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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.