Complete Guide to MH & HPS Grow Lights for Hydroponics

Complete Guide to MH & HPS Grow Lights for Hydroponics

Metal Halide (MH) and High-Pressure Sodium (HPS) lights have been around forever in indoor growing. Even with LEDs becoming more common, MH and HPS are still some of the most reliable and predictable lights you can run. They hit hard, they penetrate deep, and they grow plants fast when set up correctly.

This guide breaks down everything you actually need to know about using MH and HPS lights in a hydroponic system — spectra, wattage choices, heat management, spacing, bulb life, ballast options, reflector types, and common mistakes people make with these lights. If you’re looking for a real, no-nonsense explanation instead of vague summaries, this is it.

1. Why MH and HPS Still Matter

Growers keep using HID lights because they’re consistent. They pull real wattage, they give predictable results, and they don’t have the guesswork that comes with cheap LEDs. MH and HPS lights produce high-intensity light that drives photosynthesis aggressively — which is exactly what hydroponic plants respond to.

Even though they use more electricity and produce more heat, their raw growth performance is still tough to beat for the price.

2. The Difference Between MH and HPS

MH and HPS aren’t interchangeable in terms of what they do for the plant, even though they often fit into the same fixture. They produce completely different light spectra that push plants into different growth behaviors.

2.1 Metal Halide (MH)

• Spectrum: Bright white/blue.
• Best stage: Vegetative growth — early to mid-cycle.
• What it does: Short internodes, dense leaf production, thicker stems.
• When to use: Seedlings → young plants → mid veg.

MH produces the kind of light plants naturally get in spring. It keeps plants compact instead of stretching. If you want tight structure and solid root establishment, MH is the light that does it.

2.2 High-Pressure Sodium (HPS)

• Spectrum: Orange/red.
• Best stage: Flowering and fruiting.
• What it does: Boosts bud/fruit development, increases yield weight.
• When to use: Late veg → flowering/fruiting.

HPS mimics fall sunlight. It pushes plants to shift energy away from leaves and into reproductive growth — flowers, fruits, peppers, tomatoes, you name it.

Most hydro growers run MH in veg, switch to HPS in flower, and get the best of both worlds.

3. HID Wattage, Coverage, and Hanging Heights

The wattage you choose determines how much usable space you can actually light properly. Too weak and plants stretch. Too strong and you’re just bleaching the canopy and heating the room for no reason.

Note: These are starting points. Keep an eye on leaf temperature and adjust based on plant response.

4. Ballasts: Magnetic vs. Digital

The ballast powers the bulb, and the type you choose affects efficiency, heat, and lifespan.

4.1 Magnetic Ballasts

• Old-school, heavy, reliable
• Run hotter
• Usually cheaper

They work, but they’re bulky and inefficient by today’s standards.

4.2 Digital Ballasts

• Quieter and cooler
• More efficient
• Dimmable options (50% / 75% / 100%)
• Often compatible with both MH and HPS

Most growers should choose digital unless they’re working with older equipment they already own.

5. Reflectors and Hoods

Reflectors decide how evenly light spreads across the canopy. A strong bulb in a cheap reflector is wasted potential.

5.1 Open Reflectors

Basic metal wings. Good heat dissipation. Light spread is wide but less controlled.

5.2 Air-Cooled Hoods

Glass-sealed with duct ports. Lets you run more wattage in a smaller room.

5.3 Cool Tubes

Cylindrical glass tube with built-in ventilation. Not the best spread but unbeatable for heat control.

Tip: Some reflectors support dual bulbs or have sockets for switching MH ↔ HPS without swapping the fixture.

6. Electrical Requirements & Safety

HID lights draw real wattage, so you want to make sure you aren’t overloading circuits or running hot wires.

• A 600W HID pulls roughly 5.5 amps on 120V.
• A 1000W HID pulls around 9 amps on 120V.
• Never run more than 80% of a circuit’s capacity continuously.
• Use timers rated for at least 15 amps.
• Keep ballasts off the floor — they run warm.

Simple and boring, but it prevents fires and headaches.

7. Heat Management

HPS and MH lights get hot — that’s just the nature of HID bulbs. Hydroponic plants hate heat stress, so ventilation matters.

• Use inline fans with air-cooled hoods if possible.
• Keep grow room temps around 70–78°F.
• Use oscillating fans for leaf movement.
• Avoid blowing cold air directly onto the bulb.

The hotter the bulb runs, the faster it degrades.

8. Bulb Lifespan & Replacement Schedule

Even if a bulb still lights up, that doesn’t mean it’s producing strong PAR output. HID bulbs slowly fade over time.

• MH bulbs: Replace every 6–9 months of actual use.
• HPS bulbs: Replace every 9–12 months.

They’ll still glow past that point, but yields will noticeably drop.

Important: Never touch bulbs with bare hands. Oils from skin create hot spots that shorten bulb life or cause failure.

9. Common Problems & How to Fix Them

9.1 Plants Stretching

Usually means the light is too far away or underpowered. Lower the fixture or upgrade wattage.

9.2 Leaf Burn

Light too close or canopy too hot. Raise the fixture or improve airflow.

9.3 Hot Spots Under the Hood

Cheap reflectors cause uneven coverage. Switching to a better hood usually fixes it.

9.4 Bulb Flickering

Almost always ballast-related. Check wiring and ensure MH/HPS compatibility.

10. Final Thoughts

MH and HPS might not be the new kids on the block, but they still deliver strong, predictable results — especially in hydroponics where fast growth is the entire point. If you run them safely, keep bulbs clean, manage heat, and pair them with a good reflector, they’ll outperform most cheap LEDs with ease.

Use MH for veg, HPS for flower, and size your wattage to your grow space instead of overkilling it. Do that, and these lights are still some of the most straightforward, dependable options you can run.