On-grid domestic battery storage

JD800

I'm just about to commit to a 4kW PV installation and have found the comments on here very helpful. I think I've down selected the installer and products and having gone through numerous quotations am now happy with my choice. However, I'm hesitating over whether to go for a battery or not.
My house is electricity only running ground source heat pumps at off peak Economy 10 times backed up with solar water panels for hot water and woodburner for lounge / dining rooms. So bills lowish at less than 6000 kWh annually (big 4 bedroom barn and no gas don't forget)
With FiT and daylight savings I think I can pay for Panels and installation in about 12 years so happy with that. However, I'd rather keep the "spare" PV generated electricity to try to avoid any peak consumption and reduce off peak.
After much haggling, my chosen installer can provide 7.2 kWh of stacked Solax batteries (2.4 each)and all controls for just under £3,500 inc VAT. My simple sums seem to indicate that if I could discharge 6 kWh each day for 300 days per annum that would save me about £340 per annum (my peak rate is expensive at 19p but gets me cheap off peak at less than 7p. With 10 year battery guarantee this looks about break even (ignoring opportunity cost etc).
If you then factor in electricity price increases, the fact I get the battery system at 5% VAT on initial installation (vs 20% VAT at retrofit), the fact I might be subsidising battery development for everyone else and that the battery might last longer, I think I'm happy to proceed.
I'm not sure we will ever get to the optimum time to fit a battery system as it will always be cheaper next year.
Anyone want to unpick my logic before I commit - all comments welcome.

Martyn1981

Hiya and welcome. I can't unpick your logic, every thought I had about one sentence, you answered in the following sentence.

So can I just ask for a bit more info?

JD800 wrote: »

After much haggling, my chosen installer can provide 7.2 kWh of stacked Solax batteries (2.4 each)and all controls for just under £3,500 inc VAT.

I've looked for some specs, but I'm not sure I've found the exact same product, so could you post a link (you'll need to add a space to break it, as you probably aren't allowed to post links yet).

I just want to check discharge percentages, useable capacity, and warranty cycles v's lifetime cycles.

JD800 wrote: »

My simple sums seem to indicate that if I could discharge 6 kWh each day for 300 days per annum that would save me about £340 per annum (my peak rate is expensive at 19p but gets me cheap off peak at less than 7p. With 10 year battery guarantee this looks about break even (ignoring opportunity cost etc).

Calcs look fine, but is it possible? Can you give some specs on the PV system, size orientation etc. Trying to get a feel for if you'll have 6kWh of export, often enough in the poorer 3 seasons.

Also, after 'normal' PV consumption, will you have 6kWh of daytime import each day in the summer season to offset. For instance I export about 15kWh per day in the summer (good start) but only import 2.5kWh, so the batts can't save me more than 2.5kWh, and the bulk of that will be nightime consumption (low rate for you).

JD800 wrote: »

If you then factor in electricity price increases, the fact I get the battery system at 5% VAT on initial installation (vs 20% VAT at retrofit),

Great point, this is where you stopped me in my tracks, as I was going to go on about prices falling. Also the £3.5k installed isn't too bad, roughly in line with the Tesla PWII.

JD800 wrote: »

the fact I might be subsidising battery development for everyone else and that the battery might last longer, I think I'm happy to proceed.

My negativity on price has a lot to do with basing payback on the warranted period. If you are happy to include non-warranted years (which is of course sensible, I'm just being overly tough) then payback improves for you, and if having a battery, and boosting development/deployment adds value to you, then it should be included in your calcs. I'd spend a lot of time looking at the numbers (and chatting on here) so it does have a value.

JD800 wrote: »

I'm not sure we will ever get to the optimum time to fit a battery system as it will always be cheaper next year.
Anyone want to unpick my logic before I commit - all comments welcome.

Yep, yep, and no I can't (unpick).

Mart.

JD800

Mart,
Thanks for helpful and comprehensive reply. Responses in order of your questions:

Batteries are Solax Battery X (3 off) - the contractor's spec sheet isn't very detailed (I've asked for the manufacturer's one) but quotes a 10 year warranty and a 10 year lifespan. For each battery, 2.4 kWh of storable energy with an 80 % depth of discharge.
Note - I'm getting a good price on the batteries as an incentive to do the PV panels at the same time (one visit / one installation / one electrician / one invoice). Any standalone purchase would be £1000 more and subject to 20% VAT, not 5%.
The PV system is 14 x 290 Q Peak panels facing almost due South on an unshaded roof. I've had 5 MCS estimates of output and the average is 3930 kW hours. The house is empty 3 days a week and even when I'm in daytime usage is pretty low. Washing machine etc is all on off peak in the evening.
My GSHP set up is very controllable so I could adjust to best use up daytime PV output if needed. My last annual summary had 3000 peak kWhs (and 2800 off peak ones) used so averaging about 5 / day in Summer and 11 / day winter.
You might be correct in that after some free PV peak kWhs, I might not actually generate the full 7.2 kWh some (most?) of the year and even then may not need the stored excess. But:
I love the thought of being paid to export but actually keeping it for myself!
I'll be working from home more and more over next few years so increasing consumption
Retirement and full time at home only 5 or so years away so more consumption
Once retired then an EV might make sense and again up my requirement.

So it's not purely a financial decision based on todays numbers for me - I see it possibly covering a range of lifestyle changes in the future as well (recognising that these are impossible to numerate accurately). But I don't see panel prices dropping much or Govt incentives rising much from this point.
And whilst Theresa / Jeremy might like to talk about capping prices, in the real world the generating and distribution companies will either make their shareholders profit doing it or they'll go off and do something else instead (Return on Capital Employed is a key director metric). So prices are only going up!

zeupater

Hi

My 'over-optimism' alarm bells are chiming here .... 4kWp solar + GSHP + battery + E10 with that tariff seems to be the cause ...

I'd be looking at investigating a solution to run the HP during daylight hours and (if possible) use the £battery saving to maximise the solar kWp, using the thermal-mass in the building to reduce the need for overnight heating to a minimum, then chasing a better electricity single-rate tariff ...

Spreadsheet time before committing to a battery at that price !!

HTH
Z

Martyn1981

JD800 wrote: »

Mart,
Thanks for helpful and comprehensive reply. Responses in order of your questions:

Batteries are Solax Battery X (3 off) - the contractor's spec sheet isn't very detailed (I've asked for the manufacturer's one) but quotes a 10 year warranty and a 10 year lifespan. For each battery, 2.4 kWh of storable energy with an 80 % depth of discharge.

Hiya JD, I'm still having trouble matching this up. I can find a 6.5kWh battery (6.3kWh useable) with a 10yr warranty, but that's not what you are describing.

SolaX-Battery

They also offer a 6.5kWh (5.9kWh useable LG Chem battery).

To match the 2.4kWh modular I've found this,

the X-Hybrid using a Pylontech product

this offers 2.4kWh scaleable units upto 3, so matches what you said, but what scares me is that whilst it states >6,000 cycles, which sounds fine, it has a 5yr warranty and 10yr lifespan which won't payback I suspect (especially when I get to the next part). Now this could be down to changes in product, and changes in warranties, so no need to panic, but needs checking as 10yr warranty with greater than 6,000 cycles is decent, but 10yr lifespan, not good enough, in my opinion.

JD800 wrote: »

Note - I'm getting a good price on the batteries as an incentive to do the PV panels at the same time (one visit / one installation / one electrician / one invoice). Any standalone purchase would be £1000 more and subject to 20% VAT, not 5%.

To be fair that explains the price of £3.5k for 7.2kWh installed, and I think justifies your serious consideration of the battery based on your first post, but not your second post, nearly there (honest).

JD800 wrote: »

The PV system is 14 x 290 Q Peak panels facing almost due South on an unshaded roof. I've had 5 MCS estimates of output and the average is 3930 kW hours. The house is empty 3 days a week and even when I'm in daytime usage is pretty low. Washing machine etc is all on off peak in the evening.
My GSHP set up is very controllable so I could adjust to best use up daytime PV output if needed. My last annual summary had 3000 peak kWhs (and 2800 off peak ones) used so averaging about 5 / day in Summer and 11 / day winter.
You might be correct in that after some free PV peak kWhs, I might not actually generate the full 7.2 kWh some (most?) of the year and even then may not need the stored excess.

And here's the crux, and I suspect what is alarming Zeup too.

Based on these numbers, I don't think you'll get value from the batts.

1. Forget the offpeak units - I doubt the batt will be cheaper. Take the cost of £3,500 and divide that by 6,000 cycles x 6kWh and you'll get a cost of storage of about 10p/kWh. I suspect your off-peak is cheaper.

2. 3,000 peak kWh's is good going for an all leccy property, and the 4,000kWhs of generation sounds reasonable, but without going into too much depth, my gut feeling is you'll consume 1,500kWh's of that without trying hard. So you are left with 1,500kWh's of leccy that's worth displacing with PV/battery, down from the earlier estimate of 1,800kWh. But ....

3. Some of that 1,500kWh will be in the worst months Nov-Jan, when you may have very little export, or none. Using PVGIS and sticking a pin in Oxford for a south facing 4.06kWp system, it suggests an average daily generation for those months of 4-6kWh's, and remember that's generation, not export. Some days export will be zero, some days generation may be zero.

4. In the summer, with long generation days, import will be practically zero without a batt, then the batt will just displace those 'cheap' night units.

JD800 wrote: »

But:
I love the thought of being paid to export but actually keeping it for myself!
I'll be working from home more and more over next few years so increasing consumption.

You'll be paid for exporting a deemed 50%, so around 2,000kWh's. I suspect that even with a batt you'd probably still be exporting 1,500kWh's, though it may be lower, but only due to you displacing night units, on which you probably lose out as they are cheaper than the cost of storage.

JD800 wrote: »

Retirement and full time at home only 5 or so years away so more consumption
Once retired then an EV might make sense and again up my requirement.

That 5yr figure scares me. I suspect the savings on battery costs will be enough over 5yrs to justify waiting, combined with better products, better warranties, and possibly bigger batts (as the batts get cheaper, it's worth buying more, a bit like the PV part of a PV install).

A Tesla PWII is roughly the same cost/kWh installed, around £6.5k for 13.5kWh (useable). That price will fall, and in the summer would store enough spare to charge up that EV, and to cover good/bad days by storing more. Let's say it gets down to £4.5k in 5yrs time, but for 13.5kWh, doesn't that sound like a better package?


Sorry for being so negative now, but I don't think your remaining daytime import can be reduced enough to justify the battery expenditure at the moment. But it's only my opinion, much of it based on gut feelings.

Have a think, and bear in mind I'm a supporter of batts, not a critic.

Lastly, and this is pure speculation, but the government does seem to be talking up some sort of demand storage strategy. I'm not convinced anything will come of it, and even if it does it may be linked only to batt installs with PV, but I can't shrug off the feeling that they've started to realise the potential.

JD800

Mart and Zeup
Thanks for all opinions. I asked for advice so glad to have it.
Think the accusation of over optimism might be correct - I'm prepared to be convicted.
Concur that saving off peak units with a battery is not going to be cost effective so if I can't eradicate my peak units in total then concur the battery is an emotional decision not a rational one.
I'll take a look at my monthly consumption figures from last year split into peak and off peak and re-do the calculations more accurately. I'll also get battery manufacturer specs and post for info.
As you both imply, if the PV system will only cover the peak daytime use in Winter leaving nothing to store, then the battery is definitely a non-starter based on financial return and I'll revert to PV only but battery ready.
Thanks again for your time / comments,

zeupater

JD800 wrote: »

Mart and Zeup
Thanks for all opinions. I asked for advice so glad to have it.
Think the accusation of over optimism might be correct - I'm prepared to be convicted.
Concur that saving off peak units with a battery is not going to be cost effective so if I can't eradicate my peak units in total then concur the battery is an emotional decision not a rational one.
I'll take a look at my monthly consumption figures from last year split into peak and off peak and re-do the calculations more accurately. I'll also get battery manufacturer specs and post for info.
As you both imply, if the PV system will only cover the peak daytime use in Winter leaving nothing to store, then the battery is definitely a non-starter based on financial return and I'll revert to PV only but battery ready.
Thanks again for your time / comments,

Hi

I know it's completely different to a GSHP setup, but have you had a look at what a few of us have been doing with PV & Air/Air to provide shoulder month heating on this forum thread ? ...

To get an idea of PV seasonal performance have a look at this article on PV & Thermal systems as both would apply to your proposed setup ... Seasonal Variability - So what's it like in Winter


Also, if you do have the roof-space available and decide to increase the PV capacity to feed electrons to the GSHP, then have a look at the cost comparison analysis between <4kWp systems and larger ones in ... this site link ... it should give you an idea of the relative costs (£/kWp) of larger systems ...

HTH
Z

JD800

Thanks very much - I'll take that as homework for the weekend.
Not sure what shoulder month heating is but I'm sure the forum will educate me.
I may be back with questions :-)

zeupater

JD800 wrote: »

Thanks very much - I'll take that as homework for the weekend.
Not sure what shoulder month heating is but I'm sure the forum will educate me.
I may be back with questions :-)

Hi

In this context, shoulder heating months would be the Spring & Autumn months where lower level comfort and 'top-up' heat provision is required ...

Happy reading ....

HTH
Z

Martyn1981

JD800 wrote: »

Mart and Zeup
Thanks for all opinions. I asked for advice so glad to have it.
Think the accusation of over optimism might be correct - I'm prepared to be convicted.
Concur that saving off peak units with a battery is not going to be cost effective so if I can't eradicate my peak units in total then concur the battery is an emotional decision not a rational one.
I'll take a look at my monthly consumption figures from last year split into peak and off peak and re-do the calculations more accurately. I'll also get battery manufacturer specs and post for info.
As you both imply, if the PV system will only cover the peak daytime use in Winter leaving nothing to store, then the battery is definitely a non-starter based on financial return and I'll revert to PV only but battery ready.
Thanks again for your time / comments,

Hiya. I have to say I've enjoyed this discussion as your first post gave a good argument for considering a battery, and your second one explained why it might be too soon.

So a great mental exercise in what is currently a very tricky subject to wholly encompass. I think we all got it right this time, but it does suggest that we're getting closer to finding the 'right' person/circumstances.

Can I just ask, like Zeup, if you are maxing out roof space, or are settling for 4kWp? If you have the space then going bigger is probably sensible.