Question: I am hoping that you can answer a basic question. Gas Insert or Pellet insert - Which of these inserts is the most energy efficient? Which is the most cost efficient? Are there other important factors to consider? Thank you for your help. Answer: The Gas Insert will be cheaper to operate if you have Natural Gas- about the same if you have LP....but still the LP insert will require less service and fuss, and also does not rely on electricity to operate. <p> Then again, Pellet Fuel is a renewable resource while both Natural and LP gas are fossil fuels.