  Let $G$ be a simply connected solvable Lie group with a lattice $\Gamma$ and
the Lie algebra $\g$ and a representation $\rho:G\to GL(V_{\rho})$ whose
restriction on the nilradical is unipotent. Consider the flat bundle $E_{\rho}$
given by $\rho$. By using "many" characters $\{\alpha\}$ of $G$ and "many" flat
line bundles $\{E_{\alpha}\}$ over $G/\Gamma$, we show that an isomorphism
\[\bigoplus_{\{\alpha\}} H^{\ast}(\g, V_{\alpha}\otimes V_{\rho})\cong
\bigoplus_{\{E_{\alpha}\}} H^{\ast}(G/\Gamma, E_{\alpha}\otimes E_{\rho})\]
holds. This isomorphism is a generalization of the well-known fact:"If $G$ is
nilpotent and $\rho$ is unipotent then, the isomorphism $H^{\ast}(\g,
V_{\rho})\cong H^{\ast}(G/\Gamma, E_{\rho})$ holds". By this result, we
construct an explicit finite dimensional cochain complex which compute the
cohomology $H^{\ast}(G/\Gamma, E_{\rho})$ of solvmanifolds even if the
isomorphism $H^{\ast}(\g, V_{\rho})\cong H^{\ast}(G/\Gamma, E_{\rho})$ does not
hold. For Dolbeault cohomology of complex parallelizable solvmanifolds, we also
prove an analogue of the above isomorphism result which is a generalization of
computations of Dolbeault cohomology of complex parallelizable nilmanifolds. By
this isomorphism, we construct an explicit finite dimensional cochain complex
which compute the Dolbeault cohomology of complex parallelizable solvmanifolds.
