In later Eras more and more buildings start to depend on electricity, but it isn’t immediately clear what all you need to supply electricity. The first way of making electricity you’ll have access to is the Power Plant. To supply the Power Plant you’ll need either coal of oil. Coal is less effective but easier to obtain, Oil is more effective but you’ll need to unlock the Oil Well. Once the Power Plant is supplied with either resource it will start emitting electricity. Selecting the building will show Electricity, Network, and Network Balance. Electricity is the amount of power output from that individual building, Network is the grid it’s connected to, and Balance is the overall net electricity from the entire network.
The most important thing is to make sure everything is on the same network. If you click on Overlays, then Electricity you’ll see a green circle around the Power Plant, and red everywhere else. This is the area that is currently being supplied. To extend this you’ll need Electric Substations. Here’s where things might get confusing. The Power Plant has that green circle denoting supply. The Substations will also have their own green circle, although you cannot see it until it’s built. You need to make sure these two green circles are overlapping to be connected to the same network. If the two circles are separate when you check the overlay, they will be on different networks and the substation wont be in use. If the area of coverage ever turns yellow, you’re getting close to overloading the grid and will need more power.
In later Eras you can build different types of power plants. Nuclear Power Plants supply a lot of electricity but they need Uranium, which can be hard to find. Wind Turbines don’t need any resource but only produce a small amount of electricity and last is the Solar Plant which also needs no resources, but requires the Modern Era and is the most expensive to unlock.