Like Eric said it is great insurance with lights and fans. My fans turn on at 76 and turn off at 75, two MH turn off if the temp gets to 79, the third at 80 and VHO off at 81. That way if my AC goes out during the day the lights don't cook the tank. And all for $99 for a RKL, Temp Probe and 1 PC4, I have no idea why I waited so long to get one? With a RKE the sky is the limit, I want a MLC next, and another PC4.