Short answer: aim for an average of about 350 to 520 PPFD for 16 hours a day. For a 3 × 3 tent, that means an efficient LED drawing roughly 150 to 220 actual watts. Choose a dimmable light so you can start low and turn it up as plants grow.
How we get to that number
- Daily target. Michigan State lists 15 or more mol/m²/day as a minimum for fruiting vegetables. A lighting vendor citing Virginia Tech Extension gives 20 to 30 for productive indoor tomatoes, so we use 20–30 as the goal and 15 as the floor.
- Convert to PPFD. PPFD = DLI ÷ (hours × 0.0036). Over 16 hours, a DLI of 20 needs about 350 PPFD and a DLI of 30 needs about 520.
- Convert to watts. A 3 × 3 ft tent is 9 sq ft, or 0.84 m². We assume 2.5 µmol/J efficacy, with 80% of the light landing on the plants.
| Goal | DLI | Average PPFD | Actual watts | Monthly power cost |
|---|---|---|---|---|
| Bare minimum | 15 | 260 | About 110 W | About $9.60 |
| Good production | 20 | 350 | About 150 W | About $13.10 |
| Strong production | 25 | 435 | About 180 W | About $15.70 |
| Upper target | 30 | 520 | About 220 W | About $19.20 |
What to look for in a light
- Actual power draw of about 150 to 250 watts, with no reliance on "equivalent" wattage.
- A PPFD map for a 3 × 3 area at a stated hanging height. Look at the corner and edge values too.
- A dimmer so seedlings and young plants don't get full power.
- A bar or spread design rather than one small panel, for more even light.
- Efficacy of 2.5 µmol/J or better, if stated. Higher numbers mean lower power bills for the same light.
See 200-watt LED lights for 3 × 3 tents on Amazon
Choose the right tomatoes
A tent's height limits what you can grow. Choose dwarf or determinate varieties, which stay compact. Large indoor vines quickly reach the light. Indoor flowers also need help with pollination, so gently shake the stems or use a small brush when they bloom.
Don't forget airflow and heat
Almost all the electricity a light uses ends up as heat in the tent. Add a small circulating fan to strengthen stems and limit disease, and vent warm, humid air out of the tent. See tent ventilation kits on Amazon.
Plug timers and lights into properly rated outlets, keep water away from connections, and follow the maker's instructions on hanging height. Leaves that bleach or curl upward can be a sign that the light is too close or too strong.
