Re: contingency planning
Taking a 9.5kWh battery, at just 85% efficiency...
I use 9.5kWh at 7.5p to charge it - cost 71p - I then don't have to use (85% of 9.5kWh at 39.4p) £3.18 each day - that's £99 per month.
You could suggest that I use a "normal" tariff as my "saving", I did... but even with the pre October prices (I was on a supplier of last resort tariff) the projected saving in terms of electricity using panels/battery is enough to finance the panels/battery (with the battery actually doing the heavy lifting). If I use the current "Energy company profit guarantee" rate then I expect to be paying substantially less for electricity and appropriate finance than I would be for electricity alone.
My usage pattern actually favours the use of a time of day tariff anyway, so I'm comfortable using that direct comparison.
Projected cost per kWh, inclusive of standing charge, goes from 35.9p to 9.6p.
Ok, so how much does a 9.5kWh battery cost... About £6k installed, with a ten year unlimited cycles warranty.
It's therefore paid for itself in less than half the warranty time, whilst also giving additional protection against power cuts and increasing the value of my property...
Assume that it lasts 50% more than is warranted, and you have a 200% ROI in 15 years - or about 5% compounded annually.
Of course it also enables full usage of a home solar array - and it helps if you have other reasons to be on a time of day tariff (like an EV, or a ZEB). But there are also other benefits (like power cut protection).
There is the final point that not everything has to have an R, or be an I. You fit a new kitchen or bathroom, do you decide on an ROI, or do you get something you want, and like. Like a new kitchen/bathroom installing panels/battery systems has a direct positive impact on property value as well.
Using solar/battery does more than just give you a financial return, it also improves the grid mix, and improves your energy resilience* (protected circuits).
* I'm not going to be keeping my battery with a minimum fill level to cover any possible power cut, but I am protected in the likely event of power cuts any time up to the late evening this winter. I won't be running my oven/kettle/microwave, but that's what a gas hob is for. The boiler will have power, as will the fridge/freezer and my IT equipment (which will be behind it's own small UPS as well, to deal with the automated, but not seamless, hand over onto an isolated circuit).