What is difference between 3KW and 4.5 KW?

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KW/hr rating. By my personal experience a 3KW Instant WH is not able to give you required hot water temperature rise for a medium flow of water through shower. You at least need a 4.5KW/hr capacity. Instant WH are also similar to Storage WH only difference is that they have a small tank capacity say 1Litre to 3 Litre.