Evacuated tubes are more efficient than flat plate, and there is no reason you couldn't have a collector feeding a tank that has a heat pump for boosting when required.
I've thought about adding another tank with a bolt on HP to my system, but it's well beyond my budget for now. If you already have a tank, a bolt-on HP is the best way to go, IMO.
Depending on location and HW demand profile, a suitably sized evacuated tube system can supply your hot water for 350+ days per year without boosting in sunny areas, so the expense of a HP may not be justified. If you have very cloudy weather, then a HP is well worth it. I think Nambucca is reasonably sunny, so just an electric boost may be more cost effective.
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