How Much Wattage Do I Need Per Square Foot?
You've got your tent. Now you need to figure out how much light to fill it. The internet is full of conflicting advice — 30 watts per square foot, 50 watts, "it depends on efficacy." Let's cut through it.
The Rule of Thumb
| Growth Stage | Target W/sqft | Notes |
|---|---|---|
| Veg only | 20–30 W/sqft | Less intensity needed for leaf growth |
| Flowering | 30–50 W/sqft | More light = denser buds |
| High performance | 50+ W/sqft | With CO2 supplementation |
What This Looks Like by Tent Size
| Tent | Sq Ft | Veg (min) | Flower (target) |
|---|---|---|---|
| 2×2 | 4 | 80W | 120–200W |
| 3×3 | 9 | 180W | 270–450W |
| 4×4 | 16 | 320W | 480–650W |
| 5×5 | 25 | 500W | 750–1000W |
These numbers assume modern LED efficiency (2.0+ µmol/J). Older or cheaper LEDs with lower efficacy may need more wattage to hit the same PPFD targets.
Why Efficacy Matters More Than Raw Watts
A 600W light with 2.7 µmol/J efficacy produces significantly more usable light than a 600W light at 1.8 µmol/J. The first light delivers ~1,620 µmol/s of photons. The second delivers ~1,080 µmol/s. Same electricity bill, 50% more light.
This is why "watts per square foot" is a starting point, not the final answer. Check the efficacy rating when comparing lights — our comparison tool shows this side-by-side.
💡 Quick formula: Watts needed = tent length × tent width × 35
This gives you a solid middle-ground target for flowering with a modern LED.
Don't Forget the Electricity Cost
More watts = more electricity. A 600W light running 12 hours/day in flower costs roughly $1.15/day at the national average rate ($0.16/kWh). Over an 8-week flower cycle, that's $64 in electricity alone.
Use our Grow Cost Calculator to get the exact number for your setup — pick your light, set your schedule, and see the real cost per grow.
The Bottom Line
- Target 30–50 W/sqft for flowering with modern LEDs
- Check efficacy — higher µmol/J means more light per watt
- Match your light to your tent — use the "Browse by Tent Size" feature on our calculator page to find lights that fit
- Factor in electricity — the cheapest light isn't always the cheapest to run