Multiply the lights' watts by hours per day, divide by 1,000, and multiply by your price per kWh. Four trays of tomatoes under ten 10 W LED bars cost about $8.74 per 30 days at the 2026 US average of 18.2 cents/kWh.
Open this example in the Grow Light Planner
The formula
kWh per day = watts × hours on ÷ 1,000
cost = kWh per day × price per kWh × days
Your price per kWh is on your electric bill. If you don't have it handy, the U.S. Energy Information Administration forecasts a 2026 US residential average of 18.2 cents/kWh (Short-Term Energy Outlook).
Worked example: 4 trays of tomatoes
Four 1020 trays of tomatoes on one 48 × 24 in shelf: 10 LED bars at 10 W each, on 16 hours a day for 6 weeks, plus 4 heat mats for the first week.
- Lights: 100 W × 16 h ÷ 1,000 = 1.60 kWh a day × $0.182 = $0.29 a day.
- Per 30 days: $8.74. For the 42 days to transplant: $12.23.
- Heat mats: 72 W × 24 h ÷ 1,000 = 1.73 kWh a day, for 7 days: $2.20.
At your electricity price
The same setup at different prices per kWh:
| Price per kWh | Lights per 30 days | Season total |
|---|---|---|
| 12¢ | $5.76 | $9.52 |
| 18.2¢ | $8.74 | $14.43 |
| 25¢ | $12.00 | $19.82 |
| 35¢ | $16.80 | $27.75 |
By number of trays (tomatoes, 18.2¢/kWh)
| Trays | Light wattage | Lights per 30 days |
|---|---|---|
| 1 | 30 W | $2.62 |
| 2 | 60 W | $5.24 |
| 4 | 100 W | $8.74 |
| 8 | 200 W | $17.47 |
| 12 | 300 W | $26.21 |
Ways to spend less
- Match hours to the crop. Lettuce runs 11 hours, not 16, so 4 trays of lettuce cost $6.01 per 30 days instead of $8.74. Minnesota recommends 12–16 hours for most seedlings (Starting Seeds Indoors).
- Dim panels that overshoot. If panels give more light than the trays need, dimming them cuts the bill.
- Use heat mats only where they help, and take them away once seeds sprout.
Run your own numbers
The free BedMath Grow Light Planner works this out for your trays, shelves, crop, and lights, and shows every formula and source. This link opens it with the example above filled in.
Sources
Published 2026-11-03. Worked examples come straight from the planner's current math.