Snowball is an idealist, and truly believes in the concept of collective work to ease the workloads for everyone. In his vision, the animals would sacrifice some of their time and comfort, perhaps going without as much food in order to provide a better future for all. While building the windmill will take time and effort, it will ultimately increase the production and comfort of the farm.
...Snowball conjured up pictures of fantastic machines which would do their work for them while they grazed at their ease in the fields or improved their minds with reading and conversation.
(Orwell, Animal Farm, george-orwell.org)
His ideas threaten Napoleon's slow increase in power, and an actual improvement in the farm comfort would give the animals time and leisure to think about their position and how hard they have to work to support the pigs. Napoleon therefore opposes the windmill, claiming that the time spent building it would be better spent on normal farming and food production.
He says that there will be running water and electricity in each animal stall