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Grow Light Calculator -- How Much Light for Indoor Growing
Work out grow light wattage and coverage for indoor growing.
Canopy footprint to light, not floor area — a 1.2 x 1.2 m tent is 1.44 m²
LED is most efficient; HPS produces more heat but is cheaper upfront
Fruiting crops need ~300W/m² LED, leafy greens ~70% of that, seedlings ~50%
Daily lighting duration (14-16 hours typical for seedlings)
LED wattage per square metre for full-intensity crops (default 300)
Fluorescent wattage per square metre (default 400)
HPS wattage per square metre (default 500)
Your electricity cost per kilowatt-hour (default £0.28)
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
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).
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.
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.
Seedlings need 14-16 hours of light per day. Use a timer to maintain consistent light cycles. Most vegetables need at least 12 hours.
Yes, supplemental grow lights extend the growing day in winter. Even 4-6 hours of extra light helps greenhouse crops through the shortest days.
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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.