Reefer container is fully insulated with PU insulation thickness from 3 inches to 4 inches. It can maintain cargo temperature even when power is out. This is a case study that we experienced. The power was out for 14 hours and return air temperature increased only 0.4c. However, please note that the cargo temperature is very near the set point temperature. If the cargo temperature is not near the set point, the outcome could be different.

Before power out, container set point is 2c, return air temperature is 4.6c

The power was out for 14hours. When the power is back on, reefer container automatically starts to operate. The container set point is the same at 2c, return air temperature increases to 10.5c but quickly drops to 5c. And after 1h30m the return air temperature is back to 4.6c which is the same as before power out.