I am aware about air-cond capacity that's based on the size of the room. I am citing examples of rooms that aren't calculable, large open spaces. Imagine a big hall of irregular shape with large openings, and you just want to provide cooling for one small area only. If you want to calculate the whole area, 2.5hp also will not be sufficient. In this case, the selection of 1hp over larger capacity units will be applicable.

In typical cases of an enclosed room ie. a bedroom, for your previous case of a 1hp non-inverter being not sufficient, there can be few possibilities. Either the room size is too large (1hp being under-spec) or the unit is poorly maintained and not producing the coolness and when the temperature is set lower the electricity bill is higher. Different people have different cooling requirements. Some people may feel that the room is already cold while some may feel the same room to be warm and not cold enough. That is another factor as well.

For first floor living hall where it can be a rectangular but one side which opens to walkway and the staircase area, or a long rectangular shape, cooling is only focused at one small area and not the entire opening areas. You don't need to put a big hp air-cond at the main area just to blow the air to the other open areas where cooling is not required. Ditto irregular shape of living room as not all living rooms are rectangular as some will have odd shapes with openings to another section or few sections.

Yes, location of the room is also a factor as rooms which are facing the direct evening sun will be hotter than one that's not. A way to alleviate the hot room is to install dark heat-rejecting tints with high TSER on the windows as it may help to a certain extent.

Hi,

Links here for estimation of horsepower:-

http://airconditioner.my.panasonic.com/horsepower.html
https://www.daikin.com.my/heat-calculator/

Since your 1.5hp inverter is not cold enough for the room, likely it's underpowered. 1 unit of 2hp vs 2 units of 1hp in a room, it will be subjective to determine which is the better configuration for lower electricity consumption but ideally one unit of 2hp would be the better choice for easier control although it may yield higher electricity consumption than 2 units of 1hp.

Our current household of 3 persons also return a average monthly electricity bill of between RM170 to RM250. On rare occasions the electricity bill will be higher if there are guests and the 2.5hp air-cond is switched on for several hours during the weekends. Only 1 air-cond(1hp) is used in the house while all other air-conditioners are rarely turned on. The main contributor to the electricity bill is usually the air-cond. A colleague who has 3 persons in another household double-storey house registered a much lower monthly electricity bill, an average of RM50 per month only. I was shocked when she told me her monthly electricity bills. Then she revealed one thing - she and the husband usually do not use the air-cond at night during sleep. No wonder she can achieve the low figure.

In summary, if you want to save electricity, the most effective way is to reduce or eliminate completely the usage of air-conditioners. Easier said than done right? My own method is as said earlier, that is to adopt 1hp wherever possible(small to medium sized rooms would be applicable). If the room is huge, then there is no other choice but to use air-conditioners with higher horsepower. The only way to minimize or lower down the bill is to reduce usage wherever possible ie. instead of 10 hours, say limit to 8 or 9 hours, or set the temperature to 25 degrees and above with low fan speed instead of 16 to 18 degrees with high fan speed etc. Although minimal savings, it is still some savings.