The system referred to is new and cost a whopping £14k. What we are trying to sort out is the likely future energy costs, making these as stable and predictable as possible so that the house's running costs don't spring any surprises for sis.
Yes, the cost of a couple of extra batteries might not make initial 'sense' as such - the £6k outlay could instead be used towards covering part of the energy bill for a good number of years - but mum is prepared to cover this outlay so that her daughter's energy running costs are more predicable longer-term. (Also, as with most houses for sale, the asking price is negotiable so this could be factored in.)
(And even on purely economic grounds, battery costs will almost certainly continue to drop and electricity bills rise, so when it comes to a decade+ when the batteries will likely need replacing, chances are the system will be making even more financial sense.)
We're just trying to get the info as to how much difference a second - or third - battery would make to the current setup.
Thanks.
