High Intensity Discharge lights convert electricity into light; they are used in greenhouses, street and parking lot lighting, sports venues and by gardeners. The lights require a ballast and common sizes include 250-, 400-, 600- and 1,000-watt lamps.
Power is expressed as watts and is a function of voltage and amperage. The formula for power is voltage multiplied by amperes for volt-amps or watts. To determine amperage, divide watts by voltage.
HID lights are a two-part system: the bulb and the ballast. The ballast contributes to the power requirements and depending on efficiency, the ballast can add to the current draw in amps by as much as 10 to 20 percent.
250 and 400 Watts
A 250-watt bulb and ballast requires about 300 watts of power and draws 2.66 amps at 120 volts or 1.32 amps at 240 volts. A 400-watt bulb and ballast requires 464 total watts of power and draw 3.8 amps at 120 volts or 1.9 amps at 240 volts.
600 and 1000 Watts
A 600-watt bulb and ballast uses about 670 watts of power and draw 5.5 amps at 120 volts and 2.85 amps at 240 volts. A 1,000-watt bulb and ballast uses 1100 watts of power, drawing 9.28 amps at 120 volts and 4.68 amps at 240 volts.