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Traditional PV to power a traditional electric system will be alot cheaper.

A string of 5 with a voltage less than half the mppt maximum is not going to perform very well, especially poorly in low light, it's operating voltage is too close to the startup voltage. Sounds like one string of 10 would work fine assuming the panels are all in one orientation and no shading issues. If you want to have two strings for shading, orientation reasons then maybe look at a different inverter or optimizers?

If just securing cladding with no structural purpose any reason not to use a flexible glue (CT1)?. Seems to me that would be easier, quicker and thanks to the flexibility possibly better given wood/metal will expand at different rates in the weather.

If you are thinking about something like this then definitely also look into waste water heat recovery. Might not work well for your floorplan but if it does I doubt it would cost significantly more to install than an electric booster and could reduce the need for heat input by 40-60%. Edit: I had second thoughts about this and went to check the specs. These things have a limited flow rate range so they lose efficiency if shared between showers they won't work as well, and installing one per shower gets expensive so it's not as clear cut as I initially thought.

They are basically electric showers with better controls but installed elsewhere. https://www.flexiheatuk.com/product/electric-instantaneous-water-heaters/

I'm sorry if anything I said comes across as this. Sometimes threads on here move away from the actual question and onto the theoretical. In this case the question was a about gas boilers and the early answers were about that. Then, as is common (and generally good) on these forums, you asked how an ashp user would handle this and we diverged off into the theoretical. For what it's worth I think @Indy is being completely rational to stay with gas given the current state of the heatpump market, ie, if you are not careful you can end up with an expensive and shit install. Given that the gas market is unlikely to stick around forever, I do think it wise to allow for a future change to heatpumps but in the near term, it's not wrong to stick with gas for a high demand house. Staying on the original question, the one point that is worth considering is whether the question would be better phrased as 'Whats the best way to provide hot water for 4 people to have long showers without a wait for HW tank refill' rather than asking for unlimited water. The answer may be different to that question than unlimited water, at the very least it opens up more possibilities which can then be compared on a cost/ease of install/maintenance basis.

TBC wasn't a proposed solution to this thread and you have chosen the biggest one. Just saying that heatpumps exist in pretty much whatever capacity you want. Also heatpumps like this (top venting) won't need clearance on all sides.

There will always be a heatpump option (with other tradeoffs of course): https://clade-es.com/products/commercial-heat-pumps/elm-heat-pump/

Get new ones!

I reckon you can be confident that mains gas is available for another 20 years or so. After that I'm less sure. Gas boilers will at some point get banned from sale with incentives for people to switch away. Overtime with less people using gas the economics of maintaining the network (at least outside industrial areas) become iffy and I don't think residential cooking is enough to keep it going. I hope you will still be giving a shit at in your 70's, it's not that old any more

Not directly answering your question but I believe there are options via commerical operations. They handle the payment, etc, take a cut and you get a cut. Obviously the fees to your tenants would be higher than normal at home charging so that might be a factor. Quick google gave me this: https://www.smarthomecharge.co.uk/features/make-money-with-your-home-ev-charger/

I watched this a while ago and thought he did a good job of it. He seems very happy with the result (there are follow ups). Maybe worth a watch:

Yes, after John's last post I realised that I wasn't thinking/communicating clearly about this. I'm aware how it works but clearly got thrown off track. I think my point stands that if you set your heat source gas or ashp to run during the shower time then it will extend the amount of hot water available from a tank, priority doesn't even come in to this. The caveat is that it needs to be on and near temp before you start and thinking harder I have experienced the cold slug from the heating system coming on mid shower occasionally in the past so I should have been less surprised by John's experience.

Gotcha, well back to the main topic. If you wanted to make the tank water last as long as possible wouldn't turning on the heat pump before the shower to make sure it's running at temp get back to the situation as I originally suggested?

In a priority hot water setup the bottom of the tank shouldn't get anywhere near that low should it? The temperature sensor is in the bottom third of the tank and should start the heating process as soon as the temp drops below the setpoint + hysterisis. At least that's how I thought it worked. My current system is s-plan but plan to switch to PDHW when the warranty runs out on the install and I get around to buying a new controller.

Oh, maybe it's because @JohnMo has really long pipework to the ASHP so there is likely a lot of cooler water in the lines that initially warms from taking heat from the cylinder? Edit: If so, could probably eliminate this effect with a bypass valve near the tank that bypassed the tank until the loop was up to temp.

Well that breaks my mental model of how this works and would seem to invalidate the whole concept of priority hot water (which loads on hear swear by). Have you come up with any explanation as to whats happening? Edit to add: I have my boiler providing DHW heat when I shower as a matter of routine and have never noticed any problem with that.

One other thing to add is that if you have say a 7kw ashp in a priority hot water setup, even though it won't be powerful enough to provide continuous hot water, 7kw of heat input into a cylinder will at least slow down the rate of heat loss as you use water. So a 300L tank will go a lot further than the same tank without priority hot water. In the event you have guests it's unlikely everyone wants a shower at once. They will naturally space out a bit as people talk, people fuss over the mirror in the bathroom to shave/apply makeup/use the toilet so that 7kw heat input goes a lot further than you might initially think.

Should I read this as either: a) The prices are very good for the quality if you order in bulk but there is a premium to standard stuff b) I recommend this supplier but their prices are high What were you paying per board?

Insulation or alternatively if you have power, put a small dehumidifier in each? (drain via pipe to the exterior). Might not work too well if you don't keep the doors mostly closed.

That link includes the text Which matches previous discussion on here about this change. ie, that you have to meet the standards but don't have to be MCS approved as John says.

What @JohnMo said but also if you have a household with many people having long showers, may be well worth your time looking at whether a Waste Water Heat Recovery System might be worth it. They claim they can save up to about 50% of the energy used to heat shower water. Pricey but with your water usage might pay for itself quite quickly and much easier to install if planned in advance. The solution I liked the best when I spent some time looking at this is: https://recoup.co.uk/products/pipe-hex-range/pipe-hex/ Don't have personal experience though.

Not an electricitian but I would say no. Will it work? Maybe (what is your switching voltage?). Is it compliant with regs, no, at least not without building an enclosure for it, etc, etc. @JohnMo asks a good question, there may be alternative options to give you what you want. The existing relay you have is pretty beefy. One option is to look for a smaller relay from the same line as existing (will likely make less noise when switching), otherwise look for a DIN rail mounted SSR but if you want to pass electrical inspections in future then even then need to be careful with the details to be compliant (hopefully an electrician will be along to say more).

To avoid changing a tried and true process. Altering a production line and validation is not cheap. Neon bulbs are cheap enough that LEDs don't win. (Amazon will sell me 100 for £9, so 9p a bulb - I'm sure much cheaper in bulk).

I suspect that in the case of a single mains powered, dim, light, a neon light isn't much difference in power draw to an LED and maybe seen as more reliable. In which case isn't worth a manufacturers time to change to LED. Reasoning: LEDs are low voltage devices and require a certain current to light. Current required by an LED is similar to current required by Neon. If you build circuitry to efficiently convert mains to low voltage then the LED will be much more efficient. But in a cost optimised device with no other active electronics, then the cost of the efficient power supply is cost prohibative so you would just use a resistor to burn off the excess voltage causing a similar overall current draw/power usage to neon. While good LEDs can last a long time, the market has been flooded with poorer quality ones so manufacturers would be taking a risk to switch, for no real benefit.