As we get more and more renewables then there will soon be a time when cheap mains electricity can be used for space/water heating in domestic situations rather than gas or oil.
Why will it be cheap? The costs of the grid, or standby fossil generation, and the renewable generation all need to be paid (and the renewables increase total costs of the system). Just because something has a very low marginal cost of operation doesn't make it free - as Hinkley Point C is demonstrating.
And there's another complication that battery storage and EV charging will flatten out short term wholesale market price variations, so the concept of having a lot of surplus power pushing down prices starts to diminish. Ofgem are already proposing various network code changes that will increase the fixed cost element of residential energy bills because the excessive renewable subsidies have allowed some households to dodge paying their share of network and operation costs.
Economy 7 storage heaters are another possible example. If we had proper smart meters that is.
E7 has operated for fifty odd years without smart meters, it works with smart meters, so I don't see what your point is. Worth recalling that E7 only existed because there were no good energy storage options, and the government had built a shed-load of huge, inflexible coal power stations that had to keep running at a minimum load during the night. As government policies rush to promote EVs, the overnight demand will increase dramatically, and there won't be this magical period of really cheap overnight electricity.
And one further thought about renewable heating. Peak heating demand is after dark on the coldest stillest winter nights, and renewables aren't going to help much. Storage would only really work if it were intra-seasonal, and in that case you only cycle the asset a handful of times a year, and that's hugely expensive (which is why nobody wants to invest in gas storage).