Since some people do not want to live near a power line, and probably no people consider that a must have feature, then yes, power lines do reduce demand and thereby reduce value. So that should mean that when you buy you can get the property at a lower price, but when you sell you will also get a lower price.
For buyers the question is how much will the power lines bother them. If not at all then the house near lines would save them money. If a lot, the house probably should not be bought at any price.
There will be the same impact on renters. Some renters will not want to live near power lines, so that will make it somewhat harder to rent and/or you may have to adjust your price.