Starting tomatoes under lights costs less than most people fear, but it is worth knowing before you buy. Enter your lights’ wattage, how long they run, and your electricity price, and this calculator shows the cost for the whole seed-starting season and per seedling.
Try a preset:
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What Your Result Means: the season cost is the electricity your lights use from sowing to transplant. The default price is the recent US average; your own bill gives the real figure, in any currency.
How Much Does It Cost to Run Grow Lights for Tomato Seedlings?
Usually a few dollars for the whole season. A 40-watt light running 16 hours a day for 7 weeks uses about 31 kWh, roughly $5.60 at the US average price. Enter your own lights and rate to see yours.
- Enter the Wattage from the light’s label, and how many lights.
- Set the Schedule, hours a day and weeks before transplant.
- Enter Your Price per kWh from your electricity bill.
What Each Result Means
| Result | What It Tells You |
|---|---|
| Season cost | Electricity cost from sowing to transplant |
| Energy used | Total kilowatt-hours |
| Per week and per day | Running cost over time |
| Per seedling | Cost of light for each plant |
Cost by Light Setup
Season cost at 16 hours a day for 7 weeks and $0.18 per kWh.
| Setup | Energy Used | Season Cost |
|---|---|---|
| One 40 W light | about 31 kWh | about $5.64 |
| Two 40 W lights | about 63 kWh | about $11.29 |
| One 100 W panel | about 78 kWh | about $14.11 |
How Is the Running Cost Worked Out?
A light’s energy use is its wattage times the hours it runs, divided by 1,000 to give kilowatt-hours. Multiply by your price per kWh for the cost.
Days = weeks x 7
Cost = kWh x price per kWh
Cost per seedling = cost / seedlings
Seedlings usually spend 6 to 8 weeks indoors before transplant, and do best with 14 to 16 hours of light a day, which is why those are the defaults.
Worked Example
Say you run one 40-watt LED light 16 hours a day for 7 weeks. That is 40 x 16 x 49 days / 1,000, or about 31.4 kWh. At $0.18 per kWh the season costs about $5.64, around 81 cents a week. Spread over 24 seedlings, that is about 24 cents per plant.
Factors That Change the Cost
- Wattage: the biggest factor; check the label, not the “equivalent” figure.
- Hours per day: 14 hours instead of 16 cuts the bill by about an eighth.
- Weeks indoors: starting seeds too early adds weeks of lighting.
- Electricity price: rates vary widely by region and plan.
- Light type: LEDs use less power than older fluorescent tubes for the same light.
Understanding Your Outputs
| Output | Meaning | Best Used For |
|---|---|---|
| Season cost | Sowing to transplant | Budgeting |
| Energy used | Kilowatt-hours | Comparing lights |
| Per seedling | Cost per plant | Comparing with buying transplants |
When to Use It
Use it before buying a light, or to compare growing your own transplants with buying them. Pair it with the grow light distance calculator for setup and the seed starting calculator for how many seeds to sow.
Common Mistakes to Avoid
- Using the “equivalent” wattage: a light sold as “100 W equivalent” may draw far less; use the actual watts.
- Running lights 24 hours: seedlings need a dark rest; 14 to 16 hours is enough.
- Starting too early: extra weeks indoors cost more and make leggy plants.
- Forgetting the timer: a cheap timer keeps hours consistent and costs predictable.
- Guessing the rate: your bill shows your real price per kWh.
Accuracy and Limitations
The math is exact for the wattage and hours you enter.
What It Calculates: energy and cost for your lights and schedule.
What It Does Not Include: heat mats, fans, taxes, fixed fees, or time-of-use pricing. Actual draw can differ slightly from the label.
Methodology
Method: energy is wattage times hours used per day divided by 1,000 to give kilowatt-hours per day, multiplied by the number of days and lights. Cost is kilowatt-hours times your price per kWh. Inputs Used: watts per light, number of lights, hours per day, weeks, price per kWh, and optionally seedling count. Defaults: 16 hours a day and 7 weeks, from our grow light and planting date guidance of 14 to 16 hours and 6 to 8 weeks indoors; $0.18 per kWh, close to the US average residential price in 2026 (18.31 cents in July 2026). Rounding: kWh to one decimal and cost to two decimals.
Sources: U.S. Energy Information Administration – Average Price of Electricity, U.S. Energy Information Administration – Measuring Electricity, The Old Farmer’s Almanac – Tomatoes. Last reviewed: September 26, 2026 · Reviewed by Aiza Anwar.
Frequently Asked Questions
How much does it cost to run a grow light for seedlings?
Usually a few dollars a season. A 40-watt light for 16 hours a day over 7 weeks uses about 31 kWh, around $5.60 at $0.18 per kWh.
How do I calculate grow light electricity cost?
Multiply watts by hours per day and divide by 1,000 for daily kWh. Multiply by the number of days, then by your price per kWh.
How many hours a day should tomato seedlings be under lights?
About 14 to 16 hours, followed by a dark rest period. Running lights around the clock adds cost without helping the plants.
Are LED grow lights cheaper to run?
Yes, LEDs usually give the same light for fewer watts than older fluorescent tubes, so they cost less to run over a season.
What does a 100 W equivalent LED really use?
Less than 100 watts, often far less. “Equivalent” compares brightness to an old bulb; use the actual wattage printed on the label for cost.
How long do tomato seedlings need grow lights?
From sowing until transplant, typically 6 to 8 weeks, plus a short hardening-off period outdoors.
Is it cheaper to grow my own transplants?
Often, yes. Light costs usually come to cents per seedling, well below the price of nursery transplants, though seed, mix, and trays add a little.
Where do I find my electricity rate?
On your electricity bill, as the price per kilowatt-hour. If you pay several rates, use the one that applies when your lights are on.
Does a heat mat add much to the cost?
It adds some. Run the calculator a second time with the heat mat’s wattage and hours (usually only until seeds sprout, about 1 to 2 weeks), then add the two totals.
Related Tomato Tools
Find the Right Height with the Grow Light Calculator
This calculator estimates electricity cost from the wattage, hours, and price you enter. Taxes, fixed charges, and time-of-use rates are not included, and real power draw can differ slightly from the label. Check your bill for your actual rate. Spotted an error? Tell us here.