Grow Light DLI Calculator
Daily light integral from PPFD and hours — and the runtime or intensity a crop needs.
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Grow Light Run Cost Calculator
Monthly and yearly electricity cost to run a grow light, by watts, hours and rate.
Grow Light Run Cost Calculator
Chlorova · updated for 2026
Calculator
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Click “Calculate” to see your results.
Enter the true wall watts the fixture draws, not the 'equivalent' or the diode rating on the box — an LED marketed as 400 W equivalent may pull 200 W at the plug, and a magnetic-ballast fixture pulls more than its lamp rating, not less. The rate per kWh is your all-in delivered cost; time-of-use and tiered plans make that number move, so use your utility's real figure or a plug-in meter for anything close to a decision.
The grow light run cost calculator turns a fixture's wattage and daily hours into the electricity bill it quietly racks up each month and year, so you can compare a bright new light against what it actually costs to keep on. Enter the true wall watts, how many hours a day it runs and your utility rate, and it returns kilowatt-hours and dollars for a month and a full year. Grow lights run on timers for a third of a day or more, which is why a single flowering tent can move a household bill more than a refrigerator does.
How to Estimate Light Running Cost
- Enter the fixture's true wall watts from the spec sheet or a meter.
- Enter the hours it stays on each day from your timer schedule.
- Type your all-in utility rate per kilowatt-hour, not supply-only.
- Read the monthly kilowatt-hours and the monthly dollar figure.
- Compare that yearly cost against the fixture's purchase price.
- Add fans and pumps at their own watts for the whole tent's draw.
- Re-run the math for a time-of-use tariff if you run lights off-peak.
- Use a plug-in meter whenever your tariff is tiered or confusing.
How the Energy Cost Is Worked Out
Energy is power times time. Watts multiplied by hours per day gives watt-hours, and dividing by a thousand turns that into kilowatt-hours, the unit your utility bills. Multiply daily kilowatt-hours by thirty for a month and by 365 for a year, then times your rate per kilowatt-hour for the dollars. A 100 W light on twelve hours burns 1.2 kWh a day, about 36 a month; a 600 W fixture on the same schedule burns 7.2 a day and over 215 a month.
Why wall watts and not the marketing number: an LED box labelled with a high 'equivalent' wattage usually draws far less real power at the plug, while a high-pressure sodium lamp on a magnetic ballast draws meaningfully more than the lamp's own rating because the ballast burns watts too. Every dollar figure here scales straight off the wall-watts input, so the single most important thing to get right is measuring or trusting the true draw rather than the biggest number on the packaging.
Worked Example: A 600 W Flowering Tent
- Your flowering tent runs a light that pulls 600 true wall watts.
- It stays on twelve hours a day on a timer through bloom.
- Daily energy: 600 × 12 ÷ 1,000 = 7.2 kilowatt-hours.
- Monthly energy: 7.2 × 30 = about 216 kilowatt-hours.
- At fifteen cents a kWh, that is roughly $32 a month.
- Over a year of continuous use: 600 × 12 × 365 ÷ 1,000 ≈ 2,628 kWh.
- Annual cost lands near $394 for the light alone, before any fan or pump watts.
- A 200 W LED doing the same job would run about a third of that bill.
- The LED's higher sticker price still pays back inside a single heavy flowering season.
Monthly Cost by Fixture and Hours
Approximate monthly electricity cost for common grow setups at a fifteen-cent rate, to frame the calculator's output at a glance. Real bills scale directly with your own rate, so multiply these by your rate and divide by fifteen cents to adjust.
| Fixture | 12 h/day | 18 h/day |
|---|---|---|
| 50 W LED clip | $3 | $4 |
| 100 W LED | $5 | $8 |
| 200 W LED | $11 | $16 |
| 600 W HPS | $32 | $49 |
Key Terms
- Wall watts
- The real power a fixture draws at the outlet, including driver or ballast losses — the number that sets your bill.
- Kilowatt-hour
- One thousand watts running for one hour; the unit utilities charge you by.
- Photoperiod
- The hours of light per day a plant receives, which also sets how many hours your light costs you.
- Delivered rate
- Your all-in price per kWh after generation, delivery and fees — not the headline supply rate.
- Ballast loss
- Extra watts a magnetic ballast burns on top of the lamp rating, pushing real draw higher.
What This Means for Your Electric Bill
- Fix a meter on the outlet before you buy a big light — a ten-dollar plug-in meter reveals true wall watts and pays for itself the first time it saves you from an inefficient fixture.
- Hours matter as much as watts; trimming the photoperiod from eighteen to fourteen hours on a vegetative light cuts cost by more than twenty percent with no hardware change.
- LEDs change the math completely — a quality fixture matching a 600 W HPS's usable light often draws 200 to 250 W, so a higher sticker price can win on total cost inside one year of heavy use.
- Do not forget the heat you pay to remove: every watt the light burns becomes BTUs the exhaust fan must vent, so a hot fixture quietly raises the cooling load too.
- On a cheap off-peak rate, shifting the dark period so lights run overnight can cut a flowering tent's cost by a third without touching the schedule the plants see.
Common Mistakes & Fixes
- Entering the 'equivalent' wattage from the box instead of the real draw, which can overstate or understate the cost by a hundred watts or more.
- Pricing only the light and leaving the fans, pump and controller out of the tent's total load, so the whole-room cost comes as a surprise.
- Using the supply-only rate and forgetting delivery charges and fees that make the delivered cost per kWh far higher than the headline number.
- Assuming an LED is automatically cheap and skipping the driver-loss watts that separate the marketing figure from the true wall draw.
- Planning around a flat rate on a time-of-use plan, then getting hit when the timer has the light running hard right at the expensive peak-hour window.
When to Use This (and When Not To)
Use it when comparing lights or deciding whether to expand an indoor setup, so you budget the running cost and not just the purchase. It covers electricity only; to size the breaker and count how many fixtures safely fit one circuit, use the grow tent circuit calculator, and for the light a plant actually needs in moles per day, use the grow-light DLI tool. Greenhouse supplemental lighting that runs only a few winter hours is where this math saves the most over a full-time tent.
Grow Light Run Cost Calculator: Frequently Asked Questions
How much does it cost to run a grow light?
It depends on true watts, hours and your rate. A 100 W LED on twelve hours at fifteen cents a kilowatt-hour costs about five dollars a month; a 600 W HPS on the same schedule runs roughly thirty-two. Multiply daily watt-hours by your rate for the real answer.
Is the wattage on the box the number to use?
Usually not. A high 'equivalent' rating on an LED overstates the actual draw, while a magnetic-ballast HPS understates it once you count ballast loss. Use the true wall watts from the spec sheet or, best of all, a plug-in power meter.
Are LED grow lights cheaper to run than HPS?
Yes, typically by a wide margin. A quality LED delivering comparable usable light often draws a third to a half of what a 600 W HPS does, and it dumps less heat into the room, so your fan and cooling cost drops too. The higher purchase price frequently pays back within a year of heavy use.
How much does the timer schedule affect cost?
A lot, because cost is just watts times hours. An eighteen-hour vegetative day costs half again as much as a twelve-hour one for the same fixture, so trimming hours you do not need is the cheapest saving available — no new hardware, just a smarter schedule.
Should I count fans and pumps too?
For a whole tent, yes. Inline fans, pumps and controllers add real continuous watts, and the light's heat forces the fan to work harder. Add each device's watts to the light figure to see the true room cost rather than just the lighting line.
What electricity rate should I enter?
Your all-in delivered rate, not the supply-only headline. Look at a recent bill and divide total cost by total kilowatt-hours, or use a plug-in monitor. Time-of-use plans complicate this, so if you can run lights off-peak, model the cheaper rate.
Sources & Further Reading
- Calculating the Cost of Grow Light Electricity Use — Oregon State University Extension
- A Guide to Indoor Growing Energy Consumption — North Carolina State University Extension
- Electricity Rates and the Power Profiler Kit — U.S. Department of Energy
Chlorova Gardening Team
Builds, checks and documents every tool we publish
Chlorova editorial review
Checked against extension-service publications
Formula verified against published references · Updated
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