I have a portable heat pump/air conditioner that I want to return under warranty due to some thermostat and compressor issues. The only other unit available I can replace it with has slightly less heating and cooling capacity. My current unit is 4.1KW cooling/3.8KW heating, the replacement would be 3.5KW for both heating and cooling. What effect will I notice in the reduced capacity? Longer running to reach the desired temperature? 3.5KW should still be adequate to heat and cool my roughly 18sqm room but I'm unsure of what effect I will notice exactly in the reduced capacity.
It depends heavily
Because ultimately, you have a room with heating/cooling loads, which you're trying to meet with the heat pump. 18 sq. m. is approximately 200 sq. ft., and I've generally seen 30 BTU/hr/sq. ft. as a rule of thumb for rough sizing. With this guideline, you'd need 6 kBTU/hr, or 1.75kW. Because your heat pump is going to be twice this, it's unlikely you'll notice an impact.
Why it depends
The problem with 30 BTU/sq. ft. is that is makes quite a few assumptions about your HVAC loads. If your insulation is especially poor, you will need more HVAC power. If you live in a milder climate (ex. 20-30 °C all year) you might not need any HVAC capability. There are too many variables for us to definitively say how you will be impacted.
How to determine the impact
If you found your old unit was running constantly, that's a sign that it's incapable of keeping up with your heating and cooling requirements. If, however, it was nearly always off (or, if it used an inverter, it was running at very low power) then it was likely significantly oversized. If you don't remember, try looking back at previous electrical bills. Unless all your appliances are also electric, or you drive an EV, you should be able to tell approximately how much power your old unit used. For a ~4kW unit, you should expect 1-2kW of electrical draw while it's operating (at full power).
The only way to tell if the capacity is correct is to do a heat load calculation. If a new smaller unit works then that means that your original unit was oversized. A properly sized unit will run almost continuously during a hot day and run less on a cooler day. If it turns out that the new unit is too small then if you set it for say 74F on a really hot day it may run continuously and the temperature inside would rise until the BTUs entering the house equals the BTUs being removed. The hotter you keep your home the fewer BTUs you will need.