I bought this heater to keep the temperature in my small (6'x8') uninsulated greenhouse above freezing this winter. On the lowest setting it kept the temperature at 50 degrees when the outside temperature was in the 30s and at 40 degrees during a cold snap where the temperature stayed in the low to mid 20s. Haven't got the electric bill yet so I can't say how efficient this unit is, but the plants offered to chip in to cover any increased cost.