@z8lccda one other set of question(s) for you to think about: large house or small house? draughty old thing or passivhaus? number of people in the house? baths or showers ? teenagers with long hair? those all factor into how much heat you need for space heating vs hot water heating.  In a super-insulated house with a large family the dhw heating may be more than the space heating. in a larger less insulated house with 1 person it'll be the other way round.

I too have another question @z8lccda  🙂

I assume that your present PV panels are connected to the grid via an inverter with a G53 certificate (an approx equivalent to G98).
So that probably means you are enjoying FIT income at a relatively high rate.
If you replace/upgrade to newer hardware, then you lose those payments.

Equally, if the effective use of the roof is of greater value to you, then the FIT payments matter less.
Others are caught in this dilemma. The FIT income was wonderful until the energy crisis hit them.
That 'devalued' the FIT, and has made them reappraise the need to make their house more energy efficient.

Payment for electricity exports are now made under the Smart Export Guarantee (SEG), which can be as low as 4p/kWh 😥 

At that level it isn't 'financially viable' to install PV panels because there will never be a point in time when you recoup the outlay.

I'm unsure if you've already understood this because you haven't (yet) mentioned having storage batteries.
That technology has gained financial viability during the rising prices of the recent energy crisis.

The capital cost of PV panels and batteries are pretty low. But the costs of hiring an MCS-accredited installer have been rising.

However, if you decide not to export electricity, but only use or store it, then there's no requirement to get an MCS certificate anyway.
You'd only require it to obtain government-back grant assistance, FIT-payments and SEG.
Generic tradesmen (like electricians) are cheaper than those with MCS accreditation.

Installation costs can be lowered further by taking a DIY approach (guided by us here!).
Nor are you restricted to select the hardware from the MCS list of approved products.

There are no right/wrong answers here.
It comes down to your preferences and what you'd like to achieve.

thanks for your thoughts gentlemen, much appreciated

to clarify, i already have a solar thermal with a 210 litre tank; this was installed 10-12 year ago (we've been in the house 6 years)

one of the weaknesses of the solar thermal system, and maybe more modern ones are better, is that you can't program the solar generation to activate based on the weather (aka the solar array temperature)

Instead, you program the hot water to work at set times/days. Just like with a conventional boiler. The drawback of this for me is that if the solar array is not hot enough, you are consuming gas. And gas will be used continuously to reach the desired water temp, if the sun does not come out in that period.
The preferable method would be an option to activate solar thermal heating only when there is sufficient solar gain. Independent to the firing of the boiler. IE a programme that checks throughout the day if there is solar gain available, and exploits that when there is. And does not attempt to use gas as part of that programme.

I could probably change the current programmes through the course of the year to be more in favour of when there is *likely* to be solar gain (e.g. 9am-6pm, April-Oct), but i'd still consume a load of gas keeping the tank up to temperature, when it's cloudy / raining.

In addition, when i look at the solar thermal installation, I've got this ruddy great tank in the garage, with controllers, expansion tank, regulators, valves and an array of pipework that.. all a bit 'heath robinson' i'd rather just be rid of. Am sure i could get some money for it on eBay 🙂

Solar PV as we've mentioned, is a lot more versatile, esp. with a battery.

We'll be a two or 3 person household within a year or two, so our energy needs will change. Less DHW needed, heating will probably be about the same (and we tend to run our house fairly cool during the colder months anyway), electricity demand might increase slightly from lower occupancy offset by occasional EV car charging (no large commuting needs)

My thinking therefore, as this dilemma is prompted by major renovations we'll embark on later this year (e.g. full re-wiring) is to install a modest PV array and battery, with a combi.
If i want to feel good from using 'free' energy to heat my shower, i can fit one of the showers with an electric heated shower. Washing up - we tend to use the kettle anyway! Baths rarely used.

This way i can have a simple plumbing installation (no unvented tank, no solar thermal) and utilise electricity for general power needs as much as possible. I could expand the battery capacity in the future (if i think all is working well and we could make use of more).

Posted by: @z8lccda

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one of the weaknesses of the solar thermal system, and maybe more modern ones are better, is that you can't program the solar generation to activate based on the weather

Quite. See my comment over here about the need for a "hold off" controller.

Posted by: @z8lccda

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[...] install a modest PV array and battery, with a combi.
If i want to feel good from using 'free' energy to heat my shower, i can fit one of the showers with an electric heated shower.

An electric shower is typically a 10kW device.

It's going to be a mighty impressive inverter to supply that from a storage battery. 🤔 

I'm not saying it's impossible.
But it would warrant further discussion here before you start looking around for the required hardware!

Posted by: @z8lccda

↑

[...] install a modest PV array and battery, with a combi.
If i want to feel good from using 'free' energy to heat my shower, i can fit one of the showers with an electric heated shower.

An electric shower is typically a 10kW device.

It's going to be a mighty impressive inverter to supply that from a storage battery. 🤔 

I'm not saying it's impossible.
But it would warrant further discussion here before you start looking around for the required hardware!

You make a very good point and i had not got that far in my research 😀

From my 2min google of 'electric heated shower' this morning i stumbled a product which states 

"The powerful 9.5KW AQUAS AquaMax Flex Manual Smart Electric Shower.... you can even adjust the power to 5KW for an eco-friendly flow without compromising on your invigorating shower experience."

Either way it's a very helpful point you make - for even a 5kw shower i'd need something like a Givenergy Hybrid Inverter 5.0 / Giv-Battery 5.2?
Products - GivEnergy

On reflection i don't necessarily want to oversize a modest system, just for a short shower or two daily 🙂

Slightly off topic, but i think ovens / induction hobs work around 1-2kWh? So if the system can handle an electric shower, then would be ok for these too?

thanks 

Posted by: @z8lccda

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you can even adjust the power to 5KW for an eco-friendly flow

Wonderful... but I'm pretty sure it will still obey the laws of physics.
At 5kW you'll be showering in tepid water, however 'invigorating" is the water velocity !

I was once asked to fit an 8kW electric shower for someone... "so I won't have to pay an electrician to change the cable".

The result was inevitable.
A month later an electrician was summoned to install a 10mm² cable-run from the fuse-box. 🤨 

Excluding a heat-pump, the household devices which use most electricity are:

• electric shower
• built-in oven
• kettle
• microwave
• hob

The drain on other power-sockets is tiny in comparison, and lighting is now mainly LED anyway.

If you're considering storage battery and inverters capable of giving you 10kW, then you're most of the way towards running the whole house off-grid.
That's exactly what I'm working towards.

I'll soon have three battery banks totaling 42kWh, and 3 or 4 hybrid inverters delivering 15-20kWh.

Whatever shortfall I get from the solar-panels can be drawn from the grid using a Time-of-Use (ToU) tariff.
That's currently 12p/kWh, and will eventually be less.

But... keeping the topic title in mind... I will still retain the solar-thermal panel.

@transparent thanks very much for your input here. Very helpful.

Oh and "I'll soon have three battery banks totaling 42kWh..."  WOAH ! 

Nice 👍