You never read my post. Your technician is definately 100% wrong.

For conventional air cond, compressor on/off base on +/- 1 degree on top of setting temperature, meaning compressor will turn on when temperature rise 1 degree above setting temperature and off when temperature falls below 1 degree of setting temperature. so setting 25,26 or 23 is no difference. The only diference is for the initial turn on, setting 23 will take longer time to cool your room from 28-30 to 22 degree before it turn off. If your AC is undersize, it will never cool your room to 22, so it will be running forever. The only advantage you enjoy when setting 23 is after you turn off the AC, you still feel cool a little longer compare to setting to 25/26 degree. Heat transfer is largely depend on the medium of transfer but not degree of temperature.

For inverter type, the compressor never stop after turn on, the operating speed of compressor varies according to the setting temperature. If setting temperature to 23, the AC will drive the compressor to 23 degree and stay at the speed to maintain the temperature. Then the energy consumed depend on the actual speed of the compressor to maintain 23 degree, it could be a much higher speed compare to conventional type so end up addition consumption of electricity. As I said b4, conventional compressor running at fix speed of 50Hz from the grid whicle Inverter type running from 15Hz to 90Hz depends on " load". Eventhough at 50Hz, Inverter compressor is more efficient than conventional type due to the Compressor motor (Inverter using DC brushless motor has higher effiecientcy) but running over 70 Hz is definately taking more energy.

Do not believe anything AC technician tells you except about installation and maintenance, they are just the expert of installation, other than that they know close to nothing.

Did you ever ask what is the problem of your AC? Is it mechanical or electrical problem? I can tell you 99% of problems are gas leak (bad welding) and electronics controller (PCBA, motor, remote...etc) problems. You hardly find compressor problem, even there is, it can be caused by installation problem like under charge of gas or air/water trap inside the copper tube.

EER is a good indicatior for the electricity required to produce the same amount of cooling i'd say. but good that we are on the same page anyway

for 308 sqf, 1.5HP is enough.

It depends how you use the air-cond, if constantly set at 18C, inverter or not inverter make little difference.
But for daily >4 hours use, inverter can be the choice, consider that electricity bill become more expensive nowadays.

how bout Inverter 15Hp for a living room of 308 sqft ? sufficient or should i get 2Hp ? average use on weekdays should be 4 hours, weekend might be around 8 - 10 hours...

so Inverter or non Inverter ?

1.5 or 2 HP ? 

I'm also planning to install water pump and heater tank (7L) <-- if not mistaken the size since i wanted a rain shower feel, mine is a double story landed anyone can confirm if i only boil when use around 30min per day would it be like crazy eletric sucker ???   

worry worry