How grow light costs add up
A grow light costs money for every hour it is on. The power it draws in watts, times the hours per day, gives the energy in watt hours; divided by 1,000 that is kilowatt hours (kWh), the unit on your electricity bill. Because the light schedule changes between phases, a grow is easiest to price phase by phase: a few weeks of seedlings on a dimmed light, the vegetative phase on long days, and the flowering phase on shorter days.
Light is only part of the climate: the Plant VPD Calculator checks whether temperature and humidity suit the growth stage.
Example: one LED in a grow tent
A 300 W LED in a grow tent, with an exhaust fan of 40 W running all day, and electricity at $0.17 per kWh:
| Phase | Length | Schedule | Light kWh | Cost incl. fan |
|---|---|---|---|---|
| Seedling, light at 50% | 2 weeks | 18/6 | 37.8 | $8.71 |
| Vegetative | 4 weeks | 18/6 | 151.2 | $30.27 |
| Flowering | 8 weeks | 12/12 | 201.6 | $43.41 |
| Whole grow | 14 weeks | 390.6 | $82.40 |
The fan adds 0.96 kWh a day, 94.08 kWh over the 98 days.
Light schedules
The schedule is written as hours on and hours off. 18/6 is common for seedlings and the vegetative phase, 12/12 for flowering photoperiod plants, and 20/4 or 18/6 for autoflowering plants, which do not need the switch. Going from 18 to 12 hours cuts the light's energy use by a third, which is why the flowering phase often costs less per day even though it lasts longer.
How to use the grow light cost calculator
- 1Enter each light's real power draw and how many you run, then add fans, a dehumidifier or a heater if you use them.
- 2Set the length of each phase in days, weeks or months, the light hours per day and, if you dim the light, its power level.
- 3Enter your electricity price per kWh. The total, the cost per phase and per day appear right away; add your expected harvest for the cost per gram or ounce.
Frequently asked questions
How do I calculate the electricity cost of a grow light?
Multiply the watts the light draws from the wall by the hours it runs per day, divide by 1,000 to get kWh, and multiply by your price per kWh. A 300 W LED on an 18/6 schedule uses 5.4 kWh a day, which is about $0.92 a day at $0.17 per kWh. For a whole grow, do this for every phase and add them up, which is what this calculator does.
Which wattage should I enter for an LED?
The actual power draw from the wall, listed on the spec sheet or measured with a plug-in power meter. Many LEDs are sold as "1000 W equivalent" while they draw 100 to 200 W. If you dim the light, enter the dimming level per phase, since an LED at 50% draws roughly half the power.
What light schedule is common for each phase?
Many indoor growers run seedlings and vegetative plants on 18/6 (18 hours on, 6 off) and switch photoperiod plants to 12/12 to trigger flowering. Autoflowering plants often stay on 18/6 or 20/4 the whole time. Check what suits your plants; the calculator works with any schedule.
Should I include fans and other equipment?
Yes, for a complete picture. Exhaust and circulation fans often run all day, and a dehumidifier or heater can cost more than the light itself. Add them as equipment with their own hours per day; they run through every phase of the grow.
How can I lower the cost of a grow?
Use an efficient LED instead of HPS, dim the light for seedlings, and if your utility has time-of-use rates, run the lights during cheap night hours. A shorter vegetative phase also saves energy, though it means smaller plants.