I went down the route of a dedicated, unvented, SS, DHW, solar cylinder, and I'm very happy with the result. I get solar DHW for about 5 months with another 3 or 4 months where it occasionally needs a little immersion boost, I'm in the N of England.
For the remainder my solar DHW cylinder acts as a pre heat for my PHx input which comes into operation during the winter log gas burning season. You could do similar, but substituting my immersion and PHx for your extra accumulator fed coil. Obviously you'd need a twin coil DHW cylinder.
As for efficiency, one system over another, 6 and two 3s, IMO. There will be those who will crunch the numbers and give a more definitive answer. I wanted something simple, a cylinder temp readout with twin immersion switches next to it. For the winter burning season, a DHW flow changeover lever suitably labelled,
(summer/ winter). No fancy thermo switches linked to by passes and valves etc., all manual, and about as fool proof as I could make it, but it seems to work fine.
I also put in plenty of isolating valves so I can do any maintenance to various sections of the system, without a full drain down.

Hi Paul, Duncan here (Umbria, near Foligno but 500m agl); where are you? I have a 450m2 house that I am renovating, a circa 1960 self build ie a "casa di campagna". Roof done with 100mm EPS then ventilation for cooling. Working my way through the single pane, slide fit glass, windows at present. I am using a 40kW Wood burner with a 2500 litre tank so the first thing I have to ask is how big is your house/why do you need a 60kW monster? I too have lots of very local wood, paying E10/quintali for dry 50cm cut and split.

Now then to your question: it all depends on whether you have bought your TSs or not. The best answer by far is rarely used in the UK and that is a tank-in-tank. This a 2500 litre TS with another 350 litre tank (for DHW) inside it mounted at the top; critically the solar is connected to the TOP coil and the wood burner in the bottom. (as an aside, also contrary to UK norms, get FPs not ETs). The mains pressure cold water is simply plumbed through DHW tank, refresh time is in effect instant and the relative quantities used/involved don't cause any destratification issues. I have 5 cheap FPs and 5 living in my house; the solar is enough for all my needs for the entire heating off season (6 months for me here). If I get 2 or 3 days full cloud at the beginning or the end of the off season then I have to burn but this happens about 2 or 3 times pa - ie from mid April to Mid Oct (actually almost made Nov this year!). Even simpler than Owlman's! And with less tanks there is less of a standing heat loss. On the other hand during winter sun the tank is not really cool so you collect less free sun however, I always burn late pm early evening so the tank is down to 40 deg ish during the mid pm so I don't lose it all by any stretch.

Of course you won't want an immersion option will you! I did the whole install myself (my first proper plumbing job!) so if you are DIYing it, and not that experienced, I can tell you where I messed up.

Adding to PiH's PV comments:

+ PV for heating can be more cost effective in low light/cold outside conditions thereby extending the potential solar-heating season, particularly if adding further heat to a tank which is already warm or hot (essentially, the tank temperature doesn't affect the efficiency of a PV/immersion combination), but

- PV takes up a lot more roof space for a given power output.

Posted By: GotanewlifeI also have PV but I bought in when the tariffs were good

But you possibly paid 2 or 3 times the current installation price

Whichever way you look at it, RE is a very diffused resource. But always worth considering.
Much easier to route a few wires than water pipes.

Posted By: Peter_in_HungaryLots here now discussing whether PV is now better than ST …

By “here” do you mean on this forum or in Hungary?

Interesting. In Hungary, are there any FITs or RHI-type things to affect the decision? Electricity prices are higher in Hungary (Nov 2013: €0.2353/kWh in Budapest vs €0.1775/kWh in London [¹]) yet the € price of PV looks similar to the £ price.

How many are actually heating water with PV or are they just talking about it? Are we talking Immersun type things or dedicated PV into an immersion somehow?

[¹] http://www.bbc.co.uk/news/business-25200808

There is no support for installing PV (last year there was a grant for companies but it was not available for real people) The FIT is based on the cost of production so no profit there. The biggest drawback is that if you produce more than you use and feed the surplus into the grid you are classed as a producer of power and you have to register for VAT and do VAT accounting for all of your power production even if you are only 1kwh over! - so no one produces more than they use and most keep a monthly tally to ensure that they don't go over.

As far as I have seen most of the new solar roofs are PV, a change from a few years back when nearly all were ST. The usual set up is to heat the water then when its hot the power switches feed the grid to balance (or exceed) the usage. (Immersun style) The owners manually monitor the usage and if it looks like it is going into surplus they switch off the PV to avoid the VAT problems. Sounds good doesn't it!!

Despite this there is private PV but usually those with spare cash and the inclination to want to stop paying the power companies.

Posted By: Ed DaviesDo you mean it's set at the cost of production for the main power stations?

Yes
The VAT issue is daft, I suspect there was a bit of lobbying by the power industry to get a scheme that would deter too many people wanting to feed surplus into the grid as they see it as a bit of a nuisance.

BTW the bureaucracy here is such that you can not do VAT returns without the help of an accountant!