When we talk about supply and demand for cars, it has a time element. Units per year, units per quarter and so on. It is analogous to speed. Doing anything else

When we talk about supply and demand for cars, it has a time element. Units per year, units per quarter and so on. It is analogous to speed. Doing anything else would make it impossible to plan the supply chain. Where it could be different is in places like power generation. You might talk about peak demand in Watts, and daily demand in watt-hours.