Looking for a 400 Watt grow bulb? We have many options to choose from in both HPS (High Pressure Sodium) and MH (Metal Halide). When determining the size of grow bulb required it is important to know the two main components; size of the growing area and desired wattage per square foot. Simply determine the square footage of the grow area and multiply by the desired wattage per square foot, resulting in the bulb size needed. For EXAMPLE: 8 sq feet of grow area and a desired wattage of 50 would require a 400 watt bulb.