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

What HID means

High intensity discharge. It describes how the light is made, not what the light is for.

A household bulb glows because current heats a metal filament. An HID bulb has no filament. It has two electrodes sealed inside a tube of gas, and the current arcs across the gap between them, exciting the gas until it gives off light. That arc is why an HID bulb needs a ballast: striking it takes a much higher voltage than mains supplies, and once struck, the arc has to be held steady or it runs away.

So HID is a family, not a product. Metal halide, high pressure sodium and ceramic metal halide are all HID bulbs; what separates them is the gas and metal salts sealed in the tube. Car headlights labelled HID work the same way, which is why a search for HID grow lights and a search for headlights land in the same technical territory and nowhere near the same aisle.

This also explains the one thing that catches people out. The gas determines the colour of the light, and it determines the electrical behaviour of the arc. A ballast built to strike one kind of arc will not necessarily strike another, and that constraint runs through everything below.

MH, HPS and CMH

What does HPS stand for? High pressure sodium. MH is metal halide, and CMH is ceramic metal halide — the newer one that tries to do both jobs.

Metal halide

Metal halide runs blue-weighted. That spectrum keeps internodes short and growth compact, which is what you want from a plant that is building structure rather than fruit. MH is the vegetative lamp and the mother-room lamp.

High pressure sodium

HPS runs red and orange, and of the three it puts out the most raw light per watt. It is the flowering lamp, and it has been the commercial default for decades for exactly that reason. Run an HPS lamp through veg and plants stretch.

Ceramic metal halide

CMH uses a ceramic arc tube instead of quartz, which lets it run hotter and hold a broader, more daylight-like spectrum. The practical consequence is that one CMH lamp can carry a plant through the whole cycle rather than being swapped at flip. CMH also arrived at lower wattages than the old HID standards, which is why 150W and 315W are the numbers you see.

Those two charts are the whole argument in one picture. The HPS trace is a spike in the yellow-orange with very little below 500 nm. The metal halide trace has real energy across the blue and green as well. Neither is a complete spectrum on its own, which is the gap CMH was built to close.

Ballast, bulb, reflector

An HID grow light is three products that have to agree with each other.

The ballast

The ballast supplies the striking voltage and then regulates the arc. There are two kinds. A magnetic ballast does it with a heavy transformer coil and a capacitor: simple, durable, heavy, and it hums. A digital ballast does it with a circuit board at a much higher switching frequency: lighter, usually dimmable, and often switchable between bulb types.

Read the ballast's own label rather than a rule of thumb. Some magnetic ballasts are convertible and will run both MH and HPS; some are wired for one type only. Most digital ballasts run either, and many run several wattages. The wattage of bulb and ballast has to match in every case, and a dimmable ballast dims the whole system rather than letting you run a bigger bulb softly.

The HID ballast listings here are both 315W CMH units, one a remote ballast at 120/240V and one an electronic ballast running 120–277V; both are in the fixtures table further down. At the larger wattages the listings are complete fixtures rather than separate ballasts.

The reflector

A bare bulb throws light in every direction, including straight up. HID fixture reflectors, also sold as grow light hoods, put it on the canopy instead. Styles differ by how they trade spread against intensity: a wing is open, low-cost and spreads wide, a parabolic wraps the bulb for even coverage, and an air cooled hood seals the bulb behind glass with duct ports at each end so the heat leaves through ducting instead of into the room. Air cooled HPS grow lights are the usual answer in a tent where the lamp heat has nowhere else to go.

Whatever style you choose, size it to the room rather than to the bulb. A hood built for a 1000W lamp in a 5 x 5 space will put most of its light on the walls of a 3 x 3.

Three things to check before anything else

The wattage on the ballast and the wattage on the bulb must be the same number. The bulb types the ballast lists must include the one you are fitting. And the socket in the reflector must be the base your lamp has. Those three agree or the system does not run, and no amount of quality in any one component compensates for a mismatch in another.

What each wattage covers

Wattage is the first decision, and it follows from the floor area you are lighting.

A 1000W system covers roughly 5 x 5 ft in flower, a 600W about 4 x 4, and a 400W about 3.5 x 3.5. CMH at 315W sits near 3 x 3 and at 150W near 2 x 2. Every one of those numbers is larger in veg than in flower, because vegetative growth tolerates lower intensity at the edges while flowering does not.

Two things move those figures. Hang height is the obvious one: raise the lamp and the footprint widens while intensity at the canopy drops. The reflector is the less obvious one, and it is why two growers running the same 1000W bulb can report very different coverage.

If the room is bigger than one lamp covers, two smaller lamps almost always beat one large one. Overlapping footprints even out the intensity across the canopy, and the edges of a single big footprint are where yields disappoint.

The heat comes with the watts

An HID lamp converts most of its input into heat, and that heat lands in the room whatever the bulb type. A 1000W system is a 1000W heater running however many hours your photoperiod is, and the extraction, the air conditioning and the dehumidification all have to be sized for it. This is the cost people forget when they compare a 1000W HID against a lower-wattage alternative — the lamp is only part of the bill.

It is also the argument for air cooled hoods and for the move to 315W ceramic in smaller rooms. Taking the heat out at the reflector, before it reaches the canopy, is cheaper than removing it from the whole room afterwards.

Every HID bulb listed

Nineteen HID grow bulbs, from a 150W ceramic lamp to a tray of 1000W double-ended sodium. These are the HID grow lamps themselves; the hardware they run in is in the next section.

HID grow light bulbs are listed here by type and wattage, which are the two columns that decide it for almost everyone. Read the type column first and the wattage second. Any lamp with "double ended" in its name needs a double-ended fixture, and the rest are single-ended mogul base. That naming holds for every lamp in this table, which makes it a reliable shortcut.

The shape of that list is worth noticing. HID has concentrated at two points — 315W ceramic for the grower who wants one lamp for the whole cycle, and 1000W double-ended sodium for commercial flowering. The single-ended 400W and 600W middle that defined HID horticulture for twenty years is now the thinnest part of it.

Fixtures and ballasts

At the lower wattages the category has largely stopped being three separate parts.

A complete fixture arrives as one unit with the ballast, socket and reflector already matched. Nothing can be mismatched because nothing is chosen separately, and for anyone building HID grow light systems from scratch that removes the most common and most expensive mistake. The trade is flexibility: a fixture is replaced rather than upgraded.

Note the voltages. Several of these are three-phase commercial units at 277V, 347V or 480V, which a domestic supply will not run. Check what the room is wired for before shortlisting, because voltage narrows the field faster than wattage does at this end of the range.

FAQs

The questions that come up most often about HID horticulture lights.

HID vs HPS, and CMH vs HPS — which is better?

The first question does not quite parse, because HPS is a type of HID — an HID HPS grow light is simply a sodium lamp. The real comparisons are HPS against metal halide and CMH vs HPS, and both are answered by which stage of the cycle you are lighting rather than by which is better overall. CMH gives you a broader spectrum and one lamp for the whole cycle; HPS gives you more output per watt in flower.

Can I use MH and HPS bulbs in the same ballast?

Sometimes. Convertible magnetic ballasts have a switch for it, and most digital ballasts run either type. A ballast wired for one type will not strike the other. The label on the ballast is the only reliable answer.

What is the difference between a single-ended and a double-ended lamp?

A single-ended lamp screws into a mogul socket with one contact. A double-ended lamp is a slim tube clipped at both ends and needs a fixture built for it. DE lamps generally run more efficiently and degrade more slowly, which is why commercial 1000W is almost entirely DE now.

How long does an HID bulb last?

Output falls before the bulb fails, which is the trap. A lamp that still lights can be well down on its original output, so commercial rooms replace on a schedule rather than waiting for a failure. Follow the figure on the lamp's own documentation.

Do I still need an air cooled hood?

It depends on where the heat can go. In a sealed tent with no other extraction, an air cooled reflector is usually the difference between a workable canopy temperature and an unworkable one. In a room with proper air handling it matters less.

The bottom line

Decide the wattage from the floor area, the bulb type from the stage, and then make the ballast and reflector agree with both.

If you are running one room through a full cycle and want to change as little as possible, 315W ceramic is the answer the catalogue has converged on, and a complete fixture removes the compatibility question entirely. If you are flowering commercially, 1000W double-ended sodium is still the output leader, and the choice there is between lamps rather than between technologies.

Whatever you pick, the three constraints do not move: bulb type and ballast must agree, wattage must match exactly, and single-ended and double-ended are different fittings. Get those right and HID is one of the most predictable tools in hydroponics and indoor growing, and HID hydroponics setups are still running rooms that were built around them twenty years ago. Get one wrong and nothing lights at all.

If you want a hand matching a lamp, ballast and hood to a specific room and ceiling height, get in touch and we will work through it with you.