Tools / Operating cost

Light electricity cost calculator

Electricity is documented in grower reports as the single largest recurring cost of an indoor cannabis grow, ahead of nutrients, soil, and seed. The calculator below takes four inputs — the fixture's wattage at the wall, the daily photoperiod, the local price per kilowatt-hour, and the length of the grow in days — and returns the daily energy draw, daily cost, and the full-grow total. The math follows the standard utility-billing formula. Use the result as a planning figure for budgeting and for comparing fixture options on a true cost-per-gram basis once a harvest weight is known.

How the calculation works

Daily energy is computed in kilowatt-hours as fixture wattage multiplied by hours of operation per day, divided by 1000 to convert watt-hours into kilowatt-hours. A 400-watt fixture running 18 hours a day draws 7.2 kWh per day. Multiplying that figure by the local kWh price gives the daily cost; at $0.18 per kWh the same fixture costs roughly $1.30 per day. The full-grow total is daily cost multiplied by the number of days from seed to harvest.

The wattage input refers to the wall draw, not the rated diode output, since utility bills are documented as charging on actual power consumption rather than on photon output. LED fixtures are commonly marketed by output wattage that exceeds the wall draw — a fixture labeled "1000 W equivalent" may draw only 240 W at the wall. The published recommendation is to check the fixture's nameplate or measure draw with a kilowatt meter rather than relying on the marketing number. Inline fans, dehumidifiers, and circulation fans are not included in this calculator; documented grower budgets allocate roughly 15 to 25 percent additional draw for those auxiliary loads in a typical tent.

Reference ranges

Typical home-tent fixtures are documented at 200 to 600 watts of wall draw for a single-plant to four-plant footprint, with the upper end reserved for larger commercial-style consumer LEDs. A photoperiod schedule is documented as 18 hours during veg and 12 hours during flower, so a photoperiod grow runs at the higher draw for roughly the first half and the lower draw for the second half. Autoflower schedules are documented as commonly running 18 or 20 hours from seed to harvest, which simplifies the cost math but extends the total kWh consumed.

Average US residential electricity prices are documented as ranging from roughly $0.10 to $0.30 per kWh as of recent utility filings, with Hawaii and parts of California documented at the upper end and the Pacific Northwest documented at the lower end. International prices are documented as ranging widely, from under $0.10 in parts of the Middle East and Eastern Europe to above $0.45 in Germany and Denmark. The published recommendation is to read the actual rate off the last utility bill before sizing a fixture, since estimated rates and time-of-use surcharges can shift the full-grow figure by twenty percent or more.

Related references: Lighting · Tent setup

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