Yes this is a key issue to realise, PV is great and PV diversion can really contribute to boosting a hot water from March through to October and very occasionally on the sunniest days through the winter. Also for this reason solar PV won’t be that useful for space heating as the level of heat needed is normally on those cold cloudy days in winter.
I have some neurological issues that make daily soaking in a lovely hot bath critical! I would freeze up without it! 
@Clive_Durdle, sorry to see that you are a special case. Of course such issues should always be catered for but they will seriously impact your hot water efficiency. My late wife received a government subsidy to cover her needs but of course such extra income never meets the true need.
I think this might be the reason I am so strongly in favour of district heating as Rotterdam is doing, so that everyone has readily available the heating and hot water they require, and cooler water is available for use by greenhouses and similar
When I first read this Tim, I thought wow!! I couldn’t get my head around it. Now, having read a bit more and coming back to it, I can understand and better appreciate what you are suggesting.
re. the forced wetroom situation - it’s hideous!! When we started taking up the floor it is like mush - wet rot has affected some joists - so far it looks like we’re getting away with just treating it and we have a dehumidifier running too. The whole floor has to come up. It’s slow going - so many layers that we’re literally having to peel up and very much pace ourselves. I have a dodgy back and my husband’s shoulder is weak after a rotator cuff operation. I thought the loft was a challenge - I don’t know what is worse now.
We’re not insured either - wet rot isn’t covered and the excess for water escape is £750 which we not assured we’d have a case and could lose this money on a trace and access only to be told we’re not insured. It’s really made us wonder if it’s worth having contents insurance at all.
Is there a need to develop cooperative neighbourhood caretaking building trade solutions, like housing associations and councils have but available to owner occupiers? Age Concern etc have these types of schemes but they are not available to everyone
Where are the detailed neighbourhood asset registers that would enable decisions about what to do in what order?
Sorry to see that your wet room issue is worse than first realised 
. That is a problem with “damp proof” wall and floor coverings. You have no idea about what is happening behind them.
It looks to me as though your immediate priority has to be to stop the necessary destroy and remove phase from going into or past the ceiling below. My brother had a similar situation in a house he bought. In his case he sued the surveyor and got enough it remedy and improve his kitchen, where the problem was, but only because he did most of the work himself.
There is an issue with airtightening joist ends/walls where there has been recent damp. Do you need to rehang any joists? That deserves a topic of its own!
A lot of ground has been covered here.
Stepping back a bit, it’s worth remembering electricity is very high value energy (low entropy), it can do a lot of work before becoming heat for your home. As it’s expensive to store as electricity, storing it as heat makes a great deal of sense because it’s the cheapest form of storage we can get. Thermal energy storage is going to become more and more important as we transition to more renewable/variably generated energy.
To produce heat most efficiently, you would need to use that work through a heat pump instantaneously. The problem is that heat pumps take time to get going, about 20 minutes, and then it should stay running for at least an hour to get good COPs. Also locally generated PV can ramp down rapidly because of cloud cover. The price that energy provides will pay for PV can also change quickly, but is typically well forecast between half-hourly to one day.
The upshot is that your heat pump controller won’t care what your PV is doing, it needs to get accurate information about price forecasts from the grid, on a variable tariff, and have an algorithm to optimize when it should turn on, how hard it should work, for how long etc.
Your PV inverter won’t really play a part in this, it’s just another solar panel on the grid, a drop in the solar farm ocean.
I’ve only just seen this thread, but think it is important to mention that you are unlikely to get much solar generation during the winter. So when you want heating the solar is not going to provide much electricity.
kWh is the unit of electricity i.e its what your electric meter counts and what your are billed for. It is equivalent to using (or generating) 1,000W for one hour, 500W for 2 hrs or a 3kW heater for 20mins.
kWp is peak is the maximum (peak) number of kW that your panels can generate in ideal conditions (at noon on a mid summer sunny day).
Your invertor will also have a kWp rating which is the maximum kW that it can feed into your house/grid. The invertor is usually rated a little bit less that the sum of the kWp of all the panels.
So in mid summer a 2kWp system will generate a maximum of 2kWh each hour. (it wont manage that for every hour of sunlight, it will be less in the early morning and evening and at its maximum around noon).
In June, July and August it might manage 15-20kWh on a cloudless ideal day , but in December, January and February it’s unlikely to generate more than 1 or 2kWh per day.
Something’s not right about the details you provided. You say that you have 2kWp of panels that are estimated to generate 3200-3800kWh per year. I think you would need more like 3.5 - 4kWp to get that sort of annual generation.
I suspect your panels are 380kWp each in which case with 10 panels that would give you 3800kWp which will likely then have a 3.6kWp invertor therefore limiting your whole systems output to a maximum of 3.6kW. (I think 3.6kWp is the maximum size you can have before needing special permission from the grid/DNO)
We have 4kWp of panels and our invertor is 3.6kWp. and we generated 2,857 kWh in the last year. Admittedly our panels are not at an ideal angle but even if they were they would not generate 3,800 and our system is nearly twice the size of yours if it really is 2kWp.
I only have quarterly readings for our generation but this is how it breaks down…
Oct - Dec 216kWh total = 2.34kWh/day average.
Jan - Mar 386kWh total = 4.2kWh/day
Apr - Jun 1,177kWh total = 12.93kWh/day
July - Sep 1,078kWh total = 11.71kWh/day
On really good days in Jun/July it will generate 20-25kWh but of course not every day is sunny in Manchester so the average is much less.
Of course it makes sense from a carbon reduction perspective to move your heating to electric anyway, but don’t expect your solar PV to provide much of the required energy
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This is really great feedback Peter. Thankyou 
I particularly like your simple presentation and breakdown of the numbers - it is easy to follow and a good comparison that has given me perspective.
I can see why you’re wondering about the estimates I’ve been given with our solar array… You have got me thinking here too.
I am thinking very carefully about how best to divert our energy when not in use - it’s still a question of what to.
We did buckle with the heating situation though - having a multi fuel stove fitted for emergency heating if needed. Mostly though we’re still coping with woollen layers, gilets and blankets.
I perhaps need to look at low wattage heaters for some of our main rooms to take the chill off when it’s colder in the daytime.
We’re chipping away at insulation too - currently considering insulating the bathroom floor between the joists - since we have them up sorting out this bathroom problem we had. The bathroom floor is above our main living room so it would be a nice addition to our partial retrofit around that room.
The joist ends look good thankfully, so no rehanging is needed. 
The tops of the joists did get a little wet and have dried out nicely. We’re still going to treat them for wet rot though, because there are spores on the side of the timber.
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Treat well past any visible rot or spores. How are you going to treat the underside of the timber?
Basically … we’re not. We’re hoping the treatment from the top and sides will be sufficient. The joists have thankfully suffered only minimally on the top face. 