Grow Light Cost Calculator: Running Cost per Month

What the grow light cost calculator does

This grow light cost calculator tells you what your grow lights cost to run. Enter the power each light uses in watts, how many hours a day it's on, your electricity price per kWh, and how many lights you have. The calculator shows:

• Cost per month, year and day
• Energy use per day, month and year in kilowatt-hours (kWh)

Costs come out in whatever currency you enter the price in: pounds, dollars, euros or anything else. Enter the price as a decimal, so 28 pence or cents becomes 0.28. A month is taken as 30.42 days (365 ÷ 12), so twelve months add up to a year.

How it calculates

1. Energy per day (kWh) = watts × hours per day × number of lights ÷ 1,000
2. Cost per day = energy per day × price per kWh
3. Cost per month = cost per day × 30.42
4. Cost per year = cost per day × 365

A kilowatt-hour is 1,000 watts running for one hour. That's the unit on your electricity bill.

Worked example

A 30 W LED bar light, on 12 hours a day, with electricity at 0.30 per kWh:

• Energy per day = 30 × 12 ÷ 1,000 = 0.36 kWh
• Cost per day = 0.36 × 0.30 = 0.11
• Cost per month = 0.108 × 30.42 = 3.29
• Cost per year = 0.108 × 365 = 39.42

Add a second identical light and the cost doubles to 78.84 a year. Switch both to 14 hours a day and it rises to about 92 a year.

Typical grow light running costs

At 0.30 per kWh:

For most houseplant setups, running costs are modest. The big bills come from large tent lights used for food crops.

How many hours should grow lights be on?

Use a timer and stick to a consistent schedule, because plants respond better to regular days than to lights switched on and off at random. University of Minnesota Extension suggests total daily light of:

Iowa State University Extension recommends 12 to 14 hours for most species and no more than 16 hours a day for home growers, because plants need a dark period. If plants also get some window light, you can often use fewer hours of artificial light; University of Missouri Extension notes 12 to 14 hours may be enough with some natural light, and 16 to 18 hours if the plants get none.

Some flowering plants respond to day length. Christmas cactus, poinsettia and kalanchoe need short days, under about 11 to 12 hours of light, to set buds. Don't leave grow lights on long over them in autumn; see Christmas cactus care.

Actual watts vs "equivalent" watts

Many grow lights are advertised with a large "equivalent" number, such as "1000 W", that compares them with old high-pressure sodium lamps. The real power draw might be 100 W. Always use the actual wattage, which you'll find:

• On the label or plate on the light or its power adaptor
• In the product specification as "power consumption" or "actual power"
• By measuring with a plug-in energy monitor, which is the most accurate

Using the advertised figure can overstate costs tenfold.

Light output: what are you paying for?

The cost of running a light only matters if it delivers enough light. Two lights with the same wattage can deliver very different amounts of usable light. Look for:

• PPF (µmol/s): total plant-usable light emitted.
• Efficacy (µmol/J): PPF divided by watts. Good modern LEDs reach 2 to 3 µmol/J; older or cheap models are often below 1.5.
• PPFD (µmol/m²/s) at a stated distance: how much light reaches your plants.

Higher efficacy means more light for the same electricity. To check whether your plants are getting enough, convert PPFD and hours into daily light integral (DLI) with the plant light meter converter. Our grow lights for houseplants guide explains choosing and positioning lights.

Ways to cut grow light costs

Electricity prices

Your price per kWh is on your bill, often listed as the "unit rate". Prices vary widely by country and tariff. For reference, the US Energy Information Administration publishes average residential electricity prices by state, and Eurostat publishes household electricity price statistics for European countries. If your tariff has a daily standing charge, leave it out; that's paid regardless of the lights.

Heat and other costs

LEDs produce some heat, but much less than incandescent or high-pressure sodium lamps, so they rarely change your heating or cooling bill noticeably for a few houseplant lights. Other costs to consider:

• Purchase price of the light, which often matters more than running costs for small setups.
• Replacement: quality LEDs last 25,000 to 50,000 hours, which at 12 hours a day is six to eleven years. Fluorescent tubes need replacing more often.
• Timers and smart plugs: a one-off cost that pays back by preventing lights from running all night by mistake.

Is a grow light worth it?

For a dark flat, a north-facing room or a windowless office, a small LED grow light costing a few units of currency a month can make the difference between plants that survive and plants that grow well. It also lets you grow plants that need bright light, like succulents and crotons, away from windows. Compare the running cost with the cost of replacing plants that decline in low light. For plants that cope with dim rooms without lights, see best low-light houseplants and the low-light plant finder.

Worked comparisons

Shelf of herbs and foliage plants. Three 20 W LED bars, 14 hours a day, at 0.25 per kWh: 0.84 kWh a day, about 6.39 a month and 76.65 a year.

Single fiddle leaf fig in a dark corner. One 45 W panel, 12 hours a day, at 0.30 per kWh: 0.54 kWh a day, about 4.93 a month. See fiddle leaf fig care.

Seed starting in spring. Two 30 W lights, 16 hours a day for 8 weeks, at 0.30 per kWh: 0.96 kWh a day, about 16.13 over the eight weeks.

Office with no windows. One 15 W LED bulb, 10 hours a day on weekdays only (about 22 days a month), at 0.20 per kWh: about 0.66 a month. See plants for an office with no windows.

Setting up lights efficiently

1. Measure your natural light first, so you know how much to add. The plant light meter converter helps.
2. Choose an efficient LED with published PPF or PPFD figures.
3. Position it correctly: most LEDs work best 15 to 60 cm above the foliage, depending on power; follow the manufacturer's guidance. Iowa State suggests 12 to 24 inches (30 to 60 cm) for many LEDs.
4. Plug it into a timer and set the hours for your plants.
5. Watch the plants for two to four weeks: stretching means too little light; bleached or crispy leaves mean too much or too close.
6. Adjust hours or height before buying more lights.

Common grow light mistakes

• Running lights 24 hours a day. It wastes electricity and most plants grow worse without a dark period.
• Mounting the light too high. Light intensity drops quickly with distance, so a light 1 m above a plant delivers far less than the same light at 30 cm. You pay the same electricity for much less useful light.
• Using "equivalent" watts to compare lights or estimate cost.
• Buying the cheapest unbranded panels with no published PPF or efficacy. They often deliver less light per watt.
• Forgetting the timer. A light left on overnight for a month can add a noticeable amount to a small bill.
• Lighting empty space. Arrange plants tightly under the light's footprint so light lands on leaves, not the floor.

Cost per plant

If several plants share one light, divide the monthly cost by the number of plants. A 60 W shelf system lighting twelve plants at 7.67 a month works out to about 0.64 per plant per month, often less than the cost of replacing one plant a year that would otherwise struggle in a dark spot.

Limits of the calculator

• It uses a constant power draw. Dimmable lights use less power when dimmed; enter the actual power at your dimmer setting if known.
• It doesn't include standing charges, taxes beyond your unit rate, or time-of-use pricing. For time-of-use tariffs, use the rate for the hours the light is on.
• It doesn't estimate light output or plant growth; use the light converter for DLI.

Related tools and guides

• Plant light meter converter: lux, foot-candles and DLI.
• Grow lights for houseplants: choosing and placing lights.
• Plant humidity checker: another factor for plants in dark rooms.
• Plant watering schedule builder: plants under lights often dry faster.

Sources: University of Minnesota Extension, Lighting for indoor plants; Iowa State University Extension, Supplemental lights for indoor plants; University of Missouri Extension, Lighting indoor houseplants.