Looking to move from gas to electric water heating with solar PV.

The solar diverters seem very popular on here, but seems to me the heatpump option makes more sense, am I missing anything in my calculations here?

Currently on octopus 17c low user, but assuming both of those will not last the life of the system, lets look at high user and current 13c FIT/export offer.

So if we install a immersion heater cyclinder and solar diverter, its costing you 13c of feed in tariff that you would otherwise receive for your exported power.

If we get a heatpump, lets say 4.0 COP, reheated overnight, in auckland that's 14.8c / kw of night rate electricity, but for that you get 4kw of heat, so that's costing 3.7 cents per kw of heat.

So we're 9.3c/kw better off to export during the day, vs the extra cost of a heatpump?  

And the cost of the heatpump could shared if we go with a system that also runs the radiators in winter.