Grow light electricity cost: calculate the real monthly bill

To calculate a grow light’s monthly electricity cost, use its actual wall draw, timer hours, number of operating days, and your all-in electricity rate. The basic calculation is transparent:

monthly cost = (watts ÷ 1,000) × hours per day × days × price per kWh

Calculate your own setup

Live estimate

Use wall watts when you have them. Add fans, pumps, or controllers separately so the result describes the whole growing setup.

Grow light draw W Other equipment W Runtime each day h Days used days Electricity price /kWh Currency EUR · € USD · $ GBP · £ CAD · CA$ AUD · A$

Estimated energy cost

€6.83

30-day run

Energy used

27.3 kWh

Per operating day

€0.23

12 similar periods

€81.90

65 W × 14 h × 30 days ÷ 1,000

Cost alone does not tell you whether a fixture is useful. Pair this calculation with the measurement method in PPFD and DLI for indoor herbs so you know what the purchased electricity delivers to the canopy.

Find the four honest inputs

Actual wall watts

Use a plug-in electricity meter if possible. Otherwise, find “input power” or actual consumption in the technical specification. Ignore model names such as “1000” and incandescent-equivalent marketing. A dimmable fixture also needs a measurement at the setting you actually use.

Include associated equipment when budgeting a system. Fans, pumps, heat mats, and smart plugs may be small individually but operate for many hours. Measure them separately so you can see where changes matter.

Daily operating hours

Read the timer rather than estimating. If a light runs from 06:00 to 20:00, that is 14 hours. For a weekly schedule with different days, add the week’s hours and divide by seven.

Do not reduce hours blindly just to save money. A shorter photoperiod lowers daily light unless intensity increases. Review how many hours grow lights should run for herbs before changing the plant’s schedule.

Days per billing period

Use the actual billing period for reconciling with a bill or 30.4 days for an average month. A seedling setup running for six weeks should be calculated for 42 days rather than projected as a permanent annual cost.

All-in electricity price

Divide the relevant variable electricity charges by kWh on your bill when tariffs are complex. Fixed connection charges usually exist whether the lamp runs or not, so they should not be attributed entirely to the garden. Time-of-use rates require separate calculations for hours in each rate band.

Work through one example

A fixture measured at 60 W runs 14 hours daily for 30 days:

60 ÷ 1,000 × 14 × 30 = 25.2 kWh

At €0.25/kWh:

25.2 × €0.25 = €6.30 per month

If a 5 W fan runs for the same period, it adds 2.1 kWh and €0.53. The combined energy cost is about €6.83. Round only at the end.

Calculate annual and crop-cycle cost

Annual cost is not always monthly cost multiplied by twelve. Many growers use seedling lights only in spring or reduce indoor growing in summer. Add the actual seasonal periods.

For crop-cycle cost, multiply daily kWh by the number of growing days. A 30 W microgreens light used for 12 hours across a 10-day light period consumes 3.6 kWh. Divide by saleable or consumed harvest mass only if you measure that mass consistently.

The purpose is not to prove indoor food is cheaper than outdoor agriculture. It is to understand the convenience, learning, freshness, and cost of your particular system.

Compare fixtures without false economy

A 40 W bar is not automatically cheaper than a 60 W bar if two 40 W bars are required for even coverage. Compare the power needed to deliver a chosen PPFD across the same usable area.

Map both fixtures at canopy height. Note average and minimum PPFD, footprint, mounting height, heat near leaves, and wall watts. A useful comparison metric is watts per adequately lit shelf area, supported by the full light map.

Purchase price also matters. Calculate a simple first-year cost:

fixture price + timer + mounting + first-year electricity

Do not assume projected harvest value will repay the fixture unless you keep real yield records.

Reduce cost without starving plants

First prevent light from missing the canopy. Align bars with rows, keep plants at similar height, and use safe reflective surfaces where appropriate. Improving coverage can allow a lower dimmer setting.

Second, light only the area in use. An empty half-shelf under a full fixture wastes energy. Consolidate young plants while maintaining airflow.

Third, use a reliable timer. Manual switching leads to accidental long days and inconsistency. See best grow-light timers for the selection logic.

Fourth, manage heat as part of the room. Electrical energy eventually becomes heat. In winter that heat may partly offset space heating; in summer it may add cooling demand. Do not claim this as a saving without measuring the household system.

Common calculation mistakes

• Using equivalent watts rather than input watts.
• Forgetting to divide watts by 1,000.
• Multiplying by 24 hours when a timer runs 14.
• Comparing tariffs without taxes and variable fees consistently.
• Ignoring fans and pumps in a complete setup.
• Assuming dimmer percentage equals power percentage.
• Comparing cost without comparing delivered light and area.

Keep a one-line monthly record

Record the meter reading, timer setting, tariff, calculated kWh, and crop area once per month. If the bill changes unexpectedly, this gives you a real baseline. It also makes fixture upgrades testable rather than speculative.

Handle variable tariffs and standby power

If electricity has peak and off-peak prices, split the schedule into separate calculations. A 60 W fixture running ten off-peak hours and four peak hours uses the same energy as any other 14-hour schedule, but not necessarily at the same cost. Move the photoperiod only if the timing still provides a consistent dark period and does not create unwanted heat or light in the home.

A smart plug, timer, or driver can draw power while the lamp is off. A meter that records cumulative kWh captures this automatically. For a manual calculation, measure standby power and add its off-hours separately.

Compare cost per useful outcome

For seedlings, divide crop-cycle energy by the number of healthy transplants, not cells sown. For herbs, record usable harvest mass. For a decorative kitchen garden, the outcome may be convenience rather than the cheapest leaves.

These comparisons matter only when plant quality is acceptable. Cutting DLI until basil stretches lowers the bill while potentially increasing cost per useful harvest. Pair financial records with the light and harvest log.

Home solar needs its own marginal price. Electricity used when panels would otherwise export may cost the lost export credit; energy drawn after sunset costs the import tariff. Calling solar electricity automatically free obscures that difference.

Recalculate whenever the tariff, timer, dimmer, or growing area changes; an old estimate is not a meter reading.

Sources and further reading

• U.S. Energy Information Administration: Measuring electricity explains watts, watt-hours, and kilowatt-hours.
• University of Minnesota Extension: Lighting for indoor plants provides plant-light context for duration and intensity decisions.