Please help me with battery options and business case

[Pressure]

Hello all. I would appreciate your help in identifying battery options as well as checking that my business case / rationale for battery sizing makes sense.

Background
First some context. We built a house in 2008-9 with a highly insulated, high mass structure, and underfloor heating fed by a Daikin air source heat pump. We added 5.6KW of PV in 2017. The house is all electric for space heating, hot water and cooking so consumption is fairly high - normally around 11,000 KWH PA, plus around 2,000KWH of self consumption from PV and 1,000KWH of export.

PV has been useful but the payback has been slow. However I now believe batteries could be very useful in reducing the cost of import, principally from TOU tariffs.

Battery business case
I modelled my 2023 consumption using Agile Octopus tariff data and compared the cost of that consumption without time-shifting import with that of storing energy imported at the lowest possible cost per KWH to find a near-optimal benefit. I focused on time-shifting two periods: the highest price periods (4PM to 7:30PM, obvs) and my highest-consumption periods (7:30 to midnight = 27% of import). For my actual consumption in 2023, using Yorkshire prices the savings are:

Period.........Cost w/o battery.....Cost if importing at optimal time
4PM-7:30PM:....£770....................£67
7:30-12AM:...... £1,113............... £106

So with a big enough battery, time-shifting only these periods, I could save £1,700 on a bill of £2,700. This would be awesome .

Doing this would need a really big battery to handle the heaviest import day - 46KWH! But the average import for these periods is 6.3KWH (mode: 7.9) and 4.5KWH (mode: 5). I therefore modelled how much of the benefit of an “unlimited” battery I could get with a 13KWH battery, and it turned out that a 13KWH battery gets me 82% of the benefit.

Now, the cheapest rates in 2023 were very low - often £0, sometimes negative (I.e. they pay you to import). We don’t know if that will continue but in the short term Octopus is not moving away from these incentives. I also did a quick check of what happens to the savings if the minimum price paid is 1pence and I still got 80% of the savings.

On the other hand, my savings model doesn’t include the value of storing and using spare solar (mostly in the summer). I also haven’t included any benefits from higher-value exports. Nor have I looked at time-shifting usage at other times of the day (in particular using the battery between 6:30 and 9:00, and re-filling the battery at mid-day. So I think my business case is sound, but I would be interested in your views!

Battery options
So assuming my benefits model is accurate, there is a business case for a battery, which might pay back in say 5 to 7 years. But which one? I think my requirements are:

• 13kwh capacity - should be enough to generate savings

• 5KW+ charge / discharge rate =- so I can get enough cheap electrons in the 2-3 hours of cheap prices

• Reliable manufacturer with a good support network - I need this device to be supported in 7 to 10 years time

• Easy integration with Octopus smart TOU tariffs with a good portal / app

• Not a DIY system - sorry, I just don’t have the skills to do this

To my newbie eyes the leading candidate for these criteria is probably a Givenergy AllInOne (which is bit cheaper than a Libby or a PowerWall). But should I be looking at other options? Do please let me know!

I hope this makes sense and thank for reading this somewhat long post!

[Joeboy]

This might be worth a read.

https://camelot-forum.co.uk/phpBB3/view ... =14&t=1194

[Pressure]

Thank you JoeBoy. I have skimmed some of that thread before registering. However I do feel that a DIY approach is likely to be beyond my skills, hence stating that I would prefer not to go that route in my post .

[Countrypaul]

As a sanity check on your calculations, what prices did you use for the modeling those in force today, last week or during 2023 at some point?
What round trup efficiency are you assuming you will get from your battery?
Are you using the ASHP to directly heat through UFH or are you using any sort of thermal store? Are you assuming the ASHP will be operating during the most expensive times of day (even if driven from batteries) are wlll the ASHP be idle during those that time (relying on the thermal mass)?

Which part of Yorkshire are you in - I'm in Wharfedale.

[openspaceman]

Are you using the ASHP to directly heat through UFH or are you using any sort of thermal store? Are you assuming the ASHP will be operating during the most expensive times of day (even if driven from batteries) are wlll the ASHP be idle during those that time (relying on the thermal mass)?

Hi Pressure and welcome

Like Country Paul I would be interested in the time constant of your heating input and output as you say the house is well insulated with high mass (inside the insulation). This should mean that heat put into the under floor mass should still be emitting heat well into the day and you could look at storage radiators for boosting this as the evening arrives. Okay you will have a COP of ~4 running the heat pump off peak but the storage heater with a COP of 1 will be at a higher temperature. This with a view to minimising running the heat pump from a battery.

[Joeboy]

Thank you JoeBoy. I have skimmed some of that thread before registering. However I do feel that a DIY approach is likely to be beyond my skills, hence stating that I would prefer not to go that route in my post .

No worries, if nothing else it can give you points of reference and a jump-start to some of the salient points in batterage.

I'll put forward for a first pass. Hybrid inverter talking to Pylontech stack. Pros online bms, modular build, large hybrid inverter integration cap, decent warranty, reliability.

Cons, price in comparison to diy.

Cheers!

[Gareth J]

Only an annual ~3000kWh from a 5.6kW PV system? Seems a bit low. Apologies, it's not the reason you started the thread and there may be good reasons that you're fully aware of, if so, ignore this!

[dan_b]

Welcome to the Forum!
Is this an official business case, or more a question of trying to justify Man Maths?!

What ever the reason, you're doing the right thing.

[Colin Deng]

Maybe i can help with the system
I will try to see if my current company can provide this kind of system

[Pressure]

Many thanks for the kind and helpful input everyone, it is very much appreciated.

The “business case” is essentially an attempt to do more rational man-maths. I know I need a battery, but what is it actually worth, in kWh and £££, and just how big would I need it to be?

For pricing I was comparing my 2023 actual usage with pricing for Agile Octopus (ie per half hour pricing), which I can apply to per half hour actual usage. While pricing will be different in the future at least this model uses real-world data and I can then discount for future years (if I know enough).

Not sure if solar production is bad - it’s sheffield, with trees and houses limiting insolation.

Battery round trip efficiency - not sure - almost all battery input will be from grid at low TOU tariffs. PV generation is a minor consideration. So not worried about AC/DC conversion from solar. So I don’t have a particularly helpful answer to that question. However the ASHP feeds UFH in concrete floors across the whole house, everything is a heat store - when the ASHP has failed in the past it’s taken a couple of weeks for us to actually notice.

The heat pump is on a little-and-often cycle, so there is consumption at peak periods. This seems to be the most efficient pattern for this model. Hence wanting to leverage TOU tariffs. Water temp is low-ish, think I set it at 35 the other day. But it’s an old model. For much of its life the system was desperately badly organised - a few years ago we got a daikin expert in and they got rid of half the kit in the plant room. It’s been vastly better since then (overall import dropped by 2000 to 3000 kWh a year since the fix). Needless to say the installer in 2008/9 was rubbish and they conveniently went bust before we even moved in.

However I don’t know what the actual power usage of the ASHP, which is pathetic, but it’s hard to know what to install to measure it (or where to put the ct clamps of a monitor…).