For heating, I'm a minimalist. We are 3 season campers and we never run heat during the night. We use good sleeping bags and comforters to sleep warm and only use the heater in the morning to take the chill off. After we're up and around, our body heat and breakfast cooking keep the trailer warm until we're dressed and outside for another day of exploring and hiking.
A/C vs. Heat Pumps
Roof mount A/C's are ubiquitous in the RV and Cargo Trailer industry. These are very durable, move a lot of air, and the roof mount is convenient. But most of them are old technology and aren't particularly energy efficient. The smallest of these are around 12,000 BTU and use 17-18 amps per hour to run, and they are overkill for a trailer my size. They are fine when plugged into the RV park, but not for battery operation.
Our theme is the All-Electric RV of the future, and I'm convinced that heat pumps are the wave of the future. Heat pumps are common in residential and small commercial buildings, but rare on an RV. A heat pump moves thermal energy by absorbing heat from a cold space and releasing it to a warmer one. They use a compressor and the refrigeration cycle similar to an A/C, but they can also reverse the cycle and pump heat into the trailer. For my application, the heat pump would supply mostly A/C since I'm a minimalist on heating, but would serve as the occasional heater when needed.
The problem is that standard heat pumps you see for a residential application are too big, too unwieldy for an RV. I've seen a Duo Therm Cool Cat heat pump advertised on some smaller trailers but I'll be darned if I can find one anywhere or get any specs on it. And I think they require mounting through a big hole cut in your trailer, kind of like a window air conditioner looks.
But there's another option that I first saw when traveling in foreign countries - the Ductless Mini-Split Heat Pump. The beauty of these is no ductwork. The "split" means the outdoor compressor/condenser is separate (split) from the indoor air-handling unit. The only hole in your building is a conduit for the power cable, refrigerant tubing, and condensate drain. Comparing a 12,000 BTU mini-split unit to our 12,000 BTU roof mount RV A/C, it uses 7-8 Amps or half the Amps. While smaller than the typical household heat pump, they're still a bit boxy and not as convenient to mount as the rooftop models, but I'm going to get around that by mounting the compressor on the tongue and route the hoses into the trailer through a 3" hole and mount the interior blower up high on a wall. I'll also cover the outside unit with either a canvas cover when driving or build a metal and wire mesh cover keep the rocks off the unit.
The smallest standard mini splits I looked at were 9,000 BTU which is still more than you need for a trailer my size, but are already twice as efficient as the rooftop A/C units. Most models are rated at a 7-8 Amp draw and have a SEER Rating of ~13. The higher the SEER rating the more efficient the unit. You can find these online for around $700 which is very price competitive with a roof mount RV A/C.
While the standard mini-split is already an improvement, there are more expensive "DC Inverter" models with a SEER rating as high as 23. They cost about a third more but are 50+ % more efficient, and instead of stopping and starting periodically to maintain a temperature, they use a more sophisticated system that continuously adjusts the compressor output as needed. These are rated at 5 Amps max, but should dial themselves down to as low as a 2 Amp draw while maintaining a constant temperature. Since we're looking at every Amp, my bet is that I can cut my battery needs by one third again over the standard mini-split models and you can easily spend more than the $400 difference on one more large deep-cycle battery. I'd rather spend $1000, save on batteries and weight, and still be price competitive with roof-mount A/C's.
I don't know how Amazon does it, but they seem to have everything and as a Prime member you get free shipping so I purchased a Pioneer Ductless Mini Split INVERTER Heat Pump, 9000 BTU (3/4 Ton), 21.5 SEER 120 VAC unit. I'll cover installation in future posts and I look forward to doing some serious testing to see how efficient this unit is and I'll post results of actual Amp draw under different conditions.