Lost power when chain is not lubricated properly
New, factory-lubed chains have about 7W of losses at 250W power: 

https://www.ceramicspeed.com/en/sport/inside/test-data-reports/chain-efficiency-vs-run-time/

Looking at their other tests with various other loads it seems to be a relatively constant ~3% of losses independent of the input power. In their chain lube test even the worst lubricant only has 9W of losses and the best has 3.8W.

As Daniel pointed out in the comments, how much higher the losses of your rusty chain are depends on how bad the situation is. I’d be less concerned about efficiency and more about longevity. It sucks to have to replace a chain every 800km just because you never lubed it. In fact, longevity is probably directly related to efficiency and friction. So if your unlubed chain lasts only 1000km instead of 3000km its friction is probably three times as high.