Heat pumps are a viable alternative (not on roof) to solar collectors (on the roof).

A heat pump looks like an air conditioner and needs to be serviced like one and has a lesser life span than solar collectors generally .

Heat pumps operate most efficiently within a certain temperature range !!

Heat pumps only get water to a certain temperature  .

Heat pumps can be easier to install and more easy on the eye if your roof is visible or aesthetics are an issue .

 

So there is a definite place for :

1 Heat pumps

2 Solar collectors on roof

3 Solar collector with heat pump backup !!!! NOW WE ARE TALKING !!