Most gardeners use at least 25 watts per square foot of garden space, regardless of the type of grow light. You may need less if your light is used to supplement natural sunlight, or if you are growing a plant that does not require as much light (i.e. lettuce). However, many gardeners prefer to double or even triple the recommended wattage to achieve faster growth rates. There is really no such thing as too much light, but using a big light in a small space will sometimes result in high temperatures that are difficult to control. Keep in mind that plants need periods of darkness too. Most indoor gardeners use lighting from 12 – 18 hours per day.
An average lighting system will increase your electricity cost about $8 to $20 per month. The exact amount depends on the wattage of the system and the number of hours operated. To calculate your cost, multiply the bulb wattage by the number of hours of operation and divide by 1000. This figure is the number of kilowatt-hours of electricity used (Example: a 400 watt lamp running for 18 hours will use 7.2 kilowatt-hours). Check your power bill for the cost of each kilowatt-hour. Then multiply the number of kilowatt-hours used by the cost of a kilowatt-hour (K/hr) to figure the cost to run your light for that many hours.