Demand Response and storage - how much of each do we need? Wattstor may have part of the answer.

                                             

A new company in Cornwall - Wattstor - has a useful piece of kit which enables PV system owners to dramatically reduce the amount of electricity they need from the grid in summer. It stores surplus electricity during the day - up to 6kWh - which is then available in the evening.
I spent over £12,000 on a 4kW PV system , so I am a prime candidate for Wattstor.
Will it make any difference  to peak winter electricity demand - when there is very little daytime PV generation?
The Wattstor system costs several thousand pounds - mainly for batteries. If it shifts sufficient summer electricity from day to evening, it will pay for itself.
Delay timers and time switches will have the effect in winter of reducing peak demand at low cost with high reliability.
Perhaps someone could do the sums to calculate the ROI and carbon reduction of each approach?