Posted By: jamesingramvery quick ballpark figures
I estimate most domestic storage batteries will give a 100% profit on investment within the average 10 year warranty.

Depending on your home e.g. overnight cheap charging tariffs, Summer solar PV surplus, ( i.e. free charging ) I think it could be a lot less than that, ( maybe 6-7 years,- I'm hoping?)

Re heating: I already use my batteries for my A/A heat pump and everything else in the home. You just need to have a battery system that can charge and discharge at the appropriate levels. At the moment I have 21kW battery storage with a 9kW hybrid inverter, and I'm using this winter to decide if it's economic to add another 10kW of storage, (approx £2000) in preparation for next year.
In Summer it's not a problem I have plenty of PV. However, there are other considerations to bear in mind and to future-proof your investment. In order to produce a good plan e.g meter supply capacity, and meter tails capacity; smart EV charger ( approx £1000 ). Also, any home modifications, e.g. Home power upgrade with new consumer units with perhaps numerous circuit timers. Do you want full or partial power outage back up. All this, maybe, over and above the basic battery fit can rack up costs, and drastically affect ROI.
Regarding a 15kW inverter, the DNO will have the final say with that one. Mine knocked me back from my preferred 10.5kW, over cautiously IMO, but I couldn't be bothered to challenge it.

Limiting export on a bigger battery inverter was the bit I considered challenging on, but I thought better of it, maybe it was something to do with the fact that I already have 10kW of grid tied PV.

DNO do not like you to export normally more than 3.68 KW. That is what I was restricted to. I applied to increase export from 3.68 and I was told could increase to 5kw at no cost. If I wanted to increase over that then I would have to pay a fee of around £750 for a test and then, may not be approved. As max I can generate is 6kw it would be very rare for me to get to that level but during the summer I can get close if not using myself. So not worth it. They (certainly ours) do not like the voltage on the line climbing as it causes problems to other users with their household appliances. A friend has 17kw of panels on single phase but because of the way they are arranged they do not produce that as a peak. Where he is located the grid has the capacity to take more than we can in my area.
Since finding out how to charge my solar battery I charge it at night tariff to full, as well as a couple of portable batteries which I use to power grow lights and propagators. At same time I can charge the EV run dishwasher and washer etc. So have now effectively moved most of my consumption to 7p as oppose to 28p per kWh. So what was questionable roi on a battery, now it is a no brainer. If we have a sunny day even now in winter I can be exporting from about noon as will have started with a full battery at about 5.30am so the 7p in the battery gets 15p when I export. In December/Jan month exported 50 kWh which without the night time charging we would not have manged any.

DNOs are not concerned with your generation capacity but are very much concerned about what you export into the grid for the very reason you mention of pushing the local grid voltage too high. If you implement export limitation you can have your own solar farm so long as the limitation scheme keeps your export below the limits set by your DNO. What that limit is depends on your local grid configuration and if there's other generation on the local grid. We've got a 7.3kw export limit but we're on our own transformer and there's no other generation on our 11kv supply. Might be more restrictive for anyone else wanting to add generation, but may be not if they are straight onto an 11kv branch??

James, what would the economics look like if you had a bigger and battery and powered your heating and DHW too? Assuming SCOP of 3.

Say you had 30kWh battery to provide 90kWh of heat. Say effective night rate of 10p/kWh to allow for conversion losses. So 3.33p per kWh of heat delivered.

How much is gas right now? At least double that, so this system would halve heating costs?

Hopefully a bit cheaper than that. This is £6k, but of course inverter and installation. https://www.fogstar.co.uk/products/fogstar-energy-30kwh-48v-rack-battery-bundle

True. Battery prices are falling incredibly fast though.

Again probably silly, but what if you installed a heat pump but chose not to upgrade your emitters or install a hot water tank (let's say you're one of the many households with a combi boiler and no tank currently).

Would the savings cover a significant chunk of the cost of a 30kWh battery (charged off peak), 15kW inverter and 15kW electric boiler (or 22kW if you can do something clever and install a 3 phase unit that relied on the grid for 7kW).

You'd have to run the heat pump hotter than optimally, but who cares, because you'll be powering it with cheap (and green) overnight electricity. It would still cut your heating costs compared to gas (I think), and be greener than a standard install.

Hot water would be about the same cost as gas if powered by the battery. And of course other electricity costs would be cheaper.

Would be interesting to look at economics and technical feasibility of that (well, it would be for me!). Point is that upgrading radiators can be an expensive, disruptive faff. As is installing a hot water tank if you don't have one already. It's just a way of storing energy after all, so why not just have a single form of storage (battery) but
bigger.

Just musing

@ Gareth C:
Somewhat similar to what you're suggesting, is what I've done.
Basically, I've used existing systems and retrofitted them, plus added other bits e.g. A/A heatpump, to take advantage of Solar PV, cheaper overnight energy tariffs, newer technology, and now Battery storage. If it's your permanent home, it evolves, which for me has so far proved to be good, but then I did the majority of the work myself.
If the bones of the home are good, then retrofit/upgrade it it usually works out cheaper than ripping everything out, and starting again, every home is different.