How many watts per square metre for grow lights?

LED lights need about 300W per m² for fruiting crops like tomatoes. Fluorescent tubes need around 400W/m² and HPS lamps about 500W/m². Seed starting needs less (150-200W/m²).

Are LED grow lights better than HPS?

LEDs are more energy-efficient, produce less heat, and last longer. HPS lights are cheaper upfront and good for flowering, but run hotter and cost more in electricity.

How high should grow lights be above plants?

LED panels: 30-60 cm above canopy. Fluorescent tubes: 15-30 cm. HPS: 60-90 cm. Move lights higher as plants grow to avoid burning.

How many hours of light do seedlings need?

Seedlings need 14-16 hours of light per day. Use a timer to maintain consistent light cycles. Most vegetables need at least 12 hours.

Can I use grow lights in a greenhouse?

Yes, supplemental grow lights extend the growing day in winter. Even 4-6 hours of extra light helps greenhouse crops through the shortest days.

GardenCalcs

Grow Light Calculator -- How Much Light for Indoor Growing

Work out grow light wattage and coverage for indoor growing.

Growing Area

Canopy footprint to light, not floor area — a 1.2 x 1.2 m tent is 1.44 m²

Light TypeLED is most efficient; HPS produces more heat but is cheaper upfront

Plant TypeFruiting crops need ~300W/m² LED, leafy greens ~70% of that, seedlings ~50%

Hours Per Day

Daily lighting duration (14-16 hours typical for seedlings)

How We Calculate This

Total wattage = Growing area (m²) x Watts per m² x Plant type modifier.

LED grow lights typically need 300W/m² for fruiting crops, fluorescent 400W/m², and HPS 500W/m². Seedlings and leafy greens need less. The number of individual lights is calculated by dividing total wattage by typical fixture wattage.

Monthly electricity cost = (Total watts x Hours/day x 30) / 1000 x Electricity rate.

Frequently Asked Questions

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Last updated: February 2026

All calculations are estimates based on standard gardening guidelines. Results may vary with soil conditions, climate, and plant varieties.