We get surprisingly good power even on cloudy days, or in Winter. At least 25% efficiency on our measly 4kW of panels. Although we do have the option to top-up the sunamp unit overnight on an economy tariff too.
You could use a manual/dynamic switch to always charge your batteries to [x]% first and then start working on the DHW? Typically, the sunAmp unit will be more efficient than your grid (and will last ~40k cycles, or ~50 years).