Deep decarbonization of energy systems benefits from increased flexibility capabilities such as sector-coupling to heating through power-to-heat (P2H) conversion. Here the applicability of 2 km deep boreholes in rock for heat source and heat storage in heat pump systems supporting P2H schemes are analyzed through numerical 3D simulations. Due to higher rock temperature and deeper wells, a single 2 km hole corresponds to 30 conventional 300-m boreholes, thus enabling better use of ground heat pump systems in urban areas. A coaxial deep-heat well could produce 110 kW (55 W/m) in Finnish conditions. Regenerating the well (heat storage) for 1 month through P2H could improve the performance by over 10%. The deep-well could absorb even up to 1 MW charging power in short cycles. The deep-heat well performance is affected by geophysical parameters e.g. properties of the rock, structure of the borehole heat exchanger, flow conditions, and operational strategies. The results can also be applied to other conditions through alternative borehole configurations (e.g. using lining).