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Electricity Cost Calculator

This electricity cost calculator adds up what your grow lights, fans, dehumidifier and air conditioner cost to run, in kWh and money per day, month and grow.

Grow light

Exhaust fan

Dehumidifier

Add equipment

Use the watts on the label. For anything that switches itself on and off (an air conditioner, dehumidifier or heater), put the hours it actually runs, not the hours it is plugged in.

per kWh

days

112 days is 16 weeks. Change it to your own cycle.

More options
per kWh

Optional, for a time-of-use tariff. Empty: one price for every hour.

%

Lights on at night on a cheap rate? Put most of their use here.

Show the CO₂ from

Cost per month

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Per day
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Per grow
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Per year
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CO₂ from the electricity
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Where the money goes

    Next: Check how much of the light you pay for reaches the plants

    How it works

    Each item uses its watts × hours a day ÷ 1,000 kilowatt-hours a day. Add them up and multiply by your price per kWh. A month is counted as 30 days.

    Everything here also turns into heat; to size the cooling, use the air conditioner calculator.

    Formula
    kWh per day = watts × hours ÷ 1,000
Cost        = kWh × price per kWh
Per month   = per day × 30

    What an electricity cost calculator shows for a grow

    A grow room’s bill is a few large items and several small ones that never switch off. The calculator multiplies each item’s watts by its hours and the number you have, adds them up, and prices the total at your rate per kWh. The breakdown then shows where the money goes, so you know which item is worth changing first.

    A month is 30 days and a grow defaults to 112 days (16 weeks). Change the grow length to your own cycle, and the per-grow cost follows.

    Grow light electricity cost: a worked example

    The starting list is a 600 W light for 18 hours, a 60 W exhaust fan for 24 hours and a 500 W dehumidifier for 12 hours, at $0.20 per kWh.

    • Light: 10.8 kWh a day, $64.80 a month.
    • Dehumidifier: 6 kWh a day, $36.00 a month.
    • Fan: 1.44 kWh a day, $8.64 a month.

    Together that is 18.24 kWh a day: $3.65 a day, $109.44 a month (547 kWh), $408.58 over 112 days and $1,331.52 a year. The light is more than half of it. Moving it to a 12-hour flowering day cuts its share from $64.80 to $43.20 a month.

    The calculator takes one set of hours per item, but a grow changes its light hours halfway. For a truer figure, run it twice. With 6 weeks of veg at 18 hours and 10 weeks of flower at 12 hours, the 600 W light uses 957.6 kWh over 112 days and costs $191.52, against $241.92 if it ran 18 hours throughout.

    Air conditioner running cost

    Add the air conditioner as its own line. Its watts are on the label, or divide its cooling in BTU/h by its EER; the air conditioner calculator does this when you enter the EER. A unit rated 12,000 BTU/h with an EER of 10 draws 1,200 W.

    Enter the hours its compressor actually runs, not the hours it is switched on. At 12 hours a day and $0.20 per kWh, that unit uses 14.4 kWh a day and costs $86.40 a month. Every watt in the room becomes heat the air conditioner has to remove, so a smaller light also cuts the cooling bill.

    Off-peak rates and kWh to CO₂

    On a time-of-use tariff, enter the cheaper price and the share of your use in those hours under More options. The calculator uses the weighted average. With $0.20 by day, $0.10 at night and half the use at night, the average is $0.15 per kWh and the example drops from $109.44 to $82.08 a month. Lights on at night move the most.

    The same screen can show the CO₂ from your electricity: about 0.348 kg per kWh on the average US grid and 0.177 kg on the UK grid. The example month of 547 kWh is about 190 kg in the US and 97 kg in the UK. Your own supplier may be lower or higher.

    Common mistakes

    • “Equivalent” watts. Use the wall-socket figure, or measure with a plug-in power meter.
    • Plugged-in hours for things that cycle. A dehumidifier or air conditioner on a humidistat or thermostat runs for part of the time it is on.
    • Forgetting how many. Two fans of the same model belong on one line with “How many” set to 2.
    • Comparing LED and HPS by price per watt. A watt costs the same whatever it powers. The saving comes from light per watt, which the LED grow light calculator works out.

    FAQ

    Questions growers ask

    How do I calculate electricity cost?

    Multiply the watts by the hours a day and divide by 1,000 to get kWh, then multiply by your price per kWh. A 600 W light on for 18 hours uses 10.8 kWh a day. At $0.20 per kWh that is $2.16 a day and $64.80 over a 30-day month.

    What does a 600 W LED cost to run a month?

    $64.80 at $0.20 per kWh on an 18-hour veg day, or $43.20 on a 12-hour flowering day. That is 324 or 216 kWh in a 30-day month. Change the price to your own tariff; the cost rises and falls in a straight line with it.

    How much does it cost to run a 1000 W grow light?

    $2.40 a day, or $72 a month, at $0.20 per kWh for 12 hours a day. At 18 hours it is $3.60 a day and $108 a month. Use the wall-socket watts: a light sold as “1000 W equivalent” may draw much less than 1,000 W.

    How much electricity does a 4×4 grow tent use?

    About 18 kWh a day with the calculator’s starting list: a 600 W light for 18 hours, a 60 W fan all day and a 500 W dehumidifier for 12 hours. That is 547 kWh, or $109.44, a month at $0.20 per kWh. Your own list may be smaller or larger.

    Is LED cheaper to run than HPS?

    Watt for watt, no: 600 W for 12 hours costs $43.20 a month at $0.20 per kWh whatever the bulb, and every watt ends up as heat. An LED saves money when it gives the light you need from fewer watts, so compare µmol/J on the spec sheets.

    How much does it cost to run an air conditioner in a grow room?

    $86.40 a month at $0.20 per kWh for a unit drawing 1,200 W for 12 hours a day. Find its watts on the label, or divide its BTU/h by its EER. Enter the hours the compressor actually runs, not the hours the unit is switched on.

    Does a cheaper night rate lower the cost?

    Yes, by the share of use you move into those hours. With $0.20 by day, $0.10 at night and half the use at night, the average is $0.15 per kWh. The starting list then drops from $109.44 to $82.08 a month. Running the lights at night moves the most.

    Where do I find the watts?

    On the label or spec sheet, or measure with a plug-in power meter. Use the wall-socket watts, not “equivalent” watts. For something that cycles on and off, like a dehumidifier, use the hours it actually runs.

    Sources

    The formulas and target ranges on this page come from these references.

    1. US EPA (2025). eGRID2023 summary data, revision 2: US total output emission rate 767.209 lb CO₂ per MWh. epa.gov/egrid/summary-data
    2. UK Government (2025). Greenhouse gas reporting: conversion factors 2025, methodology paper (electricity generated: 0.17700 kg CO₂e per kWh). assets.publishing.service.gov.uk/media/6846b0870392ed9b784c0187/2025-GHG-CF-methodology-paper.pdf

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