Solar Battery - Yes/No?

Solarchaser

Petriix said:

The thing people often ignore is that much of what a battery can do could can be done manually. It's not so much about total consumption but how much of it is base load and how much is high power appliances which could be run off-peak.

My total consumption is around 5,500kWh of which 1,500kWh comes from solar, 3,000kWh is off-peak and 1,000kWh is peak. A 10kWh battery could cut the peak usage to maybe 250kWh and probably knock the off-peak in half.

Even in these expensive times, that's just £500 saved. But I'd be able to use the energy in peak times which would make it appear as though I'm saving far more. Current prices seem to be around £6k for 10kWh which makes the ROI around 12 years. However, with rates potentially falling significantly from their current high, the likelihood is a worse scenario. At best it's a massive gamble. On the other hand, paying £6k off my mortgage would be an immediate return of the capital plus a compound interest saving for the remaining term. 

Yeah I think the first paragraph is mostly valid.
I say mostly, purely because it depends on your own home environment. 
If you can control that absolutely,  then there is a good argument to say you don't need batteries.
For instance if you live alone, or are perhaps a retired couple, things like washing machines, cookers etc can probably mostly be used off peak. 
I know some forum users who are single have said this themselves.

If I was single I almost definitely would not *need* batteries,  and all of my bills would be far cheaper.

If you have a "normal" working family of 2 adults and 2.4 children, then you will very much struggle to consume most off peak, the various white goods will be used when they are required, not overnight only unless you only plan to wash and dry 7 loads of washing a week.
Some people may think that's plenty, the white goods in my home are run approximately double that,  not to mention cooking for family meals, children on various entertainment systems, electric showers etc.

Not to pick your points apart, as you have always been civil and civilised in responses, but just to explore.
At 5500kwh/ year If we just ignore that most will be winter months rather than summer, purely for simplication purposes.  5500/365 = ~15kwh / day

In the brightest 8 months you will probably cover most of the daytime usage with solar, and so for best roi you would want a system that cycles fully every day.
My suggestion would be 8kwh useable, so two of the batteries I linked a couple of pages ago around the £1600 mark and 1x sofar or lux or whatever ac inverters would bring you to roughly £4K
If you cycle that battery every day, let's say 8 months from solar and 4 months from off peak then you save around 1950kwh/year from solar.
And then move a further 975kwh to off peak from peak.

I'm not saying it would work for you, nor not work for you as I don't know your peak and off peak rates, nor your seg.
I'm simply giving you my experience of what you would likely be able to do, and if you put your prices into those sums, I wonder if it may come out more favourable for you.

If its roi you are looking for, then you really want the battery to cycle fully every day, 10kwh won't cycle every day in your summer months if you only use 15kwh/day, 8 probably won't quite either, but it was handy to just use the 4.8kwh pylontech batteries as they are a known quantity and are available off the shelf

yp70479

I am a low user (2600KW per year) with a 3.2kw array and 8.2kw battery. I'm on Economy 7 for winter import and Octopus Agile Outgoing Variable for my SEG. I tried the calculator that everyone has been recommending and it gave me a pay back period of around 9 years for panels and 18 years for panel plus battery. But I KNOW this isn't realistic. So much depends on how you use what you've got!. My system was installed at the beginning of June 2022, and between June and October I only imported 8kw from the grid. In addition I was able to turn off my gas boiler and got my water heated by the sun. This actually saved more money in gas than the panels did for electricity!. SEG at the 15p rate generated about £50. I then switched to ECO7 and Agile variable export. Didn't need to import anything or use the boiler until the end of October. I charge my battery overnight at 16p and use it all through the day/ evening. I have enough juice left to export at the Agile peak rate for an hour. At a minimum this covers two thirds of the cost of any imported electric, sometimes considerably more. Basically even in December/Jan I am only using about £20 electric per month . Between Eco 7/ energy trading, SEG and savings on hot water I figure I will save about £1000 a year at current prices - giving a pay back on my complete system in 8-9 years, instead of the 18 years suggested by the calculator. It's not what you've got - it's the way that you use it!!

Slinky

yp70479 said:

I am a low user (2600KW per year) with a 3.2kw array and 8.2kw battery. I'm on Economy 7 for winter import and Octopus Agile Outgoing Variable for my SEG. I tried the calculator that everyone has been recommending and it gave me a pay back period of around 9 years for panels and 18 years for panel plus battery. But I KNOW this isn't realistic. So much depends on how you use what you've got!. My system was installed at the beginning of June 2022, and between June and October I only imported 8kw from the grid. In addition I was able to turn off my gas boiler and got my water heated by the sun. This actually saved more money in gas than the panels did for electricity!. SEG at the 15p rate generated about £50. I then switched to ECO7 and Agile variable export. Didn't need to import anything or use the boiler until the end of October. I charge my battery overnight at 16p and use it all through the day/ evening. I have enough juice left to export at the Agile peak rate for an hour. At a minimum this covers two thirds of the cost of any imported electric, sometimes considerably more. Basically even in December/Jan I am only using about £20 electric per month . Between Eco 7/ energy trading, SEG and savings on hot water I figure I will save about £1000 a year at current prices - giving a pay back on my complete system in 8-9 years, instead of the 18 years suggested by the calculator. It's not what you've got - it's the way that you use it!!

Can I ask, how much did you pay for your panels and battery?

Petriix

@yp70479 That 8/9 year ROI is very much dependent on prices staying at their current (record high) levels. It also depends very much on how you calculate 'savings'. We've imported 2,877 kWh since 1 June 2022 but only 857 kWh of that has been peak-rate and it's cost just £309 - including ~ £30 'invested' in additional peak usage for the Saving Sessions for which I've 'earned' about £150. A battery could have brought the total down to under £150, but that's only a £160 saving. Now my prices have more than doubled, the saving could be as much as £4-500. That's still a 12 year ROI on the battery at today's prices for a retrofit, assuming electricity prices remain high.

As ever, these things depend on individual circumstances. However, a low user is unlikely to be able to argue a reliable case, especially with the recent increase in prices.

Screwdriva

What ^ Petriix said.

Also, the calculator becomes more accurate for battery owners when you hard code an incoming tariff of ~12p or whatever your E7 or Octopus night time tariff is. A "best case" scenario if you will...

yp70479

Slinky: £8750 for 8 Trina 400 panels, Solis inverter, GivEnergy AC3.0 and 8.2kw battery, iboost water heating diverter, bird mesh and EPS socket (but this was quoted in April  - so prices have gone up a lot since then).
Petrix: I know Martin Lewis says prices may fall below the current price cap by the summer, but I don't believe that they will ever fall back to their pre Russian invasion levels and with a lack of storage, less nuclear power and the grid becoming more flaky every year, the battery gives me a feeling of more security with two freezers jam packed full of home produce 
Screwdriver. I would never have switched to Eco 7 without the battery, because I simply can't shift enough of my usage off peak to make it a financially viable option. i would have stayed on the price cap rate, so that is what I am comparing my "savings" against. And, as I said quite a lot of the savings come from not having to heat water for 6 months of the year - sure I cold do that with just panels, but the battery gives me more flexibility. For example, knowing I will have enough off peak power stored for evening use, I can use daytime solar to run electric heaters, so no gas central heating during the day.

ispookie666

Even in the future there will be situations where the prices are sky high or negative. It's relatively easy to setup home assistant integration to monitor 24 hr energy prices and solar production estimates to charge or discharge. 

E 7 - I have been on it for a long time and usually around 50-60% used to night time, since the battery it has shifted to 95% off peak.  Which is still more that what I was paying Symbio