Grow Room Running Costs: How Much Electricity a Grow Light Uses

The light is usually the biggest part of a grow room's electricity bill, and the sum is simple. Tariffs differ, so this guide works in kilowatt-hours (kWh), the units on your bill. It covers the units a day for grow lights we sell from 150 W to 1030 W, what heaters, fans and a dehumidifier add, LED against HPS, and ways to cut the bill.

How to work out the cost

Watts to kilowatts. Divide the watts by 1,000. A 600 W light is 0.6 kW.
Kilowatts to units. Multiply by the hours it runs each day. 0.6 kW × 12 hours = 7.2 kWh, or 7.2 units.
Units to money. Multiply by the unit price on your bill.

The watts are in the product title or on the rating plate. Vegetative growth usually runs at 18 hours of light a day and flowering at 12, so the table gives both.

Units a day for the lights we sell

Watts from each product title or page, at full power.
Light	Watts	18 hours a day (kWh)	12 hours a day (kWh)
LUMii Switch Blade - 150W LED Fixture	150	2.7	1.8
Maxibright Daylight 200W PRO LED	200	3.6	2.4
Maxibright Daylight 300W PRO LED	300	5.4	3.6
Philips 315W CMH Lamps	315	5.67	3.78
Maxibright Varidrive 360W LED	360	6.48	4.32
Osram Vialox Nav-T Lamps, 400W	400	7.2	4.8
Maxibright Daylight 480W PRO LED	480	8.64	5.76
Sunmaster 600W Super HPS	600	10.8	7.2
Maxibright Daylight 660W PRO LED	660	11.88	7.92
Maxibright Varidrive 3.0 720W LED - Integrated Driver or Gorilla 720W Integrated LED	720	12.96	8.64
Maxibright Daylight 1030W PRO LED	1030	18.54	12.36

The HPS and CMH rows are the lamp alone; as a rule the ballast uses a little on top.

What else draws power

Wattages from each product page, title or size options.
Item	Draws	When it runs
Gorilla Tube Heaters	60, 120, 180 or 240 W	Whenever switched on
Hylite Tube Heaters	28, 55, 90, 120 or 190 W	Whenever switched on
AC Infinity - Thermoforge T7 VPD Heater	1000 W element	Adjusts its own heat output
GroWarm 2kW Fan Heater	Up to 2 kW (3 modes)	Has a thermostat, its page says
Eheim Tank Heaters	25 to 150 W, by tank size	On its own temperature control
RAM 20L Dehumidifier	280 W (27°C, 60% RH) or 330 W (30°C, 80% RH)	To a set humidity, or continuously
WindKing Wall Fan	55 W motor	Usually day and night
Extractor fans, clip fans, pumps and humidifiers	See the rating plate	Extractor: day and night

Small loads that never switch off add up: every 10 W is 0.24 kWh a day (10 W × 24 hours), so the 55 W WindKing left on all day uses up to 1.32 kWh.

LED vs HPS running costs

Watt for watt they cost the same: 600 W is 0.6 units an hour whatever makes the light. An LED saves when it needs fewer watts for the same job. The Varidrive 360W LED page says it offers comparable performance to a 600W HPS system: over a 12-hour day that is 4.32 kWh against 7.2, a saving of 2.88 kWh a day or 20.16 a week. An HPS needs new lamps as well: the Osram page says to change them roughly every 8-10 months.

A bigger LED is no saving, though: a 720 W LED at full power draws more than a 600 W lamp (8.64 kWh against 7.2 over 12 hours). Our LED wattage guide and LED or HPS? cover what you get for it.

Where the heat goes

Nearly every watt a light draws ends up as heat, and the extractor has to move it: our fan and filter guide says to go a size up for an HPS. In winter that heat is useful, so a heater is mostly a lights-off job.

Ways to cut the bill

Dim the light. Young plants need less: our LED wattage guide suggests 25 to 50% power in vegetative growth. The Daylight PRO (200W to 660W), Varidrive 3.0 720W and Gorilla 720W pages all give dimming down to 25%. As a rough guide the draw falls with the dimmer: our LED or HPS? guide counts a 720 W light at half as roughly 360 W, or 6.48 kWh over 18 hours instead of 12.96.
Put an EC fan on a controller. The Air EC Inline Fans page says they use less than half the power at half speed, and up to 40% less than AC models at full output. A controller such as the G.A.S. Enviro4 EC Fan Controller adjusts the speed automatically to hold a set temperature and humidity - ask us which fits your fan.
Run the lights at night. Some tariffs charge less overnight. If yours does, set the timer so the lights-on hours cover the cheap period. Night is also the coldest part of the day, so the heat is easier to shift in summer and does the heater's work in winter.
Heat the tent, not the room, on a thermostat. Every heater here gives the same heat per unit, so the saving is in heating less. A 120 W Gorilla tube heater left on for 12 hours of lights-off uses 1.44 kWh; a 2 kW fan heater uses up to 2 kWh for every hour it runs. The tube heater pages list no thermostat; the G.A.S Day & Night Controllers have a temperature model for heaters.
Set the dehumidifier to a target. A dehumidifier is one of the bigger loads here. Set a target humidity instead of running it continuously - see grow tent humidity.

A worked example: a 1 m tent

Take our 360W LED Grow Tent Kit - 1.0m x 1.0m x 2.0m: a 1 m tent with the Varidrive 360W LED and an Air EC fan. Add a 60 W Gorilla tube heater for the lights-off hours in winter.

From the two tables above, before the fan.
kWh	Flowering (12 hours of light)	Vegetative growth (18 hours of light)
Light, 360 W	4.32	6.48
Tube heater, 60 W, lights off	0.72 (12 hours)	0.36 (6 hours)
Total a day	5.04	6.84
Total a week	35.28	47.88

The light and heater figures are maximums: a light dimmed in vegetative growth and a heater on a thermostat use less. The fan comes on top: read its rating plate and add 0.24 kWh a day for every 10 W.

Quick answers

How much electricity does a 600W grow light use?

0.6 kWh for every hour it is on: 7.2 kWh over 12 hours, 10.8 kWh over 18. An HPS ballast adds a little.

How much does it cost to run a grow tent?

Add up the units a day for everything that runs, then multiply by your unit price. Our 1 m example, with its winter heater, comes to 5.04 kWh a day in flowering, before the fan.

Is LED cheaper to run than HPS?

Yes, when it does the same job on fewer watts. The Varidrive 360W LED page compares it to a 600W HPS: 4.32 kWh against 7.2 over 12 hours. But a 720 W LED at full power uses 8.64.

More Growers' Guides

Not sure what you need? Call the shop on 0117 971 3000 or pop in to the Bristol store and we'll help you pick.