Hot water storage tank for heat pump

I want to install a Nyle geyser heat pump for residential DHW. Currently the DHW is supplied by a propane fired boiler that also is the radiant floor heat source. I have some excess electric power due to a recently installed solar system and would like to save on propane costs. I live in a relative warm climate and our hot water use is low. I believe the heat pump can supply all our DHW but just in case I plan to plumb the system so the boiler can be a backup. My question is what kind of storage tank should I use? I could just get a cheap 40 gallon electric but I don't need the elements and there must be something better.

Thanks for the help.