An assembly includes a photovoltaic cell, an electric heater, and a heat conductor. The photovoltaic cell is configured to convert solar energy into electric power. The electric heater is connected to the photovoltaic cell. The electric heater is configured to generate heat in response to receiving electric power from the photovoltaic cell. The heat conductor is connected to the electric heater. The heat conductor is configured to conduct heat generated by the electric heater to a tubular positioned within a wellbore formed in a subterranean formation.