Power will quickly become an incredibly important resource in this game. When you get to the point of building defensive towers, you will find the power requirements are growing with every passing in game day.
Build mills as often as you can
Mills can’t be built in bunches together. Like many resource buildings, you must keep a certain distance between them. As you are expanding your settlement, build mills now and then to ensure that you have a power supply that can last you into the future.
You will eventually obtain an upgrade to these that will increase power production from 30 to 50. Not quite double, but it is a significant jump considering there is no ongoing cost to a mill.
Mills might get in the way a bit and they might not produce as much power as some of the other options do, but you have to build lots of them if you hope to build a large settlement that can survive the incoming waves.
Building Power Plants
Power plants are a bit of a double edged sword. They produce a huge amount of power. You can place them all together in groups to make the most of the space you have. The downside is that they will consume -10 wood. If you consider the fact that a well placed mill will probably generate around 10-15 wood, a power plant is going to use up all of the resources that a sawmill is going to produce.
You could probably build one or two of these and get by just fine. Once you have unlocked most of the building upgrades, you will not need much wood. You just need to be careful not to overbuild and end up with no wood to sustain your colony.