A system used to heat a subsurface formation includes an elongated heater at least partially located in an opening in a hydrocarbon containing layer of the formation. The opening extends from the surface of the formation through an overburden section of the formation and into the hydrocarbon containing layer of the formation. The elongated heater includes an electrical conductor, an insulation layer at least partially surrounding the electrical conductor, and an electrically conductive sheath at least partially surrounding the insulation layer. The elongated heater tapers from a larger thickness at a first end of the heater to a smaller thickness at a second end of the heater. The first end is at or near the junction between the overburden section and the hydrocarbon containing layer and the second end is further into the hydrocarbon containing layer.