Please help me with battery options and business case

[Gareth J]

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

That probably explains it, I expect, roughly 1000kWh per kW of panels installed on a south facing, unshaded, sensible pitch roof. But all the things you mention will be nibbling into that.

Presuming a multi string setup, you could check, on your inverter, the string voltages/amperages of the different strings on a clear, unshaded part of a day. Should be close to on another and a significant difference might indicate a problem with one of the panels.

But if that's been the output from year zero, it's probably fine.

[John_S]

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 contradictory. My understanding is that heat pumps don't like a lot of cycling on and off. Can you try increasing the delta between off and on to prevent excessive cycling? Perhaps have a timer - all of the your TOU low rate plus a couple of hours later in the day if required.

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…).

One way to do this is to get a secondhand non-smart electricity meter and wire it inline into the heat pump supply. Sure, you have to read it manually, but it is fool proof.

[sharpener]

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.

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…).

I have been trying to answer similar questions prior to getting an HP installed. As you can see from my sig there is quite a lot of kit already.

Your existing thermal storage (slab) is by far the cheapest form of energy storage and by the sound of it will carry you across the 3 hr peak periods. And meets your other criterion <Reliable manufacturer with a good support network - I need this device to be supported in 7 to 10 years time>.

So have you tried setting the HP to switch off at peak times and see if the result is acceptable?

Next cheapest is storage as hot water, do you have any scope for more of this? I am planning a 215 litre thermal store but even so the payback is 12 years if you assume 20% electricity tariff inflation see this thread.

Batteries are the most expensive option of the lot. Not least because of the limited lifespan. We settled on 10.65kWh; the financially optimal size was only 7.1 but it was running out annoyingly often in winter. Now we charge it on E7 from 0000 to 0700 to a level which depends on the next day's weather forecast and it mostly lasts until midnight. But we have a lot more solar PV and cook on an AGA in winter. So 13kWh may well be the sweet spot for you.

Here is an overview of my annual usage, there is a lot more detail here.



Yes an external meter is the only reliable way to measure actual HP consumption, I have fitted a 1-module DIN rail mounted one in my CU for £40.

Other ppl on here have got good results by going to either Cosy or Intelligent Octopus Go, worth investigating also.

[Pressure]

Excellent and interesting ideas @sharpener.
We have a honking big hot water tank at IIRC 45 degrees. I must check when the heat pump is set to produce hot water. No room for a bigger tank sadly.

Thus far all advice has been to run the ASHP continually because more efficient to maintain heat in the slab than get it up to temperature. I may ask my architect wife what the latest thinking is, she’s an academic and has lots of renewable engineers and researchers to draw on. Thus far their consensus has been to run the heat pump little and often.

That is impressively low consumption and incredible generation you have in your system!

[Pressure]

Many thanks for the Pylontech suggestion @joeboy. What hybrid inverter options might be worthwhile?

[Joeboy]

Many thanks for the Pylontech suggestion @joeboy. What hybrid inverter options might be worthwhile?

We have a goodwe 3600sbp hybrid inverter fitted and it has been fine. The WiFi dongle went intermittent on it for a while and goodwe sent a new one to me unasked for. The unit was 6 years old by then and well out of warranty. If anything I'd rather have had a 5kW unit fitted instead of a 3.6kW but I didn't know that back then.

There are loads of good inverter manufacturers out there. Look for reliability, high efficiency, a decent app/graphic and good tech support.

[Ken]

"little and often" , I take it you mean on and off often as against a little on -low setting for long periods.

No HP should be run in a on/of cycling mode. Besides this being bad for the wear and tear it is also inefficient.
It is good to run at the low temps perhaps even lower than the 35C you have.
It should have a temp range in which to operate say +/- 1-2C so that it does not keep cycling. Is your HP modern enough that it modulates down to low setting ?
Personally with your well insulated house, good thermal mass and under floor slab i would only heat the slab during off peak/night hours. I only have the central heating on in the morning the thermal mass,occupation and solar gain doing the rest during the day except for "beast from the east" moments. Same for the HW.
PS i swiched my CH off yesterday and its not a Eco house by a long way just middle of the road

Your solar is not achieving its potential because of shading. A little bit of shading unfortunately affects the whole system. Perhaps it is too late in your case but this system shoiuld have been fitted with micro inverters on each panel.

[Pressure]

Thanks again for the advice everyone.

Yes, the solar has micro inverters, which help.

And yes, the ASHP is on low constantly, it’s never “switched off”.

Ideally I’d like a battery with a charge rate of 6 to 7 kw, just to ensure I grab as much cheap import as possible, hence looking at the Givenergy AIO. More expensive, and integrated, but fast charging, good octopus integration and “one butt to kick”.

[Pressure]

Right, a quick follow-up on my journey to checking I'm not mugging myself by buying a big battery.

Following advice from members here I thought I would change the heat pump settings to see how much benefit I can get from time-shifting consumption just by doing heating and hot water at mostly-cheap rates. The change happened on 7/4/2024. I basically set heating and hot water timings to be at cheap overnight rates and avoiding 16:30 to 20:00. I also slashed the flow temperature.

Without too much detail, I compared consumption per half hour from February to the change date, with consumption both from the change date to the end of April, and to the end of May. Obviously consumption is reducing a lot from May onwards and though the rest of April was fairly cool in Sheffield, average import per day went from 40kwh between February and 7/4/24, to 33kw from 7/4/24 to the end of April. It's just about comparable but it's more of a guide than a really scientific test.

Anyway, here's the findings for what happened after making the change to heat pump settings:

• Average import per half hour in April from 4:30PM to 8PM decreased a lot: between 30% and 80% per half hour slot. Still importing on average 3KW a day in this period so there's scope to time-shift more of that with a battery

• Average import per day during the high-ish morning rates (7AM - 9AM usually) didn't change at all - average import was still 3.4kwh in this period. So again, a lot of scope to time-shift this with a battery

• Import in the cheap tariff periods of 1PM to 2:30PM (often negative pricing when it's windy) actually reduced, partly because of more solar consumption as the sun came out. Obviously this is bad, it's a big missed opportunity to load up on cheap electrons

• And here's the kicker: rates in the evening are often high-ish after 8PM although much cheaper than the peak. Import was down about 30% - but on average I'm still importing 9kwh in this period. If I can fill up a battery at lunch time I can save a lot 4-8p a kwh in the evening, if it's big enough.

Obvious caveat: this is slightly rough analysis with some loose numbers. Even so, I think it's instructive. In April there's a lot of arbitrage available: 3+kwh in the morning, another 3+kwh at peak rates, and another 9kwh with less benefit in the evening - a total of 15kwh that could be time shifted with a battery.

So a 10kwh battery with a fast enough charge / discharge rate would have enough capacity to cover most or all of this consumption. And in the depths of winter there's probably even more benefit.

Here's what I think I want to do - let me know if the logic is flawed

I'd want to use a battery with integration with the Octopus smart scheduler API in their R&D system, and I'd probably want a high charge / discharge rate of 6kw, so that probably means a Givenergy battery, preferably the AIO for the high charge rate. These are more expensive than the Pylontechs etc so payback would be longer (probably 7 years) but it's all feasible through a simple web portal, with a reasonable level of support available. I can't DIY .

Thanks for the helpful advice as ever!

[AE-NMidlands]

Here's what I think I want to do - let me know if the logic is flawed

I'd want to use a battery with integration with the Octopus smart scheduler API in their R&D system, and I'd probably want a high charge / discharge rate of 6kw, so that probably means a Givenergy battery, preferably the AIO for the high charge rate. These are more expensive than the Pylontechs etc so payback would be longer (probably 7 years) but it's all feasible through a simple web portal, with a reasonable level of support available. I can't DIY .

Thanks for the helpful advice as ever!

The Givenergy portal is apparently good (and seems a lot better than my engineer friend's Growatt one) but is quite inflexible and still seems to have bugs in it!
The smart scheduler sounds like what I am looking for, but I can't find it via my account. Their FAQ doesn't recognise smart scheduler, but Googling "Octopus smart scheduler API " gets results. I don't understand what it says though (yet!)

Given a bit of schooldays programming in Basic and Fortran 50 years ago, [correction: Algol and Basic, my offspring did Fortran for their engineering] and recently playing with an Arduino or 2, what are the chances of my getting up to speed with it?
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