Opti-idle is a system to keep you comfortable without the truck running constant. I started driving in a truck with it. I just finally bought my own truck and am planning on having it installed if I can find out where to go. Probably Freightliner will put it in. Way cheaper than an APU and less maint problems. You set a thermostat in the sleeper and go to bed, the truck will start and stop as need to maintain the temp. You get used to it quickly and soon it won't even wake you up.