Choosing the optimum number of solar panels for my roof?


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I’m planning to get solar panels for my bungalow. I have a lot of south east facing roof space, an air source heat pump, and an electric car (low mileage). We are ‘at home all day’ and I am hoping to not need battery storage.

The word on the street is to get as many PV panels as can fit on the roof, 24 would fit and the DNO is ok with that. 
How do I decide just how many panels to get ? Any advice would be gratefully received.


38 replies

Userlevel 7
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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. 

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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!

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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 

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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.

 

 

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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?

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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 

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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.

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@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!!!)

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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. 

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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?

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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?

Well as black absorbs heat better they will become quite warm/hot I would think. There will be shading to the roof of course but black also radiates more heat. Theoretically that heat should rise but no doubt it will have an effect on the roof. 
I’m sure someone can do the calculations .. the panels don’t have a silver underside do they ?

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Oh heck, I’d not considered the heat rising aspect !! So much for A level physics… oh hang on, i only got an E grade 🙄 

So it’s absorption, radiation and convection: which one wins? And I hear they don’t work so well when they’re hot!!!

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I’m out of my depth now but from what I read, the panels will ‘protect’ the roof giving a cooler effect. The fact that they will themselves be hotter is said to be compensated by the structures used in modern panels 

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That’s good news !! I’ll go with that 👍🏻😎

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With 24 panels, on a good day you will probably get around 50 kWh. That will obviously easily supply your house, plus heat pump. If you can estimate the kWh for those, the surplus will be available to charge the EV. The big question is how much will the EV take? Even though there will be days when all surplus is used charging it, you say it is low mileage, so there will also be days when it is nearly full, needing little to charge it. On these days the surplus will be exported ... but when the sun goes down you will be importing to power lights, kettle, cooker etc.

It is this import that you can avoid by having a battery. However, the savings depend on a large number of variables, including power requirements for the heat pump, your house; and charging the EV. These vary over time and season, and the weather affects the PV output.

If you want to avoid exporting (at low rate) and importing (at high rate), a battery must help. But cost savings also depend on these rates ... over several years!

You should also check if VAT is still reduced if PV and battery are bought together, but not if battery is added later?
 

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Thanks @mavison I know that I can order the battery now with 0% vat but only take delivery (and pay) at a later date (though not stretch that for too long I imagine), or cancel if I find it’s not required. It’s the battery cost that’s putting me off, if I max out on panels ! That gives me time to see how our actual generation v usage matches up.

However, I’m liking @jason.lewis ‘s idea of getting the full 15p SEG from Octopus and look at investing in Ripple rather than spend on a battery. It’s a shame OVO aren’t involved with Ripple, and I don’t think they’re ever going to match the 15p. Looks like I may well switch...

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Octopus SEG looks attractive, I agree, and we may switch to them after our Ovo fixed rate ends. And we have been happily with Ovo for many years. They must be losing customers.

Userlevel 7
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Careful with those hidden links @juliamc 

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@juliamc We're with Octopus, benefitting from 15p kWh SEG payments. We're also on their Cosy Octopus tariff which works really well in combination with a battery. We can mostly avoid using grid electricity in the expensive evening period, even on winter days.

 

There's also Octopus Flux that varies the export payments by time of day. Some people are making a fair amount of money by actively managing the charging and discharging of their battery.

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I’ve just got a quote from Octopus for their standard variable tariff and it would cost me £138 more per annum than OVO. That’s quite a lot of 15p SEGs!!

Userlevel 7
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I’ve just got a quote from Octopus for their standard variable tariff and it would cost me £138 more per annum than OVO. That’s quite a lot of 15p SEGs!!

Are you on OVOs standard variable tariff? 

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Simpler Energy - dual fuel, my annual electricity use on the OVO plan is 8332 kWh and gas just 284 kWh so maybe a ‘normal’ customer may do better.

elec unit rate   OVO 44.96p : Octopus 45.57p

gas unit rate    OVO 27.72p  : Octopus 27.47p

standing charges almost identical.

Userlevel 7
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Simpler Energy - dual fuel, my annual electricity use on the OVO plan is 8332 kWh and gas just 284 kWh so maybe a ‘normal’ customer may do better.

elec unit rate   OVO 44.96p : Octopus 45.57p

gas unit rate    OVO 27.72p  : Octopus 27.47p

standing charges almost identical.

 Be interesting to see what happens when the new rates kick in on 1st July. 

We should get the new rates soon. @Tim_OVO may know when OVO will release there rates.

As they change every quarter it does make budgeting interesting.

Do you think any of the other Octopus tariff would work for you? 

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Is that a fixed rate from Octopus? I'm on Cosy Octopus which has a "normal" electricity rate of 32.55p. The cheap rate we get early in the morning and mid afternoon is 19.53p and the expensive evening rate is 52.08p.

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No @M.isterW its their standard variable rate. I didn’t look further but will do at some stage. Solar panels not on order yet but I’d like to get a reasonable SEG payment once they’re installed.

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