You can savely assume that you will cut down electricity for A/C by 50% no matter the duration you on it for.

Given the tariff etc. you can quickly calculate how much you safe. Even if you plan to on just 3 hrs. per day, you safe  approx. 25 sen for 1 HP per day. I.e. annually you safe approx. 100 RM. Inverter aircon life is longer than normal aircon, you can assume 10 yrs easily. So price difference 500 RM - simple payback for 3 hrs usage: 5 yrs, simple savings in your pocket: 500 RM on top.

for 1 1/2 hours you still break even during the normal lifespan of the AC and you do good for the environment. If you never on the aircon, no need to buy also right. So I dont think you will use less than 1 1/2 hours in real life.

Normal usage is probably more like 6 hrs average, so simple payback is 2 1/2 yrs. so of course you should take inverter. Only ppl with too much money to waste wont buy an inverter. I am not that rich, so i buy inverter & safe.

For 24/7 your savings lets you break even after approx. 7 1/2 months. So the more you use, the more you profit from inverter. However for any usage (other than using it like display and never ever on!) you will safe with inverter in the long run.