The Solar Thread

Since early march I have exported 1000kWh more than I have used and with Octopus Flux am making >£100 a month in may, june, July …. compared with paying £120 a month. Our leccy usage is/was similar to yours, but imminent heat pump will change that!

I’d be willing to bet that May and June were the bulk of that, but it’s not normally that dry and sunny.  Compare those months to this month.  We’ve exported only 175 this month compared to almost 400 on each of the  previous two months.

Its tricky for me to work out pure solar export as we have 12.8kWh of batteries that we charge and discharge on Flux every day.  But so far in july 615kWh solar yeild. My Octopus app tells me that I have imported 369kWh and exported 644kWh.

July has been a bit manky compared with May and June. And our system was only commissioned in March and went onto Flux end of April so only decent data I have is for the late spring, summer which is obviously peak!

Total solar yeild from beginning of March is 3400kWh which I think is slightly better than I expected (estimated annual yield was 5200kWh on their calcs). As my system is AC coupled I assume a loss of c10% of generation vs usable energy (battery losses and AC/DC conversion. But the c1000kWh net generation is taken from my Octopus app so is real and billed as such, and yeah it is pretty much just may-july for export and march-july for import, so if Flux has been active in March and April my export would have been higher.

oops, i think i meant flux, not agile?

regards the types of inverters? school me? the more efficient inverter if you plan/calculate to export a decent amount?

Hmm, I’m the guy that was thinking of flooding is roof with solar panels incl batteries.  Ended up being ~£20k and out of my league.

I’ve kind of got in my head now that it’s better to throw the money at batteries on the grounds the payback on that is ~20p/Kw (cost avoidance difference between ~30p/Kw peak rate v ~10p Kw off peak, where the payback on the solar is only 10p/Kw (the cost of the off peak charge that i’d otherwise need to buy).

What am I missing please?

Using your figures, if you get a 10kwh (useable capacity) battery, your maximum earnings will be £2 a day. £730 a year.

The battery will cost you something like £7-8k

If you are buying just a battery, I think you pay VAT, so add that on (no vat if you do it with solar). Plus you need an inverter still. more £. plus installation.

Payback would be well over ten years and by that time, you’d be starting to see decrease in capacity, which will extend the payback time.

This doesn’t allow for changes in import/export prices. Could go either way really.

Don’t really understand your payback for solar calculation. If you plan to export your whole battery capacity, you need to import whatever you use – currently 30p/unit or so. Any solar generation is covering that, plus you can export excess at between 15 and 30 p depending on the time of day.

The battery idea is just wrong.  You’d be installing it only to try and make it pay for itself, it wouldn’t help your energy bill at all.  It’s basically an interest free loan for a depreciating asset.  10kW battery, inverter and installation will be £7-£8k inc vat.

Panels and a small (5kW)  battery is the way to go. £10k max and you’d be able to reduce your peak consumption, export £700 a year, provide another £700 with of power, and import in the winter to reduce your price. Say £1500/y so payback in 6.5y at current rates on Octopus Flux

Maths turns out be dead easy on Octopus Flux when you have solar and a battery and the right mindset. Your panels will make whatever income they make, and every unit of electricity you use yourself costs you 20p. Simple as.

That might be 20p of lost export during the middle of the day, or 20p that you buy overnight. But the figure is 20p. You can sell everything that you generate during peak hours, and you only buy from the grid in the night when it is cheap.

Might not remain true in winter if your batteries dont cover a full 24 hours of usage. And it gets more complicated if you discharge your batteries to the grid (can anyone do that? Powerwall doesn’t)

Mine (LG batteries via a Solis Inverter) can force discharge either dynamically  (via a raspberryPi connected to the inverter) or by schedule directly programmed into the inverter.

I don’t bother.  It’s a few £ at most (and less than you think once losses are taken into account) and it places extra wear on the batteries by charging and discharging at maximum rates for long periods.

The battery purchase can make financial sense if, and only if, the difference in price between when you would normally buy and when the battery allows you to buy, less the round trip efficiency of the battery system, gives you enough savings on lessor of the kilowatt-hours you use or the kilowatt-hours battery capacity (after efficiency) you have to pay for the battery.

So if you can buy at 10p instead of 30p (20p saving, 90% round trip efficiency, so roughly 18p saving) and you have a 13kWh Powerwall (assume £9k purchase – mine was £8.8k) and use 10kWh per day, the battery will save you £1.80 a day or £657 per annum.
On a cash cash basis that’s a 14 year plus payback period. All costs are pre VAT so that ought to net off assuming they’re all 5% rated.
But you could have stuck that £9k in savings / stock market / etc (or avoided borrowing £9k) and over 14 years it ought to be significantly more than £9k you need to make to break even (even after inflation). Google DCF or NPV for more info on that.
So on those prices and usages I wouldn’t do it.  But prices and costs change. As do interest and inflation rates.

There’s also the fact that the battery powers the inverter, so on a daily basis you can lose 1-2kWh just running the system.  Mine is 130-180W/h dependent on temperature.

When ran via solar, you just accept it, but if you’re grid charging, you’re losing 10-20% of your profitability.

Also, and I don’t know what this is like for other  battery manufacturers, but my batteries stop discharging at max rate at 20%.  At this value, the discharge rate is just above the system required power level.  So on my 19kWh usable battery, 3kWh is largely inaccessible.  At 10%, the batteries automatically force charge from the grid.

any idea where i can find a detailed solar calculator? i had one a few months back ( i think it was installable), had a quote and they have calculated based off at home all day, funny rates etc and id like to clean it up

I agree Daffy – there’s standing losses as you point out as well as round trip losses.  Good point.

Fair point on usable capacity and force charging too.

The discharging rate shouldn’t matter too much given most of the time your house is doing 200W or so. Except when it isn’t of course.

Thanks so much everyone, I really appreciate the time taken.

I apologise, I’d failed to re-mention something important: I already have a fairly inefficient 3.6Kw solar array (~2000Kw/year) already which is on a FIT agreement so I won’t have the opportunity to either expand it or export from any new array.

So what I’m hearing is I need to forget about:

1. a year round approach of importing offpeak
2. my 15-20Kw daily use.

Instead I should focus on:

1. moving to a variable rate tariff
2. shifting usage as much as possible to offpeak
3. summer: avoiding peak rate by capturing my net solar generation (i.e. gross generation less what I use) by getting a ~5Kw battery on the existing array.
4. winter: importing offpeak for onpeak use

I guess what I’m saying is I’m right 🙂  [to abandon the idea of a 2nd array and to focus on batteries] but with a different reason/scale.

I think I’m saying do the sums carefully. And be aware that tariffs, interest rates and costs will change so the calculations will change over time.
Beyond that, what works for someone may or may not work for others.

Thanks @igm. As it happens I did some rudimentary sums and came up with a 12yr min payback on either 5Kw or 20Kw battery so very much cooling from the idea.

My next steps are, as @alan1977 is seeking, is to find a realistic calculator to see if that improves me views

Just a quick PSA: I know most posters on this thread use Octopus Flux but for those who don’t, British Gas (yeah, go on, judge me…) sent me an email last night explaining that they were changing their SEG tariff for their customers (so you need to be a British Gas customer using their SEG scheme) to Export and Earn plus at 15p per kWh exported with immediate effect.

This appears to now match the basic OVO customer and Octopus fixed schemes.

Some quick calculations last night with my 3.6kWh solar install plus 3kWh battery  and this looks like it should just about cover the both the standing charge and the costs of importing power from the grid for the coming year. I don’t get hung up on the “payback period/return on investment” thing but the increased savings now being made have reduced this significantly.

My advice to anyone thinking of fitting solar panels is this: if you can afford to, and if you have the space, then even for a modest system with a few kWh of storage its a complete no brainer, just do it.

The bg tarrif has some belting hoops to jump through.

A bank certificate to prove you have an account for example.

Just logged into the solaredge app to find they have added full charge and discharge control of the battery 👍.

quotes out to 4 different companies

2 came back with issues, and when followed up on the issues asking for clarification, no reply

Ive started the ball rolling on a 3.44kw system, due to limited space and some stupid boiler flue (8x230w panels)

and a 10.4kw Givenergy battery with 3.6 inverter, for just under 11k

the Givenergy was the deal breaker as i just found out that was the first system with detailed API control which can work with Octopus Intelligent Flux, as a good chunk of my saving will be from battery management, this seems like the right way for me

july 2023 was grim

billing landed over the weekend.

imported 161 units (night charging )

Exported 300 units

total bill after standing charges – 10 quid in my favour.

changing sun and cut to tariff made a big difference for us compared with June.

Average daily export prices in june were 24-27p, now they are down to 19-23.

production: 1.04MWh

consumption: 496

export 760

import 216

net payment from octopus £76 compared to about £200 for June.

the Givenergy was the deal breaker as i just found out that was the first system with detailed API control which can work with Octopus Intelligent Flux

If Octopus ever actually make Intelligent Flux anything more than a waiting list…

The GivEnergy app, website and API are great; oodles of data and lots of control to run the system pretty much however you want to either manually or automatically.

It’s a good job screens don’t suffer from burn-in any more or the main screen of the app would be permanently visible on my phone.

July for me on Octopus  tracker

electric

Exported 474 kwh

imported 33 kwh

Gas

65 kwh

total bill after standing charges   £71 credit to account

Happy enough with that

Not as good as June but not terrible:

Exported: 646 kWh
Imported: 181 kWh (exclusively for the car)

Import will be less for August ‘cos I’ve found an ultra-rapid charger near home with a broken card reader that lets me charge for free.

Thanks everyone (re: adding more solar/a battery to my existing array).  I couldn’t even make the smallest battery work payback wise so have gone for a small investment of a solar diverter.  I think it suits our situation (limited excess, FIT so no further exporting, plug and forget) nicely and I’ve gone for a more expensive one (Eddi) almost purely to be able to report on whether I reach payback in a reasonable period.

Please can I pick your brains as a long time lurker, short time Solar stresser. Ive just had installed a 7kw system and car charger (as in the scaffolding is still up). The installer I’ve used mentions SEG payments on their website and the contract I signed states that they are a member of the mcs and the recc. I can’t have done my research very well as they are a member of the recc but not the mcs. After speaking to the installer they at first said he didn’t realise I wanted to export! Then that he had his sign off next month and wanted to bring them to our house to complete the sign off. My questions are can we delay commissioning the system until next month when he is signed off or is that done automatically at some point? (It’s already producing power). Does anyone have a system without mcs sign off and does that cause issues with support/ home insurance etc? Is it possible to get a mcs from someone who isn’t your installer? Finally, please if you are going to get a system installed please don’t get giddy like I did and not do your research.
thank you.

Apparently mcs sign off is no longer necessary for export so you may be OK. Only what I’ve been reading on Facebook Solar groups mind so check it out yourself.

FlaperonFull Member
Not as good as June but not terrible:

Exported: 646 kWh
Imported: 181 kWh (exclusively for the car)

Import will be less for August ‘cos I’ve found an ultra-rapid charger near home with a broken card reader that lets me charge for free.

Really?  That’s a lot.  How much did you generate if you’re exporting 646kWh?

From our S facing 5.5kWp array, we only generated 636kWh in July in the SW.  We exported ~210 and imported 18.

He’s got east west iirc

in summer this will yield more over a day.

welshfarmerFull Member
Apparently mcs sign off is no longer necessary for export so you may be OK. Only what I’ve been reading on Facebook Solar groups mind so check it out yourself.
I didn’t even think to check facebook groups, loads of info, thank you. As it stands British Gas, octopus etc only accept mcs accredited installs but several are looking at ways to get around this. It is a more common issue than I thought. Looks like there is a process for an mcs installer to sign a non mcs system off as well all though it may be costly. Hopefully my installer will come good.
First day of charging the car from the sun and seeing the power come through is great though. Even if too much is going to the grid for nothing!

Yay, after 6-7 weeks of being down we’ve finally started producing again. Been a slog involving a cherry picker and scaffolding to find a wire fault.

Now just need to fix the car charger which has started to refuse to charge the car.

https://p4solar.co.uk/case-study/conrad-hart-brooke-leeds-ls26/

3.9kw + 10.4kwh commissioned yesterday. Givenergy batteries and inverter.

I understand is going to take several weeks to potential months to get the DNO in place which will take me into autumn winter with limited export. In the mean time, i’m on an octopus fixed tariff at about 31p a unit, i’m wondering if i should jump onto another tarriff for cheap imports when the winter comes in, on the premise that when i can start getting paid exports, to change again. i know some tarriffs have early cancel fees. with that in mind is it worth me just jumping on agile and deal with the export part later?

Already emailed octopus to see the best way of working through it regardless

Oh, is there anything clever to be gained from using Solcast + Givenergy API ? i have seen a guide about using Home Assistant which combines the two and intelligently charges the batteries. But i don’t want to set up a home server to do this

You’re export is going to be declining anyway as we enter September, so I wouldn’t base any decision on export rates.  I’d be looking at what the best tariff is for the low yield months of October to February, something with cheap import in a fixed window to force charge the battery.  The cheapest will be Octopus Go – whether you have an EV is somewhat irrelevant even though it will be a question from them to you and is a stipulation…How would they know?

When I switched to Octopus go there was no check made. More recently I switched to Octopus intelligent and they insisted on verifying a charge to the car before completing the switch.

I was on eco 7 over last winter and I’m on tracker for both electric and gas just now with 15p export rate . I might just wait and stay on tracker

SV price is red line

whether you have an EV is somewhat irrelevant even though it will be a question from them to you and is a stipulation…How would they know?

Been folk moaning about being asked to send proof because so many people were signing up without one.

yer let’s say i do it legitimately, I’m pretty sure I cant get on flux without export being in place

But i don’t want to jump on something and get penalised by early ending charges when i need a good export rate when its all set up

so Agile was looking logical? even if it means using agile export rates from spring until now next year? in case of any early cancelation fees?

And the best way of managing this without being on top of it every day? :S

@alan1977 I think you needed to supply the DNO documentation as part of the flux signup. It then took ages after that for the export side to be sorted on Octopuses side. Flux import was connected quick quickly.

Oh, is there anything clever to be gained from using Solcast + Givenergy API ?

I haven’t done anything fancy with it but I have recently set up the solcast bit of the givenergy dashboard; normally it’s pretty accurate so it might be a good guide to how much to charge the battery overnight in spring and autumn if you want to fiddle with it. It’s definitely worth setting this bit up so you can see whether it’s worth doing anything more complicated with it.