the size of your forearm). It's a sealed tube filled with gas that creates light when the gas is ignited. A regular household light bulb is similar. A regular light bulb has a small metal filament inside that lights up, but in an HID bulb it has two electrodes that arc electric current between them to light up the gas. (think old school mad scientist laboratory).

HID bulbs are defined by the type of gas used in the bulb. The primary types used in horticulture include metal halide (MH), and high pressure sodium (HPS). Here are two pictures below showing you the visual differences between HPS and MH lamps. You can see the HPS lamps are more long and slender while MH lamps are a bit more....well....bulbous :)

High Pressure Sodium (HPS) LampHigh Pressure Sodium (HPS) Lamp
High Pressure Sodium (HPS) Lamp
Metal Halide (MH) Grow LampMetal Halide (MH) Grow Lamp
Metal Halide (MH) Grow Lamp


HID lighting systems consists of three main components, a ballast used to power the HID bulb, a reflector or hood to maximize the bulbs efficiency (by directing light over your plant surface), and a bulb.

Back to our two types of bulbs associated with HID lighting; high pressure sodium (HPS) and metal halide (MH) -- there are also "dual arc" bulbs which are a sort of combination of both. The main difference between the two bulbs is the type of light spectrum they produce (because of the gas inside). Different gases produce different colors of light when ignited, and thus we can choose which color/type of bulb to use depending on what stage of growth a plant is in.

HPS vs MH grow light spectrum differences

The HPS has a red or orange tint to it and is used primarily in the flowering phase of your plant's life. The MH bulb focuses on more of a blue spectrum and is used primarily for the vegetative stage of plant life.

HPS vs MH Grow Lamp Spectral DifferencesHPS vs MH Grow Lamp Spectral Differences

hps and MH grow area by wattage

When choosing an HID system the first thing to take into consideration is the grow area you want the lighting fixture to illuminate. Based upon this you can choose from several different wattage systems; 1000, 600, and 400 watts are the most common. A 1000 watt system will cover about a 5 x 5 ft area, a 600 about a 4 x 4, and a 400 will cover an area of about 3.5 x 3.5 ft.

choosing the correct ballast

Ballasts are integral to a bulb because an HID bulb creates an arc of electricity inside, it needs something to regulate and shape the electricity into a smooth flow for the bulbs to produce smooth constant light without flicker or distortion. There are two types of ballasts that can be used for HID lighting, magnetic and digital. A magnetic ballast is a tried and true technology that operates using heavy coils and condensers to regulate the power to the bulb. A digital ballast uses a much lighter circuit board to regulate the power at a much higher frequency than a magnetic ballast. This means a more constant stream of energy to your bulb for an increase in lumen output.

Digital vs Magnetic Grow Light BallastsDigital vs Magnetic Grow Light Ballasts
Magnetic ballast on the left, Digital ballast on the right


Another useful feature of some digital ballasts is the ability to dim the power level being used from 100% to 75% to 50%. Most ballasts these days even have a "super lumen" capability, pushing your bulb to its maximum capabilities. Due to the high frequency of digital ballasts, digital bulbs are required. This means magnetic ballast bulbs are not recommended for operation in digital ballasts due to the risk of bulb failure, and vice versa. For a system to function properly, the wattage of ballast and bulb must also be equal. It is also important to note that magnetic ballasts will not run both HPS and MH bulbs, but digital will have no problem running either.

HPS & MH Reflectors

HID bulbs require a hood or reflector to maximize their efficiency by reflecting light to where you want it. There are several different styles of reflectors, air cooled, water cooled, wing, and parabolic. Each style reflector has its niche use. Generally, the larger wattage HID system in use, the larger the reflector is recommended to maximize your growing footprint. For example, a 1000 watt bulb works well in a Sun System Magnum hood. When selecting a reflector, knowing the exact size of the space being used is imperative. Different sized reflectors are designed for specifically sized areas.

Air Cooled HPS and MH Grow Light ReflectorAir Cooled HPS and MH Grow Light Reflector
Air Cooled Magnum Reflector

When using HID lighting a considerable amount of heat is created. Therefore, when choosing a hood, be sure to make accommodations for the heat being produced. This can be accomplished by purchasing an air or water cooled reflector.

These hoods come with a sealed glass "window" on the bottom, and open ports on each side to connect ducting or hose via; the idea being to contain as much heat as possible within the reflectors and ducting. With either the passage of cool air or water through the hood-- and ventilated outside the growing area-- temperatures are greatly reduced.

best 1000w hps grow lamp

We did our own independent testing of the best 1000w HPS lamps using a photosynthetically active radiation (PAR) meter over a grow area. The top two testing single ended HPS lamps is the Hortilux. See the full test, Best 1000w HPS Lamp Comparison Test.

best 1000w MH grow lamps