The heat still needs to travel horizontally from this rock to your borehole. A 30 cm diameter borehole drilled 20 km deep with the bottom 12 km being used for h

The heat still needs to travel horizontally from this rock to your borehole. A 30 cm diameter borehole drilled 20 km deep with the bottom 12 km being used for heat collection would give you 12000 m^2 of surface area and an average temperature gradient of 135 Kelvin, which at a heat flux rate of 7 W/m^2/K would only give you 13 MW thermal, or about 4.4 MW electric when the well is first drilled and about 3 MW after 50 years. Comparable to a single windmill. You can increase your surface area through more drilling and fracking, but that adds to cost and environmental concerns. As a battery for storing heat energy generated by other methods though it's great.