Use actual wall draw
A listing that says “1000 W equivalent” may describe a marketing comparison rather than electricity consumption. Look for actual input power on the fixture label or manufacturer specification. A plug-in energy meter can measure the whole setup, including its driver and any standby consumption, if the equipment and socket are suitable.
Electrical watts do not tell you how much light reaches a plant. Two fixtures can use the same power but produce different distributions at leaf height. Keep the energy calculation separate from the PPFD-to-DLI calculation.
Three examples with the same schedule
These examples use 12 hours per day, 30 days, one fixture, and an illustrative rate of 0.20 per kWh. They are arithmetic examples, not a current tariff for any country.
| Actual draw | Energy in 30 days | Example cost |
|---|---|---|
| 20 W | 7.2 kWh | 1.44 |
| 30 W | 10.8 kWh | 2.16 |
| 60 W | 21.6 kWh | 4.32 |
For two identical lights, double the energy and energy charge. For a different rate, multiply the kWh by that rate. Use the interactive running-cost calculator to change every assumption and label the result in your own currency.
Read the bill carefully
Use the energy price that applies to the hours the light runs. A time-of-use plan may charge different rates across the schedule; calculate each period separately and add them. Standing charges usually do not increase when you add a light, so they are different from the incremental energy charge.
Taxes and fees may already be included in a quoted unit rate or shown separately. Do not count them twice. The calculator accepts your rate and does not retrieve utility prices or exchange rates.
Reduce wasted light without starving the plant
Position the fixture so light reaches the foliage rather than an empty wall. Check the manufacturer’s safe mounting distance and avoid crowding leaves against a hot surface. A timer keeps a planned schedule consistent, but a shorter schedule is not automatically better if it leaves the plant short of light.
Compare growth and actual light delivery before buying extra fixtures. The light and placement hub connects room assessment, fixture use, DLI and humidity. Record a schedule change in your journal so you can compare the next leaves with the previous setup.
Sources and further reading
Published by AI Planter Editorial Team. Read our editorial standards and tool limitations.
Frequently asked questions
Does a 30 W lamp always draw exactly 30 W?
The label is a starting point. Actual consumption can vary with settings and equipment; a suitable energy meter can measure the setup.
Should I leave a grow light on all day to maximize growth?
Do not assume more hours always help. Match the plant’s needs, the measured light level, and a suitable daily dark period.