Heating & hot water for temporary accommodation

[Nickfromwales]

As a large electric shower is utterly cold mains dependant, it would be sporadic in performance to the point where it was unusable on any setting other than minimum. May as well buy a 7.5kW baby electric shower as it would be more reliable. 
Remember that this is an annex off a residence, and therefore, as I’ve not heard any differently, it is very likely to be fed from the main house cold mains supply. So, when anyone so much as fills a kettle, the dynamic pressure to the annex will drop to near zero by comparison, absolutely nowhere near good enough a scenario to suggest a very big 10.5kW electric shower as it would be 100% reliant on a “cold mains  priority” setup which this likely will not have as residents could be in both the house and the annex simultaneously. Instantaneous DHW is a bad choice for this situation I’m afraid.

A cold mains accumulator could be introduced, but it wouldn’t be my 1st choice, eg creating solutions for mitigating against the problems that poor survey / design / execution brought about.

The only way high-powered instants would work at all reliably here would be if the annex were to benefit from its own cold mains water supply.

Then there’s the electricity supply also, as a 10.5kW shower, plus water heater, plus space heating, plus sockets & lights = a minimum of a 60a supply to a chunky CU and a lot of extra expense / complexity. That’s if the existing 1ph domestic supply can handle the extra, new electrical loads….. 🤔

[markocosic]

If there's that much pv on the roof there's already a fat electricity supply/ submain to it