How it works
kWh/day = wattage (W) × hours ÷ 1000; cost/month = kWh/day × 30 × rate ($/kWh); cost/year = kWh/day × 365 × rate
Electricity is billed by the kilowatt-hour — the energy of 1,000 watts running for one hour — so the running cost is just power times time times your price per kWh. A 240-watt fixture on for 18 hours uses 240 × 18 ÷ 1,000 = 4.32 kWh a day; over a 30-day month that is about 130 kWh, and at the 2025 US average of roughly 17.3 cents per kWh it costs close to $22 a month, or about $268 a year. The number that matters is the fixture’s real power draw, not the rating on the box — measure it with a plug-in energy meter if you can, because a light dimmed below full output draws proportionally less. The choice between LED and HPS shows up here too. Because a modern LED converts electricity to photosynthetic light far more efficiently — about 2.8 µmol per joule versus 1.7 for a double-ended HPS (Kusuma, Pattison and Bugbee, 2020) — an LED delivers the same photon output as an HPS for roughly 40% less power. Over a long flowering cycle that efficiency gap, plus the reduced cooling load because LEDs waste less energy as heat, often pays back the higher purchase price. Your electricity rate varies widely by state and time of day, so enter the figure from your own bill for an accurate result.
Sources
- U.S. EIA — Electricity explained: Measuring electricity One watt-hour is the energy of one watt used for one hour; a kilowatt-hour is 1,000 watt-hours — the unit electricity is billed in, and the basis for energy = power × time.
- U.S. EIA — Factors affecting electricity prices The 2025 US average residential electricity price is about 17.3¢ per kWh, with wide variation by state and season — the default rate used here.
- Kusuma, Pattison & Bugbee (2020), Horticulture Research 7:56 "From physics to fixtures to food: current and potential LED efficacy." Photon efficacy ≈2.8 µmol/J for the best LEDs versus ≈1.7 µmol/J for double-ended HPS — the basis for the LED-vs-HPS energy comparison.
FAQ
How much does it cost to run a grow light?
Multiply the wattage by the hours per day, divide by 1,000 for kilowatt-hours, and multiply by your rate. A 240-watt light on 18 hours a day uses about 4.32 kWh daily, so at the US average of roughly 17.3 cents per kWh it costs close to $22 a month or about $268 a year. Smaller propagation lights cost only a few dollars a month; large fixtures running long days are where the bill grows.
Are LED grow lights cheaper to run than HPS?
Yes. A modern LED produces roughly 2.8 micromoles of photosynthetic light per joule of electricity versus about 1.7 for a double-ended HPS, so it delivers the same light for around 40% less power. It also wastes less energy as heat, which trims cooling costs. The LED usually costs more upfront, but over months of daily operation the lower running cost and reduced heat load typically make it the cheaper choice overall.
What wattage should I enter?
Use the fixture’s actual power draw, not the number implied by its name or the sum of its diodes. Many LED lights are marketed by an inflated "equivalent" wattage, while the real draw is printed in the specifications or measured with a plug-in energy meter. If you run the light on a dimmer below full output, its draw falls proportionally, so enter the dimmed figure for an accurate cost.
How can I lower my grow light electricity cost?
Switch to an efficient LED, run only the wattage your crop’s target PPFD and DLI actually require, and avoid over-lighting a small canopy. If your utility charges time-of-use rates, running the photoperiod during off-peak hours can help. Good reflectivity and correct hang height put more of the light you pay for onto the plants, and matching photoperiod to the crop avoids paying for hours of light it cannot use.
Does the calculator include cooling and other costs?
No — it prices the light’s own electricity only. Fans, dehumidifiers, pumps and air conditioning add to an indoor garden’s total energy use, and HPS setups in particular need more cooling because they run hotter. Treat this figure as the lighting line item and add your other equipment separately for a full picture of what the grow costs to run each month.
Estimates only. Actual cost depends on the fixture’s real power draw, your dimming level, run time and local electricity rate, which varies by state and time of day. The 30-day month and LED-vs-HPS figures are planning approximations. Measure with an energy meter and use your own tariff for the most accurate result — general guidance, not professional advice.