In Life Cycle Assessment, the functional unit is the output(s) of the life cycle and how they are defined. The function is delivered by the life cycle, and most of the time by the use phase. The purpose of the functional unit is to make things comparable.
Type of functional units in Earthster
Earthster lets you define multiple analogous units for a given cycle. That means that you define different ways of measuring the output of your cycle.
For example, for the life cycle of a toaster, the output may be one toaster, 10 years of use, or 3000 slices of toasted bread.
We call all those numbers measures of the scale of your cycle. We distinguish between two types of unit: production units (what you produce) and functional units (what benefit or function the user gets).
The simplest functional unit is just how many products you are measuring the impact of.
Unfortunately, comparisons 'per product' are only useful in commodities (since they are exchangeable). Most product comparisons that you model will require an additional functional unit.
Production units are typically measured in physical units (either in units of your product, e.g. number of toasters; or in mass or energy, e.g. kg of steel).
In some cases, they may be also measured in currency, since you are doing a Life Cycle Assessment of a given production measured in currency (e.g. if you are measuring the total production of toasters for a given year, you might specify the total revenue from that production).
Functional units measure the functional output of the cycle. They measure the benefits delivered by the product, in a way that will make the results comparable.
The functional unit can get progressively more sophisticated as you get more information, so you are encouraged to set one, and refine it later.
An example of progressive enhancement
Let us look at the example of a diaper manufacturer, wanting to compare disposable and reusable alternatives.
A good start would be to think of a diapering (one use of a product).
If we have information about how often people use those diapers (e.g. if we know that single user diapers are changed more often than reusable diapers per week), we can update that functional unit to time units (a week of diapering).
But if we have even more information, and we know that parents potty train later when using single use diapers, we can expand the functional unit to a childhood of diapering.
In this case, the production units could be 3000 diapers, or 1000€. The functional units could be 3000 diaperings, 100 weeks, or 1 childhood.
What is new in comparison to others?
Life Cycle Assessment literature often conflates all of the terms above (production unit and functional unit) into the term "functional unit". You can still use that naming, but we encourage you to take a functional approach (the intent behind ISO defining functional units) in those.
Also, most of your impacts will be proportional to some unit in which you mention your production. You know the materials per product. Because of that, many practitioners use that as the reference flow of their analysis, and call that their "functional unit".
Our naming stays strict with ISO definitions for functional units, and places them side by side with the relevant reference flows for production (leaving other secondary reference flows as documentation, since they do not have such a primary role when comparing two cycles together).