BASIX certificate for garage converted granny flat


I am applying BASIX certificate for my garage converted granny flat.
There is one item that I am struggling to pass - the energy.
I need 40 to pass but I only got 12 now BEFORE adding the score for hot water system.
The existing electric storage hot water system (score 2) is dragging down the score.

Score 0~15
2 electrical storage

Score 16~30
16 electrical instantaneous
14~24 gas storage
16~27 electric heat pump - air sourced
28~30 gas instantaneous

Score 30+
33 ground source heat pump (direct exchange)
34 wood combustion
34 ground source heat pump (closed loop)
31~37 solar (electric boosted)
36~38 solar (gas boosted)

I need 28 more to pass.

Some of the above are either too expensive (ground source heat pump for $10k), or not practical (wood combustion or gas instantaneous as there is no existing gas line).

Is there anyone who has applied BASIX certicate before and might be able to shed some light?

It seems that solar (electric boosted) is the best bet now. Has anyone installed solar hot water (electric boosted) system before? How much does it cost for the smallest unit?

Thanks in advance.