Good question – the functional unit is one of the first steps in doing an assessment. It can be very simple, but be aware that that simplicity can be deceiving. By choosing the functional unit carefully, you leave options open for a variety of approaches and solutions – much like good framing for a design problem.

A functional unit should contain three characteristics, or precepts:

1. It should be a quantity of service that a user readily consumes.
2. It should include a physically measurable unit.
3. If the product involves time a part of the service, then a unit of time should be included.

Here are a few examples:

If you were assessing showers, for example, there would be many options for selecting your functional unit. Each would delimit the study in some way:

• If you chose to assess Okala millipoints per showerhead itself (without water or energy consumption to heat the water), then designs that affect water consumption are not factored into the assessment.
• If you chose Okala millipoints for one 8-minute hot shower, then the quantity of water would be assessed as a variable, depending on the quantity of water per minute that the showerhead releases.
• If you chose Okala millipoints per 10 gallons of 110 degree Fahrenheit shower, the quantity of water and energy would be set, not variable.

Each of these approaches is valid; it is usually a simple matter of thinking ahead to the kind of design exploration you anticipate doing (or have already done) and then deciding how to establish the functional unit so that your work is best explained and supported.

In the case of printers, you might choose a single printed sheet as the functional unit. Indeed, the printed sheet fulfills the essential characteristics of a functional unit; it is both (A) a quantity of service that a user readily consumes, and (B) a physically measurable unit. This allows a comparison to other ways of delivering the service of a single printed sheet, such as a central printing service.

If, however, you chose the amount printed in a typical office in an average week as the functional unit, you might provoke some thinking and assessment of other printing options, like a photocopier with a better preview function to reduce the total amount of wasted prints.

Here is another example of how the selection of a functional unit changes your perspective:

If you were assessing lawnmowers, the quantity of service that a user readily consumes might be the mowing of one average sized lawn (impacts per mowing of 1/5 of an acre). This would allow you to compare other options for that mowing session, such as a push mower, a solar powered mower, or a re-designed mower.

On the other hand, if you wanted to compare having the lawn maintained by a goat, or replacing the lawn with low-maintenance landscaping, you might want to look at impacts per year of 1/5 acre of a groomed yard. This would allow you to expand the system boundary to include the planning and fertilizing of the grass versus the groundcover plants, to include storage capacity needed for the mower and fuel, versus a shed for the goat and food.

For the shed or storage, these items would need to be divided by the number of years that these structures delivered their desired service. Length of time then becomes potentially important, and is the third key characteristic when determining the functional unit.

It’s a helpful exercise to develop a number of functional units for a new project, and compare the perspectives they offer you. Depending upon the project and the environmental situation, you understand up front how a functional unit is like a lens on the solution.

And, as in any good design undertaking, it’s all in how you frame the problem.