Does anybody know how much power is lost when the chain is not lubricated properly? I drive a rather heavy bike and experienced that when the chain is not lubricated it definitely has an effect. Can anyone give an estimate in percentages about the amount of power lost due to a non-lubricated chain.
New, factory-lubed chains have about 7W of losses at 250W power:
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.