continue on help on cooling/heating on extension

a) reverse cycle AC with inverter is the oz name or branding heat pump but with a DC motor drive that can do variable speed. Practically continuiously variable speed. The new model that I look at a EER or FEER or COP.... but I mean 1kw electricity can cool 4kw or heat 4 kw. vs my old 6.5 kw LG box (come with the house) which is ~2.5.
The big ducted one seems to have less efficiency. or more old models are still hanging around.
I have very little idea of how well does these heat pump work. I don't know whether I should go for a big system or multiple of small units
b) I have a central evaporative cooling in the last house also in Melbourne. Most of the time, I am running it with the water off. More of less running as the house exhaust fan like you have described. I think the average humidity is ~40%.
It seems that which other path I choose, put a reflective film on the glass of the window is a good bet.

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