Beware of solar companies pushing the sale of expensive solar panel immersion heater switching devices such as Immersun.
The maximum savings can only be the amount users would spend on hot water heating bills and that is if one heated water from 100% generated power. This would never happen all year round.
I rescued my father from falling prey to such tactics would would have taken him 15 years to recoup cost of £1,250 at 100% of heating cost. Quick estimate based on his 1.7kW PV system it would have taken 100 years to cover his normal £12 per month water heating costs.