Power
Many objects require power to function, which can be generated in a variety of ways.
Many objects require power to function, and as your society progresses the types of power will progress as well.
Early on, most power will come from calories expended by citizens. Local-power sources can also be built such as Waterwheels, powering only the immediate vicinity. When technology progresses to electricity, power can then be generated centrally, allowing groups to form together to power towns and cities as a group rather than having to individually setup power sources (community projects like this are a great fit for using the Taxes system to fund them, and Contracts and Work Parties to get them built).
Many types of power generation exist, and each has different fuel needs as well as environmental impact. The building, fueling, and operation of power plants can become a major Pollution source as technology advances, and citizens will need to carefully regulate the types of power used and how pollution is handled to prevent an ecological catastrophe.