Battery install

[DSMSMR]

So we currently have 5.6kW of solar, a zappi and an EV. Tied in with British gas EV tariff at 9p per from midnight to 5 am, then reg rate during the day.

Thinking of if its worth installing a 16kWh battery to give us all our L needs and exporting excess if poss.

To supply and install a 16kWh battery 6kW inverter DC couple it, remove old grid tied Inverter...I might get £250 as its only 3 yrs old....etc is £4500.

Payback is years. I reckon pushing 9.

We have only just got the EV, and zappi install soon. We only do 5k miles pa in the old petrol car, which will go....and maybe 2k ish in the fun car....petrol.

Is it worth gettting batteries???

My monthly DD with BG is £50

[pghstochaj]

If you only spend £50/month on electricity then why go for 16 kWh?

Is your existing inverter not a hybrid that you can add a battery to? That's a lot of solar to have with no battery, you must be exporting a lot on good days.

I wouldn't be rushing to spend £4,500 to reduce the cost from £50/month to £15/month.

You would have to do a detailed analysis to work out how you are using electricity and when.

[RizzoTheRat]

Is your payback time based on current prices or projected future prices? I'm non-uk so may well be different, but my contract expires at the end of the year my new contract for next year is about a third more expensive. Seriously considering a battery system at some point, I think I'd be happy with 9 year payback so long as the battery comes with a long enough warranty.

[superpp]

to speed up any payback, you need a better tariff.
not sure on the latest rates available, but I switched from Octopus to EDF in June.
Now getting 15p for export, 7p off peak and 30p peak.

A larger battery will let you export more, but you need to do your own sums based on a full year's bills.
Maybe ask AI to help.
I'm saving around £500 per year with the addition of a battery to my existing solar.

Additional savings available if you sign up to the likes of Axle too.

Don't use the Zappi to charge via solar, export as much as possible and only charge overnight, as you can be saving around 25%+ on your cars fuel (12p export versus 9p overnight).

Edited by superpp on Monday 5th October 13:35

[DSMSMR]

why a 16kw battery. Well yesterday as an example I used 11kW of power.....all from the grid, at 35p per.....a battery would have cost me 9p per....as I am on BG EV tarif that would charge the battery at 9p. Then theres winter. No gen after what 3/4 pm, so everything is on the high tarif.
Then in summer, exporting solar during the day at 12p...currently, then exporting a full battery between 4-7pm....at 25p per....

I have the ££ sat doing nothing...so?

[pghstochaj]

DSMSMR said:

why a 16kw battery. Well yesterday as an example I used 11kW of power.....all from the grid, at 35p per.....a battery would have cost me 9p per....as I am on BG EV tarif that would charge the battery at 9p. Then theres winter. No gen after what 3/4 pm, so everything is on the high tarif.
Then in summer, exporting solar during the day at 12p...currently, then exporting a full battery between 4-7pm....at 25p per....

I have the ££ sat doing nothing...so?

How would you export at 25p per kWh, assuming that is what you meant? Typically you will get 12 p/kWh and there is no point in exporting at 12 p/kWh to buy back at 9 p/kWh. Maybe you are on a FiT and haven't mentioned it? In which case this makes your decision more complicated as I don't think you can replace the items just as you are stating.

If you spend £600/year on electricity at roughly 30 p/kWh you are only using at 2000 kWh from the grid, so roughly 5.5 kWh/day on average. Hence the question on battery size. This is where it becomes complicated as you have a big solar install so are presumably importing a lot during 6 months and exporting a lot for the other 6 months which might be hiding quite a lot of daily usage.

It's also not clear whether your £50/month includes any import revenue (and if so, at what rate). The most you can save is about £500/annum based on the global sum hence the question on why such a large battery given your low import usage. This is unless you start using something like Agile to export at higher hourly prices but that is quite engaged.

[_CrazyIvan_]

You don't need a battery though to cover all your peak winter use as it will be wasted the rest of the year, Cheaper to export during the lighter months than charge the batteries. You just need a battery big enough to cover your average use. You'll get a better pay back calculation.

To give you an idea, our monthly electric bill is around £300 per month and we're looking at putting in a solar and battery system (around 18-26kw in storage) based on the calculations from the 4 suppliers we've had quotes from.

Edited by _CrazyIvan_ on Monday 5th October 17:07

[OutInTheShed]

superpp said:

to speed up any payback, you need a better tariff.
not sure on the latest rates available, but I switched from Octopus to EDF in June.
Now getting 15p for export, 7p off peak and 30p peak.

A larger battery will let you export more, but you need to do your own sums based on a full year's bills.
Maybe ask AI to help.
I'm saving around £500 per year with the addition of a battery to my existing solar.

Additional savings available if you sign up to the likes of Axle too.

Don't use the Zappi to charge via solar, export as much as possible and only charge overnight, as you can be saving around 25%+ on your cars fuel (12p export versus 9p overnight).

Edited by superpp on Monday 5th October 13:35

It's all very well doing the maths on full year's bills, but I don't think tariff s will look the same in 5 years time.
In the long run, EVs and heatpumps are changing the landscape.

Everybody is different, their use pattern is different their way of looking at inflation is different, their belief in how things might change is different.
Think about it your own way.

One thing I'm unsure of is how much cheaper batteries can get in real terms.
And whether the promise of plug-in batteries will actually happen.

[DSMSMR]

pghstochaj said:

DSMSMR said:

why a 16kw battery. Well yesterday as an example I used 11kW of power.....all from the grid, at 35p per.....a battery would have cost me 9p per....as I am on BG EV tarif that would charge the battery at 9p. Then theres winter. No gen after what 3/4 pm, so everything is on the high tarif.
Then in summer, exporting solar during the day at 12p...currently, then exporting a full battery between 4-7pm....at 25p per....

I have the ££ sat doing nothing...so?

How would you export at 25p per kWh, assuming that is what you meant? Typically you will get 12 p/kWh and there is no point in exporting at 12 p/kWh to buy back at 9 p/kWh. Maybe you are on a FiT and haven't mentioned it? In which case this makes your decision more complicated as I don't think you can replace the items just as you are stating.

If you spend £600/year on electricity at roughly 30 p/kWh you are only using at 2000 kWh from the grid, so roughly 5.5 kWh/day on average. Hence the question on battery size. This is where it becomes complicated as you have a big solar install so are presumably importing a lot during 6 months and exporting a lot for the other 6 months which might be hiding quite a lot of daily usage.

It's also not clear whether your £50/month includes any import revenue (and if so, at what rate). The most you can save is about £500/annum based on the global sum hence the question on why such a large battery given your low import usage. This is unless you start using something like Agile to export at higher hourly prices but that is quite engaged.

High export from the battery between 4pm and 7pm. Octopus tariffs.
My £50 pcm is my bill. I never mentioned any SEG.

[DSMSMR]

_CrazyIvan_ said:

You don't need a battery though to cover all your peak winter use as it will be wasted the rest of the year, Cheaper to export during the lighter months than charge the batteries. You just need a battery big enough to cover your average use. You'll get a better pay back calculation.

To give you an idea, our monthly electric bill is around £300 per month and we're looking at putting in a solar and battery system (around 18-26kw in storage) based on the calculations from the 4 suppliers we've had quotes from.

Edited by _CrazyIvan_ on Monday 5th October 17:07

Ummmm errrr wuh?

[pghstochaj]

DSMSMR said:

High export from the battery between 4pm and 7pm. Octopus tariffs.
My £50 pcm is my bill. I never mentioned any SEG.

That’s 16 p during 4-7 p and 9 p during the rest of the day though. It’s not 25 p, is it? With your solar set up I suspect you could be worse off being on that rather than the standard 12 p option.

But your bill will have SEG or FiT from your solar so is £50 your net or total?

You’re making this hard work.

[OutInTheShed]

DSMSMR said:

_CrazyIvan_ said:

You don't need a battery though to cover all your peak winter use as it will be wasted the rest of the year, Cheaper to export during the lighter months than charge the batteries. You just need a battery big enough to cover your average use. You'll get a better pay back calculation.

To give you an idea, our monthly electric bill is around £300 per month and we're looking at putting in a solar and battery system (around 18-26kw in storage) based on the calculations from the 4 suppliers we've had quotes from.

Edited by _CrazyIvan_ on Monday 5th October 17:07

Ummmm errrr wuh?

Ideally, you'd analyse how much you'd save for typical days each month of the year, as the solar output changes and your use changes.
But it's like herding cats if you don't have the stats for weather and your use hour by hour.
It's all nonsense anyway as soon as different tariffs come along.
You've just got an EV? That moves the goal posts.
Might you get a heat pump at some point?
Who knows what the future holds?

[theboss]

pghstochaj said:

That s 16 p during 4-7 p and 9 p during the rest of the day though. It s not 25 p, is it? With your solar set up I suspect you could be worse off being on that rather than the standard 12 p option.

But your bill will have SEG or FiT from your solar so is £50 your net or total?

You re making this hard work.

Using outgoing agile presumably

https://www.octopriceuk.app/agileOutgoing

35p tomorrow plus an Axle event at the same time (great for as long as it lasts)

The evening export rates have been consistently above 20p for the last month or so. My average on last bill (late Aug through late Sept)

Total consumption 880.2kWh @ 21.62p/kWh

[pghstochaj]

theboss said:

pghstochaj said:

That s 16 p during 4-7 p and 9 p during the rest of the day though. It s not 25 p, is it? With your solar set up I suspect you could be worse off being on that rather than the standard 12 p option.

But your bill will have SEG or FiT from your solar so is £50 your net or total?

You re making this hard work.

Using outgoing agile presumably

https://www.octopriceuk.app/agileOutgoing

35p tomorrow plus an Axle event at the same time (great for as long as it lasts)

The evening export rates have been consistently above 20p for the last month or so. My average on last bill (late Aug through late Sept)

Total consumption 880.2kWh @ 21.62p/kWh

Why did he/she mention 4-7pm which is outgoing prime though and not agile?


Edit: also, he/she will lose out on sunny days when outgoing agile is less than octopus outgoing. With that size solar and a relatively small battery that will be a regular occurrence.

It’s all a bit odd.

Edited by pghstochaj on Monday 5th October 23:28

[ARH]

surely you don't need to use the battery to power your house during the cheap rate electric times?? So you need to check how much electric you use during peak times. then use that to determine how big a battery you need.

[OutInTheShed]

ARH said:

surely you don't need to use the battery to power your house during the cheap rate electric times?? So you need to check how much electric you use during peak times. then use that to determine how big a battery you need.

1) You shouldn't assume cheap rates will be happening every day in 5 years time
2) Many Industrial LFP cells are around 300 to 310 Ah, making about 16kWh the 'natural size' for a 48V bank.
Cells 1/3 the size tend to cost more than 1/2 as much.

The bigger cells are also less stressed by higher loads.

It's a complex problem. You can attempt to pick the low hanging fruit with a smaller cheaper system, or you can go larger.
Most choices can be justified, given convenient assumptions.

[DSMSMR]

I have signed up for BG EV tarif. 9p per kWh from midnight to 5am but that just for the EV right now. And 33p per kWh during the day
Battery install. I have done some serious number crunching of various sized batteries from 5kW up to 16kW. The cost, payback etc.
Then tied that in with various tarifs, expecially Octopus's which have all different scenarios if you do or that.

I am going to apply for a G99 to tie in with a 6kW hybrid inverter to export all that. Winter export wont make much of a diff, but next spring and summer we shall see.......AND of course theres the fun thats going to kick in sometime in jan according to M Lewis.

Maybe my DNO might throw a tizzy and limit me to the 3.68kW export that I have now....shall hope they dont.

[Actual]

DSMSMR said:

I have signed up for BG EV tarif. 9p per kWh from midnight to 5am but that just for the EV right now. And 33p per kWh during the day

I hate to say this but my BG tariff has lower prices

Name
EV Power Mar27 v1
Type
Fixed
Start date
1st March 2026
End date
28th February 2027
Electric Vehicle Off Peak
5.23p per kWh
Electric Vehicle Peak
26.79p per kWh
Standing charge
52.23p per day
Exit fee
£50
Please note, all prices shown are inclusive of VAT.

All our water heating and appliances are used after midnight.

It won't last forever though.

Edited by Actual on Tuesday 6th October 10:19

[SP_]

superpp said:

to speed up any payback, you need a better tariff.
not sure on the latest rates available, but I switched from Octopus to EDF in June.
Now getting 15p for export, 7p off peak and 30p peak.

A larger battery will let you export more, but you need to do your own sums based on a full year's bills.
Maybe ask AI to help.
I'm saving around £500 per year with the addition of a battery to my existing solar.

Additional savings available if you sign up to the likes of Axle too.

Don't use the Zappi to charge via solar, export as much as possible and only charge overnight, as you can be saving around 25%+ on your cars fuel (12p export versus 9p overnight).

Edited by superpp on Monday 5th October 13:35

Agile export is far better on Octopus, however, hence I am reluctant to swap to EDF.
Will only get better in Winter.

For example, here are today's rates. We will be exporting at almost 30p/kWh this afternoon
https://agile.octopushome.net/dashboard

[SP_]

superpp said:

to speed up any payback, you need a better tariff.
not sure on the latest rates available, but I switched from Octopus to EDF in June.
Now getting 15p for export, 7p off peak and 30p peak.

A larger battery will let you export more, but you need to do your own sums based on a full year's bills.
Maybe ask AI to help.
I'm saving around £500 per year with the addition of a battery to my existing solar.

Additional savings available if you sign up to the likes of Axle too.

Don't use the Zappi to charge via solar, export as much as possible and only charge overnight, as you can be saving around 25%+ on your cars fuel (12p export versus 9p overnight).

Edited by superpp on Monday 5th October 13:35

Agile export is far better on Octopus, however, hence I am reluctant to swap to EDF.
Will only get better in Winter.

For example, here are today's rates. We will be exporting at over 30p/kWh this afternoon
https://agile.octopushome.net/dashboard

Edited by SP_ on Tuesday 6th October 10:55