Hi Matt,
Using the excess energy to run the heat pump would - except for a day with wall to wall sunshine - mean that the heat pump would be taking energy from the grid as the sky clouded over. So there will almost always be 2 or 3kWs of import.
Also, with a diverter, you’re using whatever excess is available be it 100Ws or 3kWs.
On the tariffs, we’re on Octopus Go which is roughly 9p for the cheap night time rate and 30p for the daytime rate. We are also on the export scheme, so get 8p or so for export. So we’d be paying 30p to heat the water instead of using energy we’d be paid 8p for. If you are on a higher export rate then the comparison gets more favourable of course.
We may be a special case though, as the house is almost passiv haus, so with a 4 hour run of the ASHP on the nightime 9p rate, we’ve heated the house for the coming day. If we had the ASHP on heating as well as DHW, then we couldn’t be sure that the full 4 hours would be used for heating, so we run the ASHP in heating only mode. If we could force the ASHP into a particular mode for given time periods and we could get a better export tariff, then maybe we could switch off the diverter…
Simon