It is in Chapter Five of Animal Farm that the subject of building a windmill first emerges. For Snowball, the windmill is an exciting prospect because it can be used to build a "dynamo" which can supply the farm with electricity. Once the farm has electricity, Snowball plans on building a range of "fantastical machines" which will improve the lives of the animals by giving them lighting, heating, and labor-saving devices.
In contrast, Napoleon really disapproves of the windmill. In fact, at one point, he urinates all over Snowball's plans as a means of expressing his discontent. In addition, Napoleon argues that the animals should focus on increasing food production instead of building the windmill. In his mind, the windmill represents a waste of time and energy.
Ultimately, Snowball wins over the animals, but Napoleon will not let Snowball be victorious: he runs him off the farm, never to be seen again.