Can I use pv and st?

We have a 1910 terrace with back extension with north west facing sloping roof. It is getting sunlight in the afternoons now and a flat roof on a loft extension. Just got a D for energy. Is it worth putting solar panels in these areas? We have a mega flow tank and modern condensing boiler reasonable double glazing newly insulated floor above cellar but other stuff still to do like poor loft insulation front door and cold bridges

Please explain what you mean by “st” .

For PV the quick answer is “yes”, but I’ll go into more detail when I understand the full question.

I have a split SE facing and NW facing array on my garage - 50:50 6x 360W panels on each.
The NW ones do about 50% of the energy as the SE ones, but interestingly between about 16:00 and 20:00 power the base load of my house when the SE ones are doing nothing.
So probably worth a go and it will get your EPC rating up because that doesn’t care which way they are facing!

I am guessing this is Solar Thermal

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Are you saying you already have PV and/or Solar Thermal? Or do you want to now install both?
If this roof area is only getting some hours of sun (all year round?) then it might be worth adding solar thermal. However, I am not knowledgeable enough on plumbing to know whether this can work with your meg-flow tank. It might be simpler to go all PV and then use an immersion coil in the mega-flow tank to heat HW via PV diversion. There are many options.

If @zapaman is correct and ST is solar thermal (I thought it might be a standard tariff or something else) then I have experience of both PV and ST.

My solar thermal faces due south and hogs prime insolation position. My PV faces almost everywhere else except due north.

My view of solar thermal is that facing south it is extremely efficient until the water cylinder is hot. Then it is 0% efficient. Also note that solar thermal needs regular servicing, read “expense”, which PV does not.

When my existing system needs replacing or major expenditure (hopefully not for a long time, although it is already 20 years old) then I will replace it with PV. My heat pump is most efficient at the same time as the solar thermal and I’m sure the extra PV will more than compensate for the extra time that it is running.

I doubt that you will get anything like as much hot water from any solar thermal system with an “N” in its aspect.

Regarding PV on the roof you describe then definitely it can be done. The flat roof can even have mounting racks facing the optimal direction. However, your priority needs to be in fixing the other issues you raise. The best way to cut costs is to not incur them in the first place and that is what insulation and airtightness is about.

My advice does not relate to your EPC rating, which has a good chance of being wrong anyway, but to energy efficiency. Unless you are planning to sell up soon your EPC rating is virtually irrelevant.