Grow Light Watts Per Square Foot: Complete Wattage Guide
"How many watts do I need?" is the most common question from new indoor growers. This guide breaks down wattage requirements for every light type, plant type, and grow space size so you can size your lighting system correctly.
Quick Answer: LED Watts Per Square Foot
- Leafy greens & herbs: 20-30 watts per sq ft
- Vegetables (tomatoes, peppers): 30-40 watts per sq ft
- Flowering plants (cannabis): 35-50 watts per sq ft
- Seedlings: 15-25 watts per sq ft
These are for quality LED lights (2.5+ umol/J efficiency). Budget LEDs may need 20-30% more wattage.
Understanding Grow Light Wattage
Wattage measures how much electricity a light consumes, not how much usable light it produces. This is why a 300W LED can outperform a 600W HPS - efficiency matters as much as raw power.
Actual Wattage vs. Marketing Claims
Many budget lights use misleading wattage claims. A "1000W LED" that only draws 200W from the wall is not providing 1000W of light - it's providing 200W worth. Always check actual power draw.
How to Find True Wattage
- Look for "Actual Power Draw" in specifications
- Check amperage rating and multiply by voltage (A x V = W)
- Use a Kill-A-Watt meter to measure real consumption
- Ignore "Replacement" or "Equivalent" claims entirely
Watts Per Square Foot by Light Type
Different light technologies have different efficiencies, so they require different wattages to achieve the same light output:
| Light Type | Efficiency | Veg (W/sqft) | Flower (W/sqft) |
|---|---|---|---|
| Premium LED | 2.5-3.0 umol/J | 25-35 | 35-50 |
| Budget LED | 1.8-2.2 umol/J | 30-40 | 40-60 |
| Double-Ended HPS | 1.7-1.9 umol/J | 40-50 | 50-65 |
| Single-Ended HPS | 1.4-1.7 umol/J | 45-55 | 55-70 |
| CMH/LEC | 1.5-1.8 umol/J | 40-50 | 50-65 |
| T5 Fluorescent | 0.9-1.2 umol/J | 50-65 | Not recommended |
Watts Per Square Foot by Plant Type
Leafy Greens & Herbs: 20-30 W/sqft
Lettuce, spinach, kale, basil, and cilantro need moderate light. They don't flower/fruit, so lower intensity works well. High light can actually make lettuce bitter.
- Target PPFD: 200-400 umol/m2/s
- Target DLI: 12-17 mol/m2/d
- Example: 2x4 (8 sqft) = 160-240W LED
Fruiting Vegetables: 30-40 W/sqft
Tomatoes, peppers, cucumbers, and strawberries need more light to produce fruit. Flowering and fruit development are energy-intensive processes.
- Target PPFD: 400-600 umol/m2/s
- Target DLI: 20-30 mol/m2/d
- Example: 4x4 (16 sqft) = 480-640W LED
Cannabis (Flowering): 35-50 W/sqft
Cannabis is a high-light plant, especially during flowering. Maximum light drives maximum bud development. With CO2 supplementation, even higher wattages are beneficial.
- Target PPFD: 600-1000 umol/m2/s (up to 1500 with CO2)
- Target DLI: 40-65 mol/m2/d
- Example: 4x4 (16 sqft) = 560-800W LED
Seedlings & Clones: 15-25 W/sqft
Young plants have limited capacity for high light. Excessive intensity causes stress and slows development. Start low and increase as plants mature.
- Target PPFD: 150-300 umol/m2/s
- Target DLI: 8-15 mol/m2/d
- Example: 2x2 (4 sqft) = 60-100W LED
Wattage Calculator by Grow Space
Here's what you need for common grow space sizes (using quality LED at 40W/sqft for flowering):
| Space Size | Square Feet | Veg (30W) | Flower (40W) |
|---|---|---|---|
| 2x2 | 4 sqft | 120W | 160W |
| 2x4 | 8 sqft | 240W | 320W |
| 3x3 | 9 sqft | 270W | 360W |
| 4x4 | 16 sqft | 480W | 640W |
| 5x5 | 25 sqft | 750W | 1000W |
| 4x8 | 32 sqft | 960W | 1280W |
| 10x10 | 100 sqft | 3000W | 4000W |
Why Efficiency Matters More Than Wattage
Two 300W lights can produce vastly different amounts of usable light. Efficiency (measured in umol/J) tells you how much light you get per watt:
Same Wattage, Different Output
The premium light produces 61% more usable light from the same electricity!
How to Calculate Your Wattage Needs
Step 1: Calculate Your Grow Area
Example: 4ft x 4ft = 16 square feet
Step 2: Determine Watts Per Square Foot
Based on your plant type and light quality:
- Leafy greens: 25 W/sqft
- Vegetables: 35 W/sqft
- Flowering plants: 40-50 W/sqft
Step 3: Calculate Total Wattage
Example: 16 sqft x 40 W/sqft = 640W for flowering
Common Wattage Mistakes
Mistake 1: Trusting "Equivalent" Wattage
A "1000W equivalent" light drawing 200W provides 200W of light, period. Always check actual power draw.
Mistake 2: Using HPS Wattage Rules for LED
Old rules like "50W per sqft" were for HPS. LEDs are more efficient - 35-40W/sqft of quality LED equals 50-60W/sqft of HPS.
Mistake 3: Ignoring Light Distribution
A single 600W light won't cover a 5x5 as well as two 300W lights spread out. Consider coverage, not just total wattage.
Mistake 4: Oversizing for Small Spaces
A 600W light in a 2x2 will bleach your plants. Match wattage to space - you can always dim down if needed.
Electricity Cost Considerations
More wattage means higher electricity bills. Here's what to expect (at $0.12/kWh, 12 hours/day):
| Wattage | Daily Cost | Monthly Cost | Yearly Cost |
|---|---|---|---|
| 150W | $0.22 | $6.48 | $79 |
| 300W | $0.43 | $12.96 | $158 |
| 450W | $0.65 | $19.44 | $236 |
| 600W | $0.86 | $25.92 | $315 |
| 1000W | $1.44 | $43.20 | $525 |
Key Takeaways
- Quality LED: 35-50 watts per square foot for flowering
- Always check actual power draw, not marketing claims
- Efficiency (umol/J) matters as much as raw wattage
- Match wattage to your specific plant type and growth stage
- Consider both coverage and total output when choosing lights
- Higher wattage = higher electricity costs - factor this into your budget
Get Personalized Wattage Recommendations
Our calculator considers your grow space, plant type, and goals to give you exact wattage requirements.
Calculate Your Wattage