Fuel when idling?

At 50%, plan on replacing batteries in 3 years.

You don't get energy for free. Even though you're going to see slightly better conversion efficiency of diesel to electrical energy from using your main engine vs. an APU, it will most likely be a wash, all things considered.

Personally, I'd trade the 1-2% conversion efficiency to be able to have an independent and more versatile power source (APU) than to have to be dependent on a limited battery bank to keep me cool in hot weather. Especially when you run the risk of not being able to fully charge your aux batteries from time to time due to short runs, or layovers longer than the 10 hours that the batteries limit you to.

 

I got over 6 years out of Optima AGM yellow top batteries used as starter batteries. I believe I can get 6 to 10 years out of deep cycle AGM batteries. Those deep cycle AGMs are made to do what I will be using them for and even if I did replace them in 3 years, I would be thousands of dollars cheaper than the diesel APU route.

I don`t sit more than 10 hrs anywhere and if I did, I will get paid very good to do it and get a motel room.