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Grow Light Calculator

Find the right grow light wattage and estimate electricity costs for your indoor garden

Total Wattage

512 W

Fixtures

3

Monthly

$34.56

Units

Total Wattage Needed

512 W

32 W/sqft x 16.0 sq ft

Fixtures Needed
3

200W each (600W total)

Light Schedule
16 hrs/day

9.60 kWh/day

Daily Cost
$1.15

at $0.12/kWh

Monthly Cost
$34.56

estimated 30-day period

Estimated Yearly Cost
$420.48
Daily kWh
9.60

Light Requirements

Plant TypeLED W/sqftHPS W/sqftHours/Day
Leafy Greens203012
Herbs253514
Tomatoes & Peppers325016
Flowering Plants355512
Seedlings152016

Light & Cost Comparison

Watts Needed512W
Actual Watts (fixtures)600W
Daily Cost$1
Monthly Cost$35

What You'll Need

CHRYZTAL Stainless Steel Garden Hand Tool Set 4pc

CHRYZTAL Stainless Steel Garden Hand Tool Set 4pc

$20-$304.5
View on Amazon
KINGLAKE Plastic Plant Labels T-Type 100pcs

KINGLAKE Plastic Plant Labels T-Type 100pcs

$6-$104.5
View on Amazon
CHRYZTAL Stainless Steel Garden Hand Tool Set 4pc

CHRYZTAL Stainless Steel Garden Hand Tool Set 4pc

$20-$304.5
View on Amazon
KINGLAKE Plastic Plant Labels T-Type 100pcs

KINGLAKE Plastic Plant Labels T-Type 100pcs

$6-$104.5
View on Amazon

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Example Calculations

14x4 ft LED Tomato Grow

Inputs

Grow Area4 x 4 ft (16 sq ft)
Plant TypeTomatoes & Peppers
Light TypeLED
Electricity Rate$0.12/kWh

Result

Total Wattage Needed512 W
Fixtures (200W each)3
Monthly Cost$34.56

A 4x4 LED tomato setup needs 512W (3 fixtures at 200W each = 600W actual), costing about $34.56/month.

22x4 ft Herb Garden

Inputs

Grow Area2 x 4 ft (8 sq ft)
Plant TypeHerbs
Light TypeFluorescent
Electricity Rate$0.12/kWh

Result

Total Wattage Needed240 W
Fixtures (54W each)5
Monthly Cost$13.61

An herb garden under fluorescent lights needs 240W (5 tubes at 54W each = 270W actual).

35x5 ft HPS Flowering Room

Inputs

Grow Area5 x 5 ft (25 sq ft)
Plant TypeFlowering Plants
Light TypeHPS
Electricity Rate$0.15/kWh

Result

Total Wattage Needed1,375 W
Fixtures (600W each)3
Monthly Cost$97.20

A 5x5 HPS flowering room needs 1,375W (3 fixtures at 600W each = 1,800W actual), costing $97.20/month.

Frequently Asked Questions

Q

How many watts per square foot do I need for grow lights?

It depends on the plant type and light technology. LED lights need 20-35 watts per square foot, while HPS lights need 30-55 watts. Leafy greens need less light (20W/sqft LED) than flowering plants like tomatoes (32W/sqft LED).

  • Leafy greens and herbs: 20–25 W/sq ft (LED) or 30–40 W/sq ft (HPS)
  • Tomatoes, peppers, and fruiting plants: 30–35 W/sq ft (LED) or 45–55 W/sq ft (HPS)
  • Seedlings and clones: 15–20 W/sq ft (LED) – lower intensity prevents stretching
  • Measure actual draw watts, not "equivalent" watts – a 200W LED replaces a 400W HPS
  • Use a PAR meter to verify 400–600 µmol/m²/s for vegging and 600–900 for flowering
Light TypeWatts/sq ft (Veg)Watts/sq ft (Flower)Efficiency
LED20–2530–352.5–3.0 µmol/J
HPS30–4045–551.5–1.8 µmol/J
Fluorescent25–35Not recommended0.8–1.2 µmol/J
CMH25–3535–451.8–2.1 µmol/J
Q

Are LED grow lights more efficient than HPS?

Yes, LED grow lights are significantly more efficient. They produce more usable light per watt, generate less heat, and last 50,000+ hours versus 10,000 for HPS. While LEDs cost more upfront, they save 30-50% on electricity over time.

  • LEDs produce 2.5–3.0 µmol/J vs. 1.5–1.8 µmol/J for HPS – 40–60% more photons per watt
  • LED lifespan: 50,000–100,000 hours vs. HPS bulbs needing replacement every 10,000 hours ($50–$100 per bulb)
  • LEDs run 50–70°F cooler, reducing HVAC costs by $20–$50/month in sealed rooms
  • A 200W LED replaces a 400W HPS while producing equivalent PAR output
  • LED ROI breakeven: 12–18 months from electricity savings at $0.12/kWh
Q

How many hours of grow light do plants need?

Most vegetables and flowering plants need 12-16 hours of light daily. Leafy greens and herbs thrive with 12-14 hours, while tomatoes and peppers prefer 16 hours. Seedlings also benefit from 16 hours to promote strong growth.

  • Lettuce, basil, and herbs: 12–14 hours/day with a consistent dark period
  • Tomatoes, peppers, and cucumbers: 14–16 hours/day for maximum yield
  • Seedlings and clones: 16–18 hours/day to prevent stretching and promote stocky growth
  • Use a timer – inconsistent light schedules can stress plants and reduce yields by 15–25%
  • Provide at least 6 hours of uninterrupted darkness for proper plant respiration
Q

How much does it cost to run grow lights?

Running costs depend on wattage and electricity rate. A typical 4x4 ft LED setup (two 200W fixtures) running 16 hours daily costs about $1.15/day or $35/month at $0.12/kWh. HPS setups cost 50-80% more due to higher wattage requirements.

  • Monthly cost formula: (watts × hours/day × 30) ÷ 1,000 × $/kWh
  • A 400W LED running 16 hrs/day at $0.12/kWh = $23.04/month
  • Same coverage with 800W HPS = $46.08/month – double the electricity bill
  • Add 10–20% to electricity cost for cooling if lights raise room temp above 80°F
  • Off-peak electricity rates (nights) can cut costs 20–40% – run lights overnight
Q

How far should grow lights be from plants?

LED lights should be 12-24 inches above plants, HPS lights 24-36 inches, and fluorescent lights 6-12 inches. Start higher and lower gradually. Watch for light burn (bleached leaves) or stretching (light too far away).

  • LED: 12–18" for flowering, 18–24" for veg – adjust by watching leaf tips for bleaching
  • HPS: 24–36" minimum – use the hand test (if too hot for your hand, it is too close for plants)
  • Fluorescent/T5: 6–12" – low heat allows very close placement for seedlings
  • Raise lights 2–4" if you see leaf curling, yellowing tips, or bleached spots
  • Lower lights 2–4" if stems stretch thin and internodes exceed 3–4" apart

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Last Updated: Mar 26, 2026

This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.

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