Water heater take a lot power. Typically, 1KW-1.5KW at least and above
So switch on half an hour per day, already cost you roughly 30 cents or we take 1.5KWH
1.5 x 30 days already cost you 45 KWH
If there are 2 unit, 90 KWH gone which translate about Rm35-40.

Oven also another one that power hungry, those generate heat a lot one generally is power hungry equipment.

Please do not buy inverter a/c if you are going to set the temp at 18C and room size is relative large.

An underpower A/C will consume a lot of electricity as well, because the comressor is running non-stop at all, which in this kind of situation, inverter or non-inverter make no difference.

Not mandatory, but as an advertising point.

With electricity cost escalating, those being labelled "energy saving", "green" can make you more sales.

Even HDD with label "green" sell well, although the saving is actually negligible
Instead saying running at lower speed, label it as green when require lower power. <--- a nice marketing strategy.

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.