We study rank 1 flat bundles over solvmanifolds whose cohomologies are
non-trivial. By using Hodge theoretical properties for all topologically
trivial rank 1 flat bundles, we represent the structure theorem of K\"ahler
solvmanifolds as extensions of Hasegawa's result and Benson-Gordon's result for
nilmanifolds.Comment: To appear in Int. Math. Res. Not. IMR