Load your photo

Open up your photo in Photoshop, then go to Windows > Layers. 

Remember to set up your file for non-destructive editing by clicking the (+) icon in the Layers panel to add a new layer you can mess with. Your original layer will remain completely intact.

Select the Spot Healing tool

Head to the Tools menu (Window > Tools) and select the Spot Healing tool. Set the mode to Normal and the type to Content-Aware, then click the box for Sample All Layers. 

Choose a brush size that matches the size of the power line, with a Hardness setting of 100% and Spacing setting of 1%. Fatter power lines need bigger brushes. 

Tip: Use the bracket keys to adjust the size of your brush.

Brush along the power line.

Using one clean sweep, drag the brush along the power line from left to right and watch it vanish. Hold down the Shift key to constrain your brush to a perfectly straight line.

Tip: You can use the Eraser tool to undo your Spot Healing Brush strokes on your working layer.

Tidy up your photo

Drag the Spot Healing tool along any power lines in your shot, even ones that go across tricky backgrounds, like trees. The brush will take care of it easily, leaving you with a clean image.