Choosing the optimum number of solar panels for my roof- Any advice?

Hi @juliamc , having had PV panels for over 10 years now and helped others on their way, I would agree that as large a number of panels as you can afford (and fit) would be useful. I realise that you have other power consumption than ‘standard’ but without batteries, solar will obviously only help during sunlight hours. 
We got batteries around 18 months ago and it has made so much difference to how much we use compared to before. 
Solar is a better investment than batteries but the combination is a great way of maximising your solar production and shifting your demand to off peak as much as possible. 
It will be possible to add batteries later but it is worth considerable thought before you invest. 
Currently, in this latest sunny period, not only is the house demand covered but the batteries are charged by mid morning even with a quite low (by todays standards) PV system. So that covers all of our evening, night and early morning use. 
We have just 17 panels - split east and west to get a good overall coverage during the day. 

With @juliamc’s EV charging and air source heat pump, is it not likely that, by shifting hot water heating and EV charging to the sunny points of a day, Julia will use every watt of energy her panels capture? As a novice myself I agree with Julia’s approach of getting all the panels that can be fitted and avoiding, for now, the battery. 

Would be interesting to hear other heat pump + solar + EV owners if they agree @M.isterW @hydrosam @jason.lewis @sylm_2000 

OVO are fitting solar panels by the way. More content on the forum coming very soon on that!

I'd agree that you should get as many panels as you can. The installation costs are pretty much fixed so the cost per kW reduces the more panels you have.

I would 100% recommend getting batteries. I nearly didn't include them in our installation but that would have been a huge mistake. Having a battery means you make much better use of the electricity you generate, even if you're home all day (which we are). We have 16 panels, split east and west, and a 6.5kWh battery. The battery is just big enough to get us through the night at the moment (including heating the hot water early in the morning). Although I specified the battery without any real analysis of cost / benefit it is about the right size for us. If I'd gone any bigger I'm not sure we would have recouped the cost 

This might be useful. It's the graph for today from our solar app. The turquoise is the house load. The green is solar generation. You can see how much spare solar electricity would have gone to the grid without a battery and where we would have drawn from the grid. Instead we've used almost nothing from the grid today.

Nice graph thanks 😎. What is the payback period for your systems @BPLightlog and @M.isterW ? If I was to get pv now and not get  battery storage till next year, is there anything should be sure to get now in preparation?

Ours was under 6 years @juliamc. Having said that, it does include the FIT scheme. 
As others will tell you, there are some inverters which can be ready for battery fitment and others where you would need an additional controller. Ours is the latter type and does incur a slight delay on battery switch-in .. I presume due to the command chain between the systems. Most come with a suitable monitoring system. I think I’ve shared before, but ours for today looks like this 

Orange - household consumption

Green - solar production

Red - grid in/out

Blue - battery charging

(batteries were fully charged after overnight use around 10:30 this morning 

I don't know exactly what our payback time is likely to be. We've had our heating system reconfigured since the solar was installed so our usage has changed. We're also on a tou tariff (Cosy Octopus) which makes it harder to calculate payback time. But, we paid £9k for the system and I think we'll recoup the cost in about 7 years.

If you don't get a battery now but think you'll get one in future it's worth considering fitting a hybrid inverter. This allows you to fit a battery without needing an additional inverter. Many of them allow for "plug and play" battery connections so a confident DIYer could do it, or it would take an installer minutes.

@juliamc we have had solar PV for a year now - 5.6kwp made up of 14 Jinko 400W panels, Enphase micro inverters (good for efficiency and reducing impact of part shading).

Having been through the winter with these I have calculated that in the worst months December and January we still offset 13% of our energy use (mostly the heat pump during the day). February was a massive 30% as that month was quite sunny, but cold, this year.

So, I would say that to offset as much as you can during the winter you should go for the maximum possible number of panels. That way you can up the % offset during deepest winter, and a heat pump is a great fit for that (without one you just wouldnt be able to use what you generate in the winter even, if on gas). 

Re battery - I’m not sold mostly because of cost. I have gone with Cosy Octopus which is a heat pump tariff, which pays back a handsome  15p per kWh, so I can make the PV work hard through the summer to build a £££ buffer that I can use in the winter. Tariffs are a good way of offsetting at different times of year, and I have also found that there is no way to cheat the system (the energy Co’s have covered every angle).

I have however taken a 120% membership in Ripple’s Kirk Hill wind farm, and I took the view at 25 years that will outlast a battery at a cheaper cost. Certainly well into retirement.  I have also run out of ‘New Toys Credit’ with my wife (LOL!!!)

Another one in favour of including battery here if you can. If you can get a reasonably big battery setup, for heat pump owners the payback will be shorter, assuming you go on to the right tariff.

Our January bill was around £150, April was just £25, and we are looking at less than £500 for the year. This is whole house use inc. hot water, heating and an electric car, (and not even a well performing heat pump at that, with the correctly designed pump it’d drop another £100+. 4-bed house for some context of scale).
Regarding panels, I have 3.3kw, I’d love more. But in the winter their output is relatively small compared to usage.

Export tariffs are evolving now, Octopus Flux pays a reasonable rate.

Lots to consider. Payback, environmental impact, both saved and consumed in construction, and wanting to chase the best tariffs. 

Thanks for all those answers.. I’ll have more questions now of course! In the meantime do you think a roof load of black panels will make the loft cooler because they’ll cast a  shadow on the tiles, or hotter from them radiating heat so close to the tiles?