Horizontal waterwheel mills are part of Portuguese vernacular heritage, but due to their obsolescence, they are generally in a poor state of repair or in ruins. The fact that these structures are often isolated and disconnected from the power grid increases the cost and complexity of their repurpose for new uses. However, the conversion of these mills’ hydraulic systems—from the weir to the turbine—into microgeneration systems is an opportunity to operate these buildings off-grid. To assess this hypothesis, two horizontal waterwheel mill clusters, N1 and N9, located in the parish of Atalia, Gavião, were studied. It was considered the available head, ranging from 5 to 6 m, an estimated flow rate of 10 l/s, a potential hydraulic power of 420 W for N1 and 340 W for N9 and annual energy demands of 232 kWh and 121 kWh respectively. The installation of two Turgo turbines with an efficiency of 70% and two battery banks shows the feasibility of seasonal or intermittent occupancy, operating totally off-grid. Permanent use of the buildings would be possible by reinforcing the microgeneration capacity, through the addition of more turbines and batteries. However, this scenario will need to be assessed in the future through a cost-benefit analysis.