Plants are much more sensitive to decreases in light intensity and color spectrum than the human eye, so it can be tough to judge when grow bulbs need replacing.
For optimal growth, Hortilux recommends replacing HPS lamps at 9-10 months and metal halide lights at 6-8 months. After that, the light begins to diminish, slowing growth and impacting crop yields. The old bulb doesn't need to go to waste. You can use it as supplemental light for mother plants or other plants that do not require aggressive growth.
So how do you know how much your electric bill is going to go up after installing your light system? Sunlight Supply offers this helpful formula:
How much energy will my light use?
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 X 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.
If you're thinking it's time to stock up on bulbs, replacement grow lamps are 10% off at GreenTree through the end of April!