Looks like a fairly standard setup you see in most new houses. Small set of Solar Panels, small Inverter tied into the meter to feed back to grid... the minimum for whatever spec or regulation the home builds are aiming for these days.
I got told all sorts of different stories from my Agent doing the sales, from the construction company and one of the Site Engineers. At the end of the day, unless the builder you talked to is the Electrician who installed or certified the setup, there's a good chance it's not their area and they really don't understand it.
As Zenith said, very unlikely there is a diverter that would be needed to heat the water only from excess solar power.
It will instead (assuming it's turned on, on both switches in your attic) generate solar power, feed it through your mains box and either to power demand in your house, or out through meter to grid if there's excess.