
Midea
4-in-1 PortaSplit 3.5 kW Portable Air Conditioner and Dehumidifier
Quiet, efficient split; but mold issues and bulky.

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Midea or clones there of. The model with the big giant oval hose is really a 2 hose model. If you have electric heat that you pay for, get the heat pump model instead. It absolutely cranks out the warm down to -5C. Then rely on your electric heat for the coldest days.
The first rule of portable air conditioners is get a 2 hose model (or the Midea with the giant oval hose that is really 2 hoses. 1 hose hose models are an america only thing. They blow out the very air you are trying to cool. The only reason they exist is 'stupid people are confused by 2 hoses'. 2 hoses balance the pressure and use outside air to cool the condenser. They are the correct choice.
I also have horizontal windows! I have the Midea Duo and I love it. Super quiet and efficient. The remote acts as a thermostat too. It does need line of sight and it’s a bit finicky but that’s my only complaint.
Yep, I have 3 different systems, a Midea 14,000 duo portable heat-pump in the living room, a Midea 8,000 window heat-pump in the master bedroom, and a 12,000 mini-split heat-pump in the backrooms. I've had them all on an Emporia energy monitoring system for the last 5 years and they are all similar in operating costs considering the room dynamics and individual heat loads, basically too close for me to call when dealing with the 'real' world. The mini-split is the best at efficiency, but with the installation cost, it has a negative return on investment. With the newer technology on window heat-pumps going from 40F to 5F, I will be heading that way just for the $500 and 30 minute plug & play option - ascetics be damned. I only went with the portable duo because it was the only option at the time, plus a year ago I permanently mounted it in the window and got rid of the oval hose-in-hose for a COP boost. Plus a perfectly hermetical sealed system. Even though I'm retired from hvac and have all the equipment to install any system - I look at the ease of the future and the 12 year lifetime of these overpriced installed machines. Contractors just got too greedy with the stealing of rebates and tax credits from poor people that I hope the fire of hell will be a fitting end for them...
10,000btu would be over-sized unless it is an inverter, which it sounds like it is if there is a reported whine. Most can throttle down to about 30% of max, so you'd be good there - if it is indeed an inverter machine. Whines can be highly depended on your electrical & grounding system. So kinda no rhyme or reason if or when it can happens. I've had the Midea Duo portable for 3 years now and have no problems with any sounds- tho' I'm kinda old. When I owned my repair shop I had to get my son to tell me when I fixed a corona discharge whine on the old ass CRT televisions...
I have both, the Midea is going to be best for your situation, it has more of a variance in capacity than the Hisense, Hisense tops out at 12000btu but is rated for 8000, Midea tops out at 16200btu but is rated at 12000. Also, the Midea has a drain pump to pump the condensate out which is especially important in the heating mode, specially if it is going to go into a defrost. Overall, I think the Midea just has overall better functionality and futures than the Hisense and is closer to a mini split. The Hisense is perfect for my smaller rooms though.
I have the Midea. Combined with the inadequate central AC in the bedroom it's in, the room got down to a lovely 62 degrees last night. This is in Alabama.
i got this like last week! working great!

Midea
4-in-1 PortaSplit 3.5 kW Portable Air Conditioner and Dehumidifier
Quiet, efficient split; but mold issues and bulky.

Midea
Duo Series
Efficient dual-hose inverter; but high-pitched noise, sealing issues.

Whynter
ARC-1230WN
Smart dual-hose inverter; struggles in extreme heat.

Whynter
ARC-14S
Durable dual-hose; but drainage, power, struggles in heat.

LG Electronics
10,000 BTU (SACC) / 14,000 BTU (ASHRAE) DUAL Inverter Smart Wi-Fi Portable Air Conditioner (LP1419IVSM)
Very quiet dual inverter; not for whole-house.

Ranked #1
Midea - 4-in-1 PortaSplit 3.5 kW Portable Air Conditioner and Dehumidifier

Ranked #1
Midea - 4-in-1 PortaSplit 3.5 kW Portable Air Conditioner and Dehumidifier

Ranked #1
EcoFlow - WAVE 2 Portable Air Conditioner