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How do I accurately measure my Grow Room Efficiency?

Smokin Moose

Fallen Cannabis Warrior

How much juice you burn to grow a gram..

Standard methods of reporting garden yields are imprecise and don't take the factor of time into account. This is a summary after discussion on a new standard formula for reporting yields and efficiency.

The GE Rating is based on the amount of kilowatt hours used to produce one gram. This factors in how long your strains flower and the resulting number makes it simple to compare 4000 watt gardens with 400 watt gardens. The calculations are easy, here is an example:

2000 watt flowering area
60 days of flowering
12 hours on light cycle
total yield: 49.5 ounces

1) Divide the total wattage of your lamps by 1000 to determine the kilowatts per hour:

2000 / 1000 = 2 kw/h

2) Multiply the previous result by the hours of the light cycle (usually 12):

2 x 12 = 24

3) Multiply the previous result by total days in flowering to determine how many kilowatt hours you used:

24 x 60 = 1440

4) Make sure your yield is in grams, so:

28.4 x 49.5 ounces = 1405.8 grams

5) Now simply divide grams by kilowatts:

1405.8 / 1440 =

0.97 grams per kilowatt hour, or a GE Rating of 0.97! Good job to the hypothetical grower, approaching a GE of 1 is the first plateau to be reached and is a sign of an experienced grower.

The secondary additions to the GE formula would be for the "Whole Garden GE Rating" which must include time spent in cloning and in veg. Figure it out the same way, with days spent under cloning lights, hours per day, and add those totals to the watt usage before dividing. This will give you a much lower rating than the standard Bloom GE Rating, but is more accurate for SCROG gardens, and anyone who is involved with extra vegetation time.

We have had a few contributors that have posted their monstrous GE Ratings..

OT1 - average 1.43, high of 1.89
Cobber - 1.75
Sharrina - 1.63
Quimik - 1.58


New Member
There was no mention of the kw/hrs used for fans, supplemental lighting, pumps (if hydro) etc.
Using only the lighting for your calculation is misleading.

Be Irie

Nug of the Year: 2009 - Plant of the Month: Aug & Sept 2008, Dec 2008, Jan 2009 - Member of the Month: Feb 2009 - Nug of the Month: May 2008, Jan & Feb 2009, Nov 2012
He gives you an example and a formula. so all you have to do is add the wattage in for whatever else you are using??? Let's use our heads a little bit here, it's not rocket science... :smokin2:


New Member
You are right BE,
I should not have used the term "misleading" sorry about that.
What I meant to say did not come out in my post, sorry about that bro.
Sometimes I look back at things I say and wish I would have chosen my words more carefully.


New Member
My sproutling didn't even make it to the month mark and bugs and root nasties are killing it already. Im pissed.:helpsmilie::icon_roll


New Member
Is that 49 oc. wet or dry. I would think dry but needed to ask.


New Member
The basic information provided in the original post is useful and a good start, but with all due respect I'd like to suggest that the only real measurment of the electrical efficiency of a grow is kilowatt hours (KWH).

If you use a 600w HPS from seed/clone to harvest, your total electrical usage will be very different than if you veg for a while with CFL, then move to 2ft T5 then switch to the 600w HPS.

And has been prviously mentioned, blowers, filters, heater, air conditioning, pump or no pump, it will all affect the total energy usage.

Fortunately for us, there is an inexpensive device that will accumulate, measure and calculate all the information we need.
The Kill-A-Watt plugs into your outlet and displays voltage, amps, KWH etc.
It's also very nice to use just to monitor your amperage from time to time to make sure your wiring is safe and happy.
They are rated at 15 amps, so just plug a 15 amp plug strip into it and everything else into the plug strip.

I used mine on my old freezer and was surprized to find out that I could buy a new efficient freezer that would save enough electricity to pay for itself in a little over 1 year.

P3 - Kill A Watt

Amazon, Home Depot etc have them for $25 or less.

Hope this helps,
an old gardener
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New Member
They work ok, I do wish they made one for 220v my air lights ets. are all 220v
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