Electricity cost

Formula — Electricity cost

Every device is billed for the hours it actually runs in each stage. A device tied to the light schedule runs for the stage’s photoperiod, a night-time device for 24 h minus that photoperiod, and cycling loads (heater, AC, humidifier) are scaled by a duty cycle because a thermostat only powers them part of the time.

• kWh = watts × qty × hours/day × duty% × days ÷ 1000
• hours/day = photoperiod | 24 − photoperiod | 24 | custom
• HPS draw = lamp watts × 1.08 (ballast overhead)
• energy cost = Σ kWh × price per kWh
• standing charge = monthly fee × season days ÷ 30.44
• total = (energy cost + standing charge) × (1 + VAT%)
• heat load [BTU/h] = peak watts × 3.412

Worked example — Electricity cost

Take a 240 W LED on 18/6 for 28 veg days and 12/12 for 63 flowering days, plus a 45 W exhaust fan running around the clock. The LED contributes 240 × 18 × 28 ÷ 1000 = 121 kWh in veg and 240 × 12 × 63 ÷ 1000 = 181 kWh in flower, so 302 kWh. The fan adds 45 × 24 × 91 ÷ 1000 ≈ 98 kWh. That is 400 kWh for the season.

At 0.30 per kWh the energy alone costs 120. Add a 10/month standing charge over 91 days (10 × 91 ÷ 30.44 ≈ 30) and 23% VAT: (120 + 30) × 1.23 ≈ 184.50 for the full run. Yielding 350 g puts the cost at about 0.53 per gram and 1.46 g/W against the 240 W of lighting. Swapping the LED for a 400 W HPS (billed at 432 W with ballast losses) would push lighting energy to about 544 kWh and the bill past 280 for the same photoperiods.

Typical ranges — Electricity cost

Peak load matters separately from consumption: it decides whether a single 16 A circuit can carry the room. The calculator reports the worse of the lights-on and lights-off halves of the day, ignoring duty cycles, because a circuit must survive the moment everything switches on at once.