My existing hot water cylinder needs replacing within a week or so.

The dilemma I have, is that I would like to fit solar thermal panels at some stage, but the fitting of the panels, for various reasons is reliant on a planned extension At the moment the possibility of going ahead with this extension is about 50/50.

So I'm not sure whether to fit a solar cylinder of around 250 litres and have the possibility of not being able to fit the solar thermal and so have and oversized cylinder , or going with 175 non solar cylinder and finding that I can fit panels but have a compromised cylinder.

I am not sure if it is relevant, but our water here is very agressive so will be going for a Megaflo

At the moment I am minded to go for the smaller cylinder, but would be grateful for any views to clarify my thought process.

Hi Tony, Solar PV is not really a viable option here unfortunately, no subsidies and a buy back price of 5p does not make sense financially.

Brian

Yep, solar cylinder. Relatively small extra charge, no prob to use without 2nd coil 'activated', and you'll kick yourself if you get SWH and you had decided *not* to get a twin-coil cyl - major cost and disruption.

Thanks everyone, solar cylinder it will be.

Brian

Without subsidies and just for water heating it's really close between PV and solar thermal - if you've got the space for PV. If you're only planning a small system to heat water for 6 or 8 months of the year then whatever is locally convenient for installers seems best.

If you want a larger system to heat water for more of the year then you'll have excess energy available in the summer so perhaps the choice should be made depending on what you can do with that. Heating a pool of some sort seems the only sensible use for excess heat whereas excess electricity is more flexible, particularly if you're home during the day to make use of it.

As Steamy says, you have to do the sums.

I would go for a low wattage one like 1kW or less

The lower the power the more often that you can use it, there will always be too much except in the winter

Get yourself a thermalstore, with 1 x boiler flow and 3 x boiler returns. That way you can open valves 1,2 or 3 and heat 1,2 or 300 litres - typically only the 100 in summer and then have the 200 available for shw.
With a thermalstore and evac tubes, you can run direct (no coil, no antifreeze saving circa £350) and if you are 50/50 then future proofing with 2 x direct tappings and isolators only about £40 investment.
You want shw to heat from bottom up - and (the much less efficient but potentially high temp) PV using a dump controller to heat 3 elements from top down consecutively.
Immersun or iBoost 3 stage dump controller is what someone here called a diverter.
Solar Dave - tech at TMS (thermal management specialists)