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GPW, a tool for comparing harvests.

louise

By louise
in Cannabis Harvesting

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louise Grand Master Kush

louise

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I thought I would put this up as there is some confusion about what 'gram per watt' means and how to calculate it. 
 
GPW is a statistical measure of the total amount of lighting energy used to produce each harvest. Thus GPW isn't simply -  I used a 400 w lamp and harvested 400 g... I have an efficient grow.
 
How then would we compare to grows, each using a 400w lamp and yielding 400g but differing in other factors?
 
To be meaningful, we must take account of energy use over time. Once we have done that we can compare grow cycles that vary in a range of factors (lamp size, time, lighting schedule, and harvest). The worked example is imaginary and answers the question above... 
 
Grow A  Vegetative Phase

400w lamp
20 days
24hrs/day
 
Grow B Vegetative Phase
 
125w lamp
50 days
18hrs/day
 
 
Calculate energy used during the vegetative period (kw/vp).
 
Step 1. Divide lamp wattage by 1000 to determine the energy used each hour (kw/hr)
 
A 400/1000 = 0.4kw/hr
 
B 125/1000 = 0.125kw/hr
 
Step 2. Multiply kw/hr by daily light cycle to determine energy used each day (kw/day)
 
A 0.4KW x 24hr/days = 9.6kw/day
 
B 0.125kw x 18hrs/day = 2.25kw/day 
 
Step 3. Multiply kw/day by the number of days in vegetative phase to determine energy used during the entire vegetative phase (kw/vp)
 
A 9.6 x 20 = 192 kw/vp
 
B 2.25 x 50 = 112.5 kw/vp
 
 
Now we need to do the same calculations for the flowering phase.
 

 
Grow A Flowering Phase
 
400w lamp
50 days 
12hrs/day
 
400g harvest
 
Grow B Flowering Phase
 
400w lamp
70 days 
12hrs/day

 
400g harvest
 

First we will calculate the kw hrs used during flowering period (kw/fp)...
 
1. Divide lamp wattage by 1000 to determine kw/hr
 
A 400W/1000 = 0.4kw 
 
B 400W/1000 = 0.4kw
 
2. Multiply kw/hr by daily light cycle to determine kw/day
 
A 0.4kw x 12hr/day = 4.8kw/day 
 
B 0.4kw x 12hr/day = 4.8kw/day
 
3. Multiply kw/day by the number of days in flower to deterime kw/flowering period
 
A 4.8 x 50 = 240 kw/fp
 
B 4.8 x 70 =336 kw/fp
 
Okay, now we need to add the kw/vp to the kw/fp to get the total kw/grow cycle, before calculating the final GPW figure. Exciting hey?

 
A 192 + 240 = 432 kw/gc 
 
B 112.5 + 336 = 448.5 kw/gc
 
Drum rollllllllll :yahoo: last calculation....
 
Divide the harvest in grams into the kw/gc
 
A 400grams/ 432kw = 0.926 GPW
 
B 400grams/448.5kw = 0.892 GPW
 
Any questions?
 
Credits, I first heard about the GPW at cannabis.com at the beginning of the century and to refresh my memory for the formula, I referred to a post at rollitup.

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louise Grand Master Kush

louise

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Great post Lou. I brought this up a few years ago and got flamed to death lol

 

Yeah, things have changed in the forum over the last few years and I thought people here were ready for this now... 

 

I expect you were flamed by exactly the same members who flamed me for suggesting a slightly salty solution is better for flushing than plain water.... the good ol' days lol

 

thanks for the post Lou - I love doing stoned maths

doubly so when it's stoned, stoner maths

 

guess I should try and figure my GPW  for my last grow

but first, must get into the stoned stoner maths mood :bong:

 

No worries Matti, I found ganesh spirit to be a good choice for stoner maths... when it came to editing/formatting I had swapped over to the kush and I think it shows in the format... resisting the urge to re edit.

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