Inverter only saves electricity when it's partially turn on, which means full blast. If it's in full blast, it does not save electricity. Rain water shower needs high pressure, maybe around 1.5 bar onwards. Make sure ur mixer can take the pressure. Check ur water heater electricity consumption, ?? Kw/hr, den times 0.36cents. Never never judge air condition capacity by guessing. U can run a software analysis or by rule of thumb , 60btu/hr sqft would be usually sufficient but depends on direction and glass quartet too.