The formula
kWh per day = watts × hours ÷ 1,000. Multiply by the number of days, then by your rate. For 100 watts over 16 hours: 1.6 kWh a day, 48 kWh per 30 days, and about $8.74 at 18.2¢.
Which rate should I use?
The default of 18.2¢ is the EIA's forecast for the 2026 US residential average. State rates vary widely, so use the rate on your own bill. Add delivery charges if they're billed per kWh.
Use actual watts
Use the power the light really draws from the wall. "Equivalent" or "replaces" wattage is a marketing number. If you want to measure a light, a plug-in electricity monitor shows true draw.
Typical monthly costs
| Setup | Actual watts | Per month |
|---|---|---|
| Herb shelf | 40 W | About $3.49 |
| Seedling shelf | 80 W | About $6.99 |
| 3 × 3 tent of tomatoes | 200 W | About $17.47 |