HPS/MH 300-400 Watts Lamps: The perfect power for your medium-sized grows

HPS and MH lamps in the 300-400 watt range represent the sweet spot for many indoor growers. This power range offers an excellent compromise between light output and heat management, perfect for medium-sized grow spaces. Whether you opt for an HPS during flowering or an MH during vegetative growth, this power guarantees solid results without skyrocketing your electricity bill.

These lamps effectively cover areas from 80x80cm to 100x100cm depending on your setup height. With a PPFD of around 400-600 µmol/m²/s at 60cm distance, you are in the optimal zone for efficient photosynthesis.

Why choose a 300-400W HPS/MH lamp?

Frankly, this power is the right balance. Unlike 600W lamps that heat up like ovens, 300-400W lamps remain manageable in terms of temperature. You can run them in a room without turning your apartment into a sauna, especially if you have proper extraction that moves at least 150-200 m³/h.

The spectrum of HPS at this power delivers approximately 55,000 lumens, more than enough to boost your yields during flowering. MH lamps of the same power excel in vegetative growth with their blue spectrum that promotes compact and vigorous development. Light penetration easily reaches 80-100cm deep, allowing you to grow taller plants without losing density on the lower branches.

The other big advantage is their lifespan. A well-maintained 400W HPS lamp will easily last 2-3 full cycles before losing efficiency. And in terms of consumption, we're talking about 0.4 to 0.45 kWh actual with the ballast, which is about €10-12 per month when operating 12h/12h depending on your electricity tariff.

How to choose your 300-400W lamp

First, define your space. For an 80x80cm grow tent, a 300W is more than enough. If you go up to 100x100cm or more, opt for a 400W. Height also matters: with less than 1m80 total height, stick to 300W to avoid burning your plant tops.

Regarding ballasts, magnetic ones are still indestructible but generate more heat. Electronic ballasts are more expensive but more efficient and often allow you to vary the power (250W/400W/600W on the same ballast). For the reflector, a good Adjust-A-Wing or a hammered reflector distributes light better than a simple smooth reflector.

Don't forget extraction: plan for a minimum of 200 m³/h to dissipate the heat from a 400W, and 150 m³/h for a 300W. A good silent extractor will help avoid noise pollution, especially if your setup runs at night.

Frequently Asked Questions

What size grow tent can a 300-400W HPS/MH equivalent lamp cover?

A 300W lamp effectively covers an area of 80x80cm, or 0.64m². For a 400W, you can go up to 100x100cm (1m²) with a good reflector. The optimal hanging height is between 40 and 60cm from the plants depending on the power. The higher you raise the lamp, the larger the covered area but the intensity decreases. The ideal is to aim for 400-600 µmol/m²/s at canopy level.

How many plants can I put under a 300-400W lamp for good growth?

Under a 300W, 4 to 6 plants in 7-11L pots yield good results. With a 400W, you can go up to 6-9 plants depending on your growing technique. In SOG (Sea of Green), count 9-16 small plants per m². In SCROG, 1-4 plants are sufficient if you train them correctly. The important thing is to uniformly fill the space under the lamp without creating significant shadows between plants.

What is the difference between a 300-400W LED and HPS/MH lamp in terms of heat and yield?

A 400W HPS lamp emits about 1200-1400 BTU/h of heat, compared to 800-1000 BTU/h for an equivalent LED. In terms of efficiency, HPS lamps achieve 1.2-1.4 µmol/J compared to 2.0-2.8 µmol/J for good LEDs. A 200W LED can replace a 400W HPS in terms of PPFD, while generating half the heat. However, HPS lamps remain unbeatable for deep light penetration and are much cheaper to purchase.

How to install a 300-400W lamp in my tent without overheating the room?

Extraction is key: a minimum 200-250 m³/h extractor with a carbon filter if needed. Place the fresh air intake at the bottom of the tent and the exhaust at the top, on the opposite side. An oscillating fan helps to homogenize temperatures. Hang the lamp with yo-yos to easily adjust the height. If the room gets too hot, schedule a nocturnal cycle (lamp on at night) to take advantage of cooler outdoor temperatures.

How to maintain a 300-400W HPS/MH lamp so it lasts a long time?

Change the bulb every 2-3 cycles or when you lose 10-15% light intensity. Clean the reflector and protective glass every 2-3 weeks with isopropyl alcohol. Regularly check electrical connections, especially on magnetic ballasts that vibrate. Avoid repeated on/off cycles that prematurely wear out the bulb. An electronic ballast with soft start significantly extends the lifespan of your bulbs.