Grow Light Cost Calculator

Grow lights turn a basement shelf or a spare corner into a seed-starting station, and they keep tender plants alive through the winter. They also appear on your electric bill, and most people have no idea by how much. This grow light cost calculator tells you what it costs to run your lights for any number of lights, wattage, hours per day and days, at your own electricity price. It shows the total for the period, the cost per day and the cost for a 30-day month. It uses US dollars and kilowatt-hours, like the rates on your utility bill.

The math is simple, but it is easy to get wrong, because bulbs are sold by watts and bills are written in kilowatt-hours. The calculator does the conversion for you, so you can compare different lights, different schedules and the cost of starting your own seedlings against buying transplants.

How to use the grow light cost calculator

1. Enter the number of lights. Count each fixture or bulb you will run together.
2. Enter the power of each light in watts. Use the actual wattage printed on the label or the power supply, not the equivalent wattage on the packaging. The hint under the box explains the difference.
3. Enter the hours on per day. Seedlings are usually given a long day, and the guide below suggests typical schedules.
4. Enter the number of days. For a seed-starting season, that is usually 6 to 10 weeks.
5. Enter your electricity price per kilowatt-hour. Your rate is on your electric bill. Use the all-in rate, including delivery charges, if you want the true cost.
6. Read the results. You get the total cost for the period, the energy used in kilowatt-hours, the cost per day, the cost per 30 days and the total power draw.

The grow light cost math

A kilowatt-hour is the energy used by 1,000 watts running for one hour. Your bill charges you for each one.

Energy per day (kWh) = lights × watts each × hours per day ÷ 1,000
Total energy (kWh) = energy per day × number of days
Cost = total energy × price per kWh

Worked example: a seed-starting shelf

You run two 40 watt LED shop lights for 14 hours a day over 60 days, at 17 cents per kilowatt-hour. The energy per day is 2 × 40 × 14 ÷ 1,000 = 1.12 kWh. Over 60 days that is 67.2 kWh, and at $0.17 the cost is $11.42. That works out to about 19 cents a day, or $5.71 for every 30 days.

Worked example: a bigger setup for overwintering

A larger rack runs four 100 watt LED fixtures for 16 hours a day over 90 days, at 20 cents per kilowatt-hour. The energy per day is 4 × 100 × 16 ÷ 1,000 = 6.4 kWh, which is 576 kWh in 90 days. At $0.20, the cost is $115.20, which is $1.28 a day or $38.40 for every 30 days.

Worked example: a single small light

A single 24 watt LED strip runs 16 hours a day for 45 days at 15 cents per kilowatt-hour. It uses 24 × 16 ÷ 1,000 = 0.384 kWh a day, and 17.3 kWh in 45 days. The cost is $2.59.

Actual wattage and equivalent wattage

Lighting is often labeled with an equivalent wattage, which tells you how much light the product gives compared with an old incandescent bulb. A bulb marked as equivalent to a 100 watt bulb may actually draw only 15 watts or so. Your bill is based on what the light really draws, which is the actual wattage. You will usually find it printed on the base of the bulb, on the fixture label or on the power supply, and it is often written as W or watts. Some grow lights advertise a large number that is the power of the diodes at their maximum rating, not what the light draws from the wall, so check the draw if you can.

If you cannot find the wattage, a plug-in electricity meter, sold inexpensively at hardware stores, measures what any device actually draws.

How long to run grow lights

Seedlings are commonly grown under lights for about 14 to 16 hours a day, with a rest of 8 to 10 hours in darkness. Plants use the dark period too, so running lights around the clock is not recommended. For many tender plants kept through winter, shorter days of about 10 to 12 hours are usually enough, since they are not trying to grow fast. A cheap outlet timer keeps the schedule steady, and it is the easiest way to save money by making sure the lights never stay on by accident.

How many lights do you need?

Light intensity falls off quickly with distance, so the aim is to keep lights close and cover every plant. A standard four-foot LED or fluorescent fixture generally lights a strip about as wide as one seed tray along its length, and wider fixtures or several in a row cover more. Many gardeners hang lights from chains so they can raise them as plants grow. A common starting point is to keep fluorescent tubes a few inches above seedlings, while stronger LEDs may need to sit farther away, so follow the manufacturer’s advice for the distance.

If plants stretch thin and tall, the light is too weak or too far away. If leaves look bleached or scorched, it is too close. The seed tray planner can help you work out how many trays you need to light, and from there how many fixtures.

LED and fluorescent lights

LED lights have largely replaced fluorescent tubes for home seed starting. They use less electricity for the light they produce, run cooler and last for many years. Ordinary four-foot LED shop lights, in the range of 30 to 50 watts for many models, are a popular and low-cost choice for seedlings. Fluorescent T5 fixtures also work well and are still sold, though they use more power for the same light and the tubes need replacing. Purpose-built grow lights add colors tuned to plants, but for starting seeds, cool white LEDs work well. Because electricity is the running cost, an efficient LED often pays back its higher price over a few seasons.

Ways to lower the cost

• Use a timer. It stops lights running longer than planned.
• Light only what you need. Group trays under fewer fixtures, and turn off a shelf when it is empty.
• Use reflective surfaces. White walls, mylar or foil around a rack bounce light back onto plants, so the same fixture does more.
• Start seeds at the right time. Starting too early means weeks of extra light for seedlings that are not ready to go outside. Our seed starting calculator and the guide on when to start seeds indoors help you time it.
• Choose efficient lights. Look at light output per watt, not just price.
• Use a heat mat only when needed. Heat mats add to your bill. The heat mat capacity calculator shows how many you need, and most seeds only need heat until they sprout.

Overwintering tender plants

Lights are also a good way to keep tender plants alive through winter, such as geraniums, rosemary, citrus trees, fuchsias and houseplants that spend summer outside. A few practices make the move easier.

• Bring plants in before frost. Many tender and tropical plants suffer when nights drop to the low 50s Fahrenheit, which is well before frost. Check your first frost date with the frost dates by ZIP code tool and move plants in a week or two earlier.
• Inspect and clean them. Check leaves and stems for aphids, scale and spider mites, rinse foliage with water, and keep new arrivals apart from your other houseplants for a couple of weeks.
• Cut back as needed. A light trim makes plants easier to fit and cuts the number of leaves they have to support.
• Expect some leaf drop. Plants moved from bright outdoor light to a room often shed some leaves. It is normal, and new growth adapts to the lower light.
• Give light and cool conditions. Many overwintered plants do best with bright light and cooler temperatures than the rest of the house, and need less water than in summer.

Tubers and bulbs are handled differently. Dahlias, cannas and gladiolus are usually dug after frost has blackened the foliage and stored in a cool, dry, dark place. They do not need lights at all.

Is starting your own plants cheaper than buying them?

Starting seeds under lights is usually cheaper per plant than buying transplants, particularly for vegetables you plant in numbers. Use the calculator to see what the electricity adds. If the first example above, at $11.42, grows 72 seedlings, the light costs about 16 cents per plant, and you would also need to count seed, mix, trays and your time. You can compare it with what the plants would cost at a garden center. Our homegrown food savings calculator estimates what a garden saves overall, and the garden cost calculator covers the full budget.

If you would like to skip lights entirely for hardy plants, see the winter sowing calculator, which starts seeds outdoors in containers, and the safe transplant date calculator and hardening off schedule calculator for moving seedlings grown indoors out to the garden.

What a season of seed starting costs

Real numbers help put the bill in perspective. A single 40 watt four-foot LED light run for 14 hours a day over an eight-week seed-starting season uses 0.56 kWh a day, which is 31.4 kWh in 56 days, and costs about $5.33 at 17 cents per kilowatt-hour. A shelf unit with four of those lights, running on the same schedule, uses 62.7 kWh and costs about $10.66. In a state with pricier power, say 24 cents per kilowatt-hour, the same four lights would cost about $15.05 for the season.

These are modest sums compared with what a tray of transplants costs at a garden center, but they grow quickly with bigger setups and longer schedules, which is why it pays to check the numbers before you buy a rack of high-powered lights. Run the calculator with your own figures, try a few schedules and compare lights before you decide.

Common grow light mistakes

• Using equivalent wattage. It overstates what the light draws and throws off the cost. Use actual watts.
• Leaving lights on all day and night. Plants need a dark period. Use a timer.
• Hanging lights too high. Seedlings stretch and become weak. Keep them close, following the manufacturer’s advice.
• Forgetting delivery charges. Your bill may include fees on top of the energy price. Use the all-in rate for the true cost.
• Starting too early. Extra weeks under lights add cost and leave you with overgrown seedlings.
• Ignoring the extras. Heat mats, fans and heaters add to the total. Count them if you run them.
• Bringing plants in without checking for pests. Insects thrive indoors in winter. Inspect and isolate.

Frequently asked questions