In case of A/C, bigger is better. In fact over 10% better in that case. Ask folks camping in the heat wave.
2010 Ford Expedition TV
2010 Outback 230RS Toybox, 5390# UVW, 6800# Loaded Not yet camped in Hawaii, 2 Canada Provinces, & 2 Territories I can't be lost because I don't care where this lovely road is going
We had both options from the factory on our small class C. They obviously had no concerns about freezing, etc. We chose the 15. It had the added bonus of also having a heat pump mode. Several mornings on our trip to the UP of Michigan we used the heat pump to take the morning chill off. That means we used the campground electric not our LP with the furnace. Now we are back in the heat and the AC is running. Mostly runs on low fan speed but when max cooling is needed it has the horsepower. All things considered I am very happy with the 15.
Bigger is better. There is no chance of a single 15K btu AC running cold enough to short cycle. The salesman wants to sell you the 13.5 unit because that's what he has in stock and he wants to get rid of it. The difference in run current draw is a couple of amps, and the LRA at startup is less than a second, which except on the smallest generator is not going to make any difference.
Throttling back? The only thing you can regulate is the fan speed and the thermostat that determines how long the compressor runs during a cycle. The compressor is going to run at the same speed all the time. You can change the running time by changing the temperature setting, but you have no control over the temperature coming out of the register. Sometimes the lack of understanding of air conditioning equipment in these threads amazes me.
In the summer parked out in the sun as you would be in an rv park you cant have too much cooling and even 15,000 btu's at times is still not enough I doubt if you will ever have to cut it back because of "over cooling" I doubt if you could feel 1500 btu difference. But I would still go for the 15,000 btu one. Why? I suppose its all mental..You think the larger is cooling better when its really not, out in the direct sun.. The secret is called shade which is very difficult to find in any rv ghetto.....
If it requires 15k to cool the coach in the heat of the day, that's one matter. If a smaller unit can do it, and does it reliably, then the 15 is oversize. That will show up in two places. First, more current draw. Yes, on a hookup site you're paying only to be ON electric, not by how much you use, at least in most cases. But AC needing more amps can translate to breakers tripping etc. We have far more breaker (coach and pedestal) issues with our 15k than we did in our first coach with 13.5. Second, an oversized unit won't run the compressor long enough to adequately dehumidify the area being cooled. Translation: Clammy Coach. We get that now with 15k in our 31'. So I'd say unless your're delivering flowers, maybe meat, 13.5 is adequate in the smaller units.
There even used to be an 11k but I think I'd be reluctant for that in any C coach 20-some foot trailer.
If God's Your Co-Pilot Move Over, jd
2003 Jayco Escapade 31A on 2002 Ford E450 V10 4R100 218" WB
The difference in compressor current draw for the Dometic non-heat pump units is 1 amp. If 1 amp gives you breaker problems either at the pedestal or at your breaker panels, then you have other issues and you might do well to look at some kind of energy management system. The heat pump versions will draw a little more because they have a reversing valve that is energized in cool mode.
also, I seriously doubt that a single 15K unit is going to cycle often enough, even at night, that it is not going to dehumidify the space. If you don't believe that, walk outside and look at how much condensate is dripping off of your trailer roof. These trailers and busses are not insulated as well as you might think or would like.
Right now I'm sitting in an Arkansas SP in 100 degrees and my trailer is just comfotable and the AC will not shut off all day. At night it will cycle as it should and keep the trailer comfortable. A 13.5K simply would not keep up, even at night. My first trailer was a Jayco 22FB with the small AC and it was miserable in the summer. I can't imagine anything smaller than the biggest unit you can buy in the Texas or Florida heat, and I have yet to have a customer call me and complain that I sold them an AC that was too big. That's like waking up and wishing you had a smaller truck