Split air-conditioner in shop...

[Chris09]

Looking on Amazon there is a Senville brand 18,000 single zone mini split for $1,300.00 wondering if anyone has used that brand. A lot cheaper than anything I can get locally.
[added] Senville Manual shows that it can control to 8C in some sort of low limit mode so maybe that's the 8C that I was guessing around. It seems to say you can leave it off, and if it senses 8C, it will start up.
Charts appear to show that for R410A, in an outdoor air condition of -8.33C DB, and -10.56 WB, Port Pressure [which I have to assume is Suction Pressure] will be 302, 339 and 362 psi at in indoor [air entering the coil] at 55, 65 and 75F respectively. Assuming zero pressure drop those pressures translate to about 34C, 38.5C and 41.35C SST or mean evaporator temperatures. The chart, I assume is a COP chart, which indicates a COP ratings of 1.95, 2.25 and 2.35 at 55, 65 and 75F.
Similar COP with their R32 charts. And not much reduction in performance at -17C DB and -19WB.
Two kilowatts [or more] for the price of one down to about -10C outside is pretty good.

 

[Chris09]

Looking further into the Senville manual, there are similar performance charts for R22 which I guess might have been used in these units before 410A.
Jumping right down to the corresponding COP chart for that refrigerant, you have 1.25 to 1.40 for the same [-8.33C or 17F] drybulb outdoor air temperature.
That is remarkable. I had no idea how R410A changed the heat pump world [for colder climates].
Fully reviewing the 190 pages, I'll say this of it, it's like a lot of boiler manuals, lots of good information combined with brilliant obfuscation or outright absence of what you really need or want to know; the units of pressure and temperature are convoluted. The charts below the tables must be COP charts, but are not specifically labelled as such [I guess they're COP charts], and there are no corresponding heating values [what a surprise, how many times have I not seen that].
I seem to recall in one separate document that this or another heat pump brand had a nominal 18,000 btu/hr rating [which is cooling mode] but in heating mode had a 16,000 to 4,000 btu/hr heating capacity. What I'm unable to find in google land, is a simple direct comparison of heat pump output in btu/hr for various manufacturers. [Lie, I just did it now, but I knew it would dazzle me with stupid distracting bs].
So it appears that at 17F outdoor air temperature, a nominally rated 18,000 btu/hr heat pump will run the outdoor fan at high speed, run the compressor at high speed [to achieve maximum pressure and SDT] and then trim the indoor fan motor speed to achieve a warm discharge air temperature. Could be wrong, but I'm guessing you're only getting about 4,000 btu/hr.
...which is like a kilowatt

 

[ST1100Y]

I had no idea how R410A changed the heat pump world [for colder climates].

With R32 you can transfer more energy with even less charge then R410A...
Sure, from a pure technical POV were R12 & R22 brilliant, stable, reliable "safety refrigerants"... seen environmentally not...
R410A cranked the COP noticeable, R32 even more... lower charges, smaller HEX volumes, narrower piping diameters, lower energy consumption... and lower losses at long pipe runs...
(one can build VRF systems for office and hotel buildings with a total pipe run of up to 1000 meters and a head/elevation of up to 90 meters... and no worries about oil transport, the lubricants have foam additives, enabling safe oil retrieval even during minimum compressor revolutions & gas flow, thus no "oil siphons" needed...)

To stay with your 18,000 BTU single unit, I attached the performance chart of a Toshiba Seiya R32 series residential split type air2air model (of course in kW and °C...)
Mind that they all are Inverters, variable speed rotary type compressors with PMV expansion valves... thus operating in part-load the majority of the time, so EER & COP increase even more after the first cooling down or heating up of the space due to the then quasi "oversized" heat-exchangers...

Over here, with our brick/concrete build houses, radiators and floor heating systems, with constant raising energy costs and amplified by the never-ending CO² propaganda/debate, air2water heat-pumps are a big thing now... the outdoor units absorbs energy from the ambient air (+ the absorbed electrical energy) and warms your heating system as well as your domestic hot water tank...