Second thoughts on batteries

[Paul_F]

So we're just kicking around a few aesthetic questions before applying for planning permission on a new-build Passivhaus, and the architects have suggested we start thinking about heating, MVHR, PV, etc.
Tentative plan is:

1. 3-phase power to site in order to allow for lots of PV and car charging in future.
2. Heating and hot water from a small (3kW) heat pump. GSHP or ASHP is TBC - GHSP preferred but the groundworks may be too expensive. Modelled heat load (PHPP) is about 1.8kW.
3. 300 litre unvented hot water cylinder, possibly a Mixergy one for faster recharging.
4. Underfloor heating and cooling plus a water to air heat exchanger in the MVHR outlet, running at very low temperature (25°C max).
5. Lots of PV - at least 8kW, maybe 10kW at a stretch on the SW facing roof. NE facing roof is up for grabs and may be feasible.

I had been assuming that a home battery wouldn't make sense as they're simply too expensive. However, I've just realised that Givenergy do 10kW 3-phase hybrid inverters for almost the same as an equivalent standard inverter, and an 8 kWh battery can be had for £2k ex-VAT. Since that gives a whole-house UPS and does full integration with e.g. Agile Octopus (both incoming and outgoing), it's significantly more interesting than other battery systems.
If the current high electricity prices continue I could see it being cost-effective, but I'm not expecting that to happen. At 10p/kWh difference between charging and discharging costs though it starts to be justifiable over 10 years and we get a whole-house UPS for free, as well as a partial hedge against very high electricity prices.

I know a bunch of you run battery systems at home, so could you comment on how realistic this thinking is and how it fits with your experience?

[Joeboy]

Best of luck with the build, batteries are ace. I'm sure this is the 3rd of 4th time you've asked the same questions. Why so hesitant over such a timeline? Nobody (sadly) has a crystal ball for prices. I have not heard anyone ever say they regret getting batteries.

It's your personal financial & mental hurdle to get across and I hope you do.

[Paul_F]

Best of luck with the build, batteries are ace. I'm sure this is the 3rd of 4th time you've asked the same questions. Why so hesitant over such a timeline? Nobody (sadly) has a crystal ball for prices. I have not heard anyone ever say they regret getting batteries.

It's your personal financial & mental hurdle to get across and I hope you do.

It's a slow and painful process to go through I'm afraid, and my personality type is to over-analyse everything anyway. We've also had an abortive start on this ~4 years ago, where we tried to plan out a retrofit which had something of a cost-blowout so we had to stop before we really got started.

I did have some questions about it on the other place, essentially that was when I was working off my own sketch design - we've now got one from an architect which is significantly better and more mature. Some of the underlying design assumptions have changed though, which is why I'm thinking this over again.

[Joeboy]

Fair enough entirely. It would simplify if you had a think and bullet point the first two reasons that come to mind that stop you on batteries and share them with us.

Lots of different reasons pro & con, personal situations and beliefs out there.

For example I originally invested to cut utility bills, this developed into a more green awareness and urge to improve this aspect of life and I am now into the Don Corleone way of thinking. 'I refuse to dance on the end of a string for those big shots'.

If I was building a passivhaus I would ensure my power array had head room to expand in the main components without changing out, instead modular growth at a later date.. Sadly I don't know the give energy system or their reliability. Most of the LFP guys here run Pylontechs to a good result. Doesn't set it in stone but it is a strong indicator.

8kW for 2.4k is great, just make sure you dig down for system reviews on the company, equipment and support. .

[Oldgreybeard]

I can really understand the dithering, especially over battery storage. I designed our house in 2012, and decided then that we were going to have as much solar as I could possibly fit in the roof, and that we were going to have battery storage, so I ran cables in ready for it. Money was tight during the build, and battery prices looked too high to be worth fitting, so they got left out.

Now, some five years after we finished the build, I've decided the time is right to install a battery system, so everything is on order and we should be up and running with them by mid to late October (there's a long lead time on batteries at the moment, it seems.

The thing that swung the balance for me was the recent hike in electricity prices and finding an Economy 7 tariff that offered a cheap overnight rate of just 7p/kWh, fixed until June 2023. Because both our cars are now EVs, and we already use E7 for heating and hot water (when the solar panels aren't generating enough) it was a no-brainer to try and get practically all of our grid consumption switched to the overnight off-peak period.

Payback time looks to be around 7 to 8 years, by my estimates, but anything could happen with energy pricing after this fixed tariff runs out. It may be I need to add more batteries to increase the capacity if prices go up, in the hope that we can make more use of excess solar generation, but that's a lot cheaper than the initial installation and adding another battery pack is an easy DIY job.

[Paul_F]

Fair enough entirely. It would simplify if you had a think and bullet point the first two reasons that come to mind that stop you on batteries and share them with us.

Two key concerns:

1. Up-front costs: funding the build requires us to borrow a significant amount of money from a close family member. This means I'm reluctant to spend significant amounts of money on nice-to-haves we don't need up front unless they come close to paying for themselves and taking advantage of the VAT saving makes sense.
2. Grid integration: most battery systems aim to minimise import from the grid, and do so by soaking up any PV that would otherwise be exported. At a grid level that isn't terribly helpful - for instance with today's prices you'd want to discharge from battery to grid during the evening peak, charge from PV (+ grid too maybe) during the day and a bit from the grid overnight. GivEnergy have a fairly basic integration with the Octopus API which is an improvement (can force discharge if the export rate is above a certain level), but still not great.

If I was building a Passivhaus I would ensure my power array had head room to expand in the main components without changing out, instead modular growth at a later date.. Sadly I don't know the give energy system or their reliability. Most of the LFP guys here run Pylontechs to a good result. Doesn't set it in stone but it is a strong indicator.

I'm fairly relaxed on reliability - LiFePO₄ cells seem to be pretty robust, and the rest of the system isn't that complex or highly stressed.
Expansion is an open question - GivEnergy batteries are DC-coupled so can run multiple batteries in parallel, but it's a closed ecosystem so realistically expanding it would be problematic. Having said that, it's questionable on the value of adding extra batteries past a certain point - the returns diminish quite strongly with bigger batteries, and cost / embodied carbon don't.




The values are calculated for a Passivhaus Plus (very similar to what we're looking at) with the difference in imports only being 2-300 kWh/year. Given that new-build is zero rated for VAT, the thinking is that we do it right first time or not at all.

8kW for 2.4k is great, just make sure you dig down for system reviews on the company, equipment and support.

Haven't got that far yet, still trying to work out if it's even worth considering. It's currently £2100 ex VAT, but you have to use their inverters which are a little more expensive (+£500 for 4kW). That's what makes the battery cost-effective incidentally - it gets to share the inverter electronics, but ties you to their inverter as a result.

The thing that swung the balance for me was the recent hike in electricity prices and finding an Economy 7 tariff that offered a cheap overnight rate of just 7p/kWh, fixed until June 2023. Because both our cars are now EVs, and we already use E7 for heating and hot water (when the solar panels aren't generating enough) it was a no-brainer to try and get practically all of our grid consumption switched to the overnight off-peak period.

Yeah, the recent spikes in prices (particularly export prices) coupled with the fact that LiFePO4 cells are hitting $100/kWh in bulk got me looking again. They're mostly still very expensive, but definitely coming down.

[bxman]

Hi

Not to sure of the etiquette in these matters but could you please post or possibly PM the details of this particular tariff .

7p sounds an awful lot better than 15.55p for the E 7 period

TIA

[Oldgreybeard]

Hi

Not to sure of the etiquette in these matters but could you please post or possibly PM the details of this particular tariff .

7p sounds an awful lot better than 15.55p for the E 7 period

TIA

Hopefully doing it won't be considered out of order if I mention the supplier, although it is an EV Driver E7 tariff, so you need at EV to qualify for it. The supplier is Good Energy and the tariff is their EV Driver, Green Driver 7 Hour June 2023 one, at the bottom of this page: https://www.goodenergy.co.uk/our-tariffs/#

[nowty]

If money is tight for the build, I would concentrate on getting as much Solar PV fitted first as a priority. A battery system can be retrofitted later once you know where you are once you are living in it.

In your position, If I had to make a choice of more PV vs less PV and a battery, more PV wins, especially when you can now get paid for export through the SEG scheme.

I have 1.7kW on my NE facing roof, helps you get through those cloudy summer days. It was a DIY job using a second hand inverter and was very cheap to install, at about £1000 all in.

[Oldgreybeard]

We did exactly as @Nowty suggests. Money was tight for our passive house build, so I dropped the idea of installing batteries and prioritised installing solar. I managed to get 25 panels into the South facing roof, as many as would physically fit, using all the roof area, but went for in-roof panels, so saved around £2,000 worth of slates, and the associated cost of fitting the slates.

One other thing I did was put as many extra cable runs in as I thought we'd ever need, so lots of outdoor armoured cables in ducts, just terminated at the ends with wiska boxes. This has been a real boon, as it means all those circuits were tested and are included within our Part P building regs chit, so I can legally DIY connections to the ends of them.

This has saved a lot of money, as it meant I could connect car charge points, outside lights and now the battery shed, all without needing to get additional Part P sign off, and the associated cost The cost of cable is virtually nothing in the overall scheme of things, and if run in ducts, with additional draw strings, it can save loads of hassle when adding things in future. My only regret is that I didn't add extra ducts for things like data cables, as it would have been really handy to be able to get Ethernet and CT transformer connections to some places. In particular, I definitely should have run Ethernet underground to our garage (which is the other end of the garden) as I've had a lot of hassle setting up a point-to-point Wi-Fi link, that's cost a load more than 50m of direct burial rated Ethernet cable would have cost.