GuidesLighting & costs
How much does a grow light cost to run?
One formula, four numbers. Here's how to work out what an LED grow light really costs in electricity — and the mistakes that make estimates wrong.
You'll learn
- The difference between watts, kW and kWh
- The formula from power draw to monthly cost
- How to get honest numbers for your own light
Who this is for: anyone deciding whether to add an LED grow light for seedlings or indoor plants, and wanting to know the running cost before buying.
Understanding grow-light electricity usage
Electricity is billed by energy, not by power. A light's wattage tells you how fast it uses energy; how long it runs decides how much energy it uses in total.
Power — a rate
How fast the light draws electricity right now. 100 W = 0.1 kW.
Energy — an amount
Power × time. 0.1 kW running for 10 hours uses 1 kWh. This is the unit on your bill.
The formula, explained
- W ÷ 1000Power in kilowattsThe light's actual electrical draw in watts, divided by 1000.
- × hHours per dayHow long the light is really on each day.
- × daysNumber of daysThe period you're costing — usually a month.
- × €/kWhPrice per kWhWhat you pay for each kilowatt-hour of energy.
The first three terms give energy in kWh. Multiplying by the price per kWh turns energy into money.
A worked example
These are illustrative assumptions — not measurements from my office. €0.25/kWh is a round example price, not the tariff of any particular country or supplier.
- Light100 W
- On16 h/day
- Period30 days
- Price€0.25/kWh
- Convert watts to kilowatts100 W ÷ 10000.1 kW
- Energy used each day0.1 kW × 16 h1.6 kWh / day
- Energy used each month1.6 kWh × 30 days48 kWh / month
- Monthly running cost48 kWh × €0.25€12.00 / month
How to calculate your own cost
Find the light's actual power draw
Many lights are sold with an “equivalent” wattage (for example “1000 W equivalent”) that is far higher than what they draw.
Do this: Look for the actual power consumption in watts in the specifications — not the equivalent figure.
Measure it at the outlet when you can
A plug-in energy meter between the socket and the light shows the real draw, and many can total kWh over a week.
Do this: Plug the light into a meter and note the watts at your normal brightness setting.
Use your real daily hours
Cost depends on how long the light actually runs, not the maximum it could run.
Do this: If a timer runs the light 14 hours a day, use 14.
Find the price you actually pay
Your bill shows the price per kWh. Some tariffs add per-kWh network charges or taxes on top.
Do this: Use the full per-kWh price. On a time-of-use tariff, work out the hours in each price band separately.
Calculate the cost
Put the four numbers into the formula — or let the calculator do it.
Do this: Multiply kW × hours per day × days × price per kWh, and label the answer as an estimate.
Common mistakes
Trusting the advertised wattage
An “equivalent” wattage describes brightness compared with older lamps, not electricity use. Use the real power draw.
Assuming maximum hours
Costing a light at 24 hours a day when a timer runs it for 14 overstates the bill.
Guessing for dimmable lights
Dimming usually lowers the power draw, but not necessarily in exact proportion to brightness. Measure at the setting you use.
Using the wrong price
Tariffs differ between suppliers, plans and times of day. Use your own per-kWh price, not a headline average.
Forgetting other equipment
Fans, heaters, pumps and dehumidifiers can use more than the light. Cost them separately and add them up.
Mixing estimates with measurements
A calculated figure is an estimate. Energy you've measured with a meter is a measurement — keep them labelled differently.
Understanding real-world limitations
The formula is exact for a light that draws a constant amount of power. Real costs differ with dimming, driver efficiency, standby draw and other equipment on the same circuit.
Your bill may also include charges that don't depend on how many kWh you use, such as fixed standing or connection charges. Treat the result as a solid basic estimate, not a reproduction of your bill — unless you measured the energy used.
Use our free calculator
This guide teaches the method. The calculator does the arithmetic with your own numbers.
- Enter watts, hours per day, days and your price per kWh
- See daily and monthly kWh, and monthly and yearly cost
- Any currency — and the formula it used, shown in full
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