The windmill is one of Snowball's great ideas, a way to take some of the workload off the animals, allowing them more leisure time and so more time to enjoy their lives. His idea is entirely in keeping with Old Major's original philosophies, that the animals, working together as a team, will benefit everyone and be more free because of their shared labor. However, Napoleon sees the windmill in a different light, and when it is finally finished, it does not help to relieve their work:
The windmill, however, had not after all been used for generating electrical power. It was used for milling corn, and brought in a handsome money profit. The animals were hard at work building yet another windmill...
(Orwell, Animal Farm, msxnet.org)
Instead of using the shared labor of the animals to create electricity, taking the burden of work off the animals, Napoleon uses it to make money for his own use. Since he and the pigs are living as dictators, keeping and consuming far more than their fare share, the windmill no longer represents shared labor, but slave labor.