Light and energy
Grow Light Electricity Cost Calculator
Enter the actual input wattage per fixture, the number of lights, and the operating schedule to estimate energy use and cost for any period. Replace the example electricity rate with the rate from your own bill.
Calculator
Estimate grow light energy use and cost
Enter the light's actual input wattage and your operating schedule. Results update as you type.
Before you calculate
What this estimate can tell you
Watts (W) describe the rate at which a light uses power, while electricity bills charge for energy accumulated over time in kilowatt-hours (kWh). Running the same 40 W fixture for 12 hours uses twice as much energy as running it for 6 hours. This calculator estimates energy use; wattage alone does not describe the light reaching a plant.
How grow light energy use and cost are calculated
Period energy use (kWh) = watts per light × number of lights × hours per day × days ÷ 1,000. Period cost = period kWh × the electricity rate you enter. The annual estimate keeps the same daily schedule and rate for 365 days.
Assumptions and limits
- The fixture is assumed to draw the entered wattage for the full operating period; dimming, power variation, and label accuracy can change actual use.
- Cost uses one user-entered rate per kWh and does not model tiers, time-of-use prices, seasonal rates, taxes, or other bill charges.
- Only the grow light is included. Fans, humidifiers, timer standby power, and other equipment are excluded.
Worked example
Example: one 40 W grow light for 12 hours a day
A 40 W grow light running 12 hours a day for 30 days uses 40 × 1 × 12 × 30 ÷ 1,000 = 14.4 kWh. At an example electricity rate of $0.15 per kWh, the estimated 30-day running cost is $2.16.
FAQ
Common questions
Which wattage should I enter for a grow light?
Use the fixture's input power or actual power draw, not an incandescent-equivalent or marketing wattage. For a dimmable light, a plug-in energy meter can provide a value closer to the setting you actually use.
What is one kilowatt-hour?
One kilowatt-hour is 1,000 watts used for one hour. A 100 W light running for 10 hours also uses 1 kWh. Watts measure power; kWh measures energy used over time.
What electricity rate should I enter?
Use the per-kWh rate shown on your bill or supplied by your utility. Rates can vary by location, season, usage tier, and time of day, so the calculator uses an editable example instead of claiming one universal rate.
How do I calculate two grow lights with different wattages?
If they run for the same hours, add their wattages and enter the total as one light. If they run on different schedules, calculate them separately and add the kWh or cost results.
Does a higher-wattage light always help plants more?
No. Wattage is electrical input, not PPFD or DLI at plant level. Fixture efficiency, spectrum, hanging distance, coverage area, and obstructions all affect the light that reaches the leaves.
Calculation references
- U.S. Energy Information Administration: Measuring ElectricityUsed to verify the definitions of watts, kilowatts, and kilowatt-hours and the relationship between power and operating time.
- Taiwan Power Company: Electricity Bill EstimatorUsed to confirm that an estimated appliance cost can differ from an actual bill because of usage and tariff structure.
- Taiwan Power Company: Residential Rental Electricity QueryUsed to verify the bill-level average price-per-kWh concept; the calculator therefore keeps the rate editable.