Energy harvesting from flow-induced vibrations (FIV) of an elastically-supported cylinder (diameter D) immersed in the wake of another cylinder with diameter d is systematically investigated. The diameter ratio of the two cylinders is d/D = 0.4. The incoming flow velocity considered is U∞ = 1.54–9.0 m/s, corresponding to Reynolds number Re = 3.3 × 103 to 1.95 × 104. A controllable magnetic damping system that can add harnessing damping ratio ξ is used to convert wind to electrical energy. The ξ is varied from 0.00091 to 0.00249. The cylinder spacing ratio L/d is varied from 1.0 to 4.0, where L is the distance from the upstream cylinder center to the forward stagnation point of the downstream cylinder. The vibration responses comprise combined vortex-induced vibration (VIV) and galloping vibration for small ξ and separated VIV and galloping for high ξ. The galloping offers harnessed power P three times larger than the VIV. The maximum efficiency η is 11.2–14.6% in the VIV regime but is only 1–2% and almost independent of reduced velocity Ur and ξ in the galloping regime.