Probably the best way I can think of to do this is via the creation of a new thematic style. If your file is already constructed as lines (which it sounds like it is) - all you need to do is go into the layer properties and create a thematic map (colour bands) based on the breaks you want to represent (i.e gigawatts).
Next double click on each symbol to bring up the Symbol selector, click on change (under the image of the current symbol) to bring up Symbol Properties box and select "Simple Line" from the Symbol Layer Type drop down.
From here you can edit the line for width, colour and transparency.
Do this for each of the lines representing the breaks.
Finally, when you've got a style your happy with - use the "Save Style" button (bottom right of the layer properties box), see image below. This will save the style as a "QGIS Layer Style file" (*.qml) - which you can load for different maps in the future.