Related guides: wattage and PPFD guide · indoor grow-light guide
Lighting cost is easy to estimate and useful before buying equipment. It is separate from whether the fixture provides the right photons for your crop.
Convert watts to kilowatts
Divide fixture watts by 1,000. Multiply by hours per day and days in the billing period to estimate kilowatt-hours. A 100-watt light running 16 hours per day for 30 days uses about 48 kWh.
Use the real rate
Multiply kWh by your electricity rate. If your utility has time-of-use pricing, use the rate for the actual schedule or calculate separate periods. Add fans and pumps if you want a system estimate.
Keep performance in view
A cheaper light is not automatically better if it covers less area or produces weak growth. Compare cost per usable canopy area and use PPFD or a manufacturer’s map where available.
The basic calculation
Estimated energy use in kilowatt-hours is watts divided by 1,000, multiplied by hours per day, multiplied by the number of days. A 60-watt fixture running 14 hours per day for 30 days uses 25.2 kWh before any dimming or standby draw. Multiply that energy use by the rate on your electricity bill to estimate the lighting portion of the cost.
For the next step, see the DLI calculator guide.
Budget the real schedule
Use the schedule you actually expect to run, not a fixture’s maximum rating if it will be dimmed. If a system has separate drivers, pumps, fans, or a heater, calculate them separately so lighting is not mistaken for the full operating cost. Rates, taxes, and time-of-use plans vary by location.
Cost is only one part of the decision. A lower-watt fixture that does not cover the canopy can create stretch or uneven growth; a higher-watt fixture may add heat that requires ventilation. Compare energy with useful coverage using the wattage and PPFD guide.
Keep a simple log
- Fixture wattage or measured draw.
- Photoperiod and dimmer setting.
- Days in the billing period.
- Local energy rate and any separate equipment.
The tools hub contains the current calculation tools. Use the result as a planning estimate, then compare it with the actual bill after a full growing cycle.
