A system like Sanden CO2 or Chiltrix will heat your water with outside heat, so it's always 2x+ the efficiency of resistance heat. If your climate is predominat

A system like Sanden CO2 or Chiltrix will heat your water with outside heat, so it's always 2x+ the efficiency of resistance heat. If your climate is predominately hot then thermal collectors are far more efficient, but interior hot water heat pumps would be an okay choice where that's not possible.