Tracking Productivity & Expressing Yields
A yield expression is a mathematical fact expressed as a rating - a measure of productivity or efficiency. In its simplest form a yield expression looks like this.... LE=Y/L (Light Efficiency=Yield÷Light). With Yield normally expressed as grams, and Light as lumens, it's written simply as grams per lumen or g/l. The result is a gram figure, the magnitude of which denotes a more or less light-efficient garden. A similar expression is used to discover the efficient use of canopy space (g/square foot). The use of energy is often ballparked with the g/watt expression, however, a more advanced and precise measure of energy efficiency is obtained using g/kilowatt hour. What growers want to learn from these basic yield expressions is which crop, garden or grower showed a better ability to produce from the light, space or energy resources each had on hand.
About the Basic Calculator
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The information entered is for flowering facilities, meaning the flowering room or area from which the harvest was made. This includes single-area gardens that are used for both the vegetative and flowering phases, or multi-area gardens where information for just the flowering portion is entered. For users willing to spend more time entering data, greater detail can be found using the advanced version.
In the Basic Yield Expressions calculator form below, information is broken down by a harvest's three major players - weight, space, and illumination. Illumination is further broken down by the two factors most growers are interested in - light and energy (or lumens and watts). After entering data for your indoor garden and crop production, click the Calc button to return three measures of garden efficiency based on its yield. A fourth figure, lpw (not based on yield), was added for reference sake, it returns the electrical efficiency of the lamp(s) being used.
Formulas used in the calculator are shown in parenthesis under each expression's name. Although lumens are expressed as kilolumens (kL or 1000 Lumens) to keep decimals manageable, you should enter your lumen information as total lumens and the calculator will make the conversion to kL for you. It's best to enter the initial available lumen rating for your particular lamp. If you don't know its rating, you can Google the manufacturer's web site, or click the link in the form and use one of the generic ratings.
Should any physical changes be made to your garden's space or illumination between crops, a second line is included to provide a means for tracking the before & after results and comparing them in side-by-side fashion. Caution should be exercised, however, crop-to-crop yields can vary as much as +/-15% even when conditions haven't changed, and data from several crops may be needed before the import of changes are fully appreciated. Following the calculator is some commentary on scaling, and why the calculator is set up the way it is.
Scale versus Skill
The crop yield produced from a garden is largely a matter of the two major resources it has at its disposal - canopy space and light. The handshaking or interplay occurring between these two resources alone will influence total production more than any other gardening parameter. And though big total production quantities can be impressive, especially to new growers, seasoned growers know that total weight alone doesn't address some of the more important and challenging aspects of indoor cultivation. Big lights, big spaces and the resulting big production numbers haven't as much to do with a grower's ability as they do with the scale of his operation. Quantity alone is a matter of scale - the same quantity from fewer resources is a matter of skill.
Sooner or later a grower's attention turns from production to productivity. Either he'll want to produce more from the space and light he already uses, or produce the same while reducing those resources or costs in time, labor, or expense. For a grower to learn better ways from other growers he needs something more than just total weight to work with. To do this he balances the economies of scale (production) with the efficiencies of skill used to manage that production (productivity), and adopts productivity skills from other growers who make more efficient use of their resources. Yield expressions allow us to scale down the hyperbole of big production numbers in order to examine the skills underlying those numbers.
Scale (the size of garden resources) is reconciled by expressing yield as grams per resource unit rather than just total grams. For example, grams per unit of area or grams per square foot is used to scale space, and for light grams per lumen is used. These equitable expressions of productivity put both large and small indoor gardens on the same level playing field. A level playing field insures that one grower can learn and benefit from the skills another grower used to construct or operate his garden, regardless of the size of their gardens. Even though a small scale grower can't come close to matching the total production of a large scale operation, once the spatial differences are accounted for he can match, and often exceed, its productivity per unit of space or light.
Skill (the use of garden resources) is elusive, often clouded by the infinite variety of garden sizes and lighting options available to those spaces. Though there are actually several skills, or efficiencies, involved in a garden operation that go beyond the basic expressions examined on this page, the Garden & Crop Production information used here will allow the two major resources responsible for yield to be examined. These two resources - space & light, are the foundation on which every garden is built, the skillful use of which every indoor gardener shares in common.
Using Light versus Using Energy
Though electrical energy creates the indoor light we use, it's a mistake to think it can be used to decide which grower is more or less skilled at using light to produce grams. Rating how one uses light to grow more, and how he uses energy to spend less, can give remarkably conflicting facts. Click the Insert Examples button and notice the only difference between the two gardens, the first's 400w lamp (an HPS) produces more light than the second's 400w lamp (a MH).
If one grower had a way to use light better than the other, it isn't being shown in the Energy expressions. However, the Light rating of 7.4 grams/kilolumen reveals important detail about light not found in the g/w Energy rating - the second grower is managing his garden in such a way as to produce 28% more from the light his lamp generates. That, despite the first grower having 10,800 more lumens and a better yielding spectrum as well.
Seeing the better 7.4g/kL productivity, should the first grower adopt skills used by the second grower he would yield almost 3 ounces (28%) more, a fact not learned from an energy rating. Should the second grower adopt the first grower's lamp, he too would see a yield increase, another fact not learned from an energy rating. The obvious implication is that grams/kL can be used as a cross-lamp rating while grams/watt cannot. The benefit is in revealing something superior about the second grower's use of light, for unlike g/w, g/kL ratings are immune to the vast electrical differences between lamps.
Take a moment to look at the electrical efficiency specifications (LPW) for HID lamps used by growers. Notice how the lumens per watt efficiency rating is different for every lamp, especially how the Hi/Lo figures range from 65 to 150. Once aware that a watt from one lamp can produce only 65 lumens of light while a watt from another lamp can produce as many as 150, it becomes apparent that there is a significant disconnect between lumen and watt ratings. And the use of watts cannot bridge this LPW-gap between different lamps without introducing sizable errors. Unlike the 28% error exposed in the example's 96 and 123 LPW lamps, a 65 to 150 range opens the possibility for watts to misrepresent light by as much as 230%. This inability for an energy rating to detect a more sophisticated and direct connection between light and growth does not serve the discriminating grower, and errors of these magnitudes have no justification for one wishing to draw realistic conclusions about the use of light.
For those reasons, when dealing with light, electricity and growth, a lamp's electrical properties are kept separate from its lighting properties in yield expressions. Having a cross-lamp light rating gives us the best of both worlds, and allows us to discover what growers have to offer in both the light and energy departments. Together with the rating for Space, we can evaluate productivity for the two major resources every grower shares in common. And we can do it in a equitable way that doesn't mistake the skillful application of light and space for the scale at which a grower chooses to grow, nor mistake the lamp's wattage for his skillful application of its lumens. More examples can be found here.