AC Vs DC coupled batteries

[Hammertime]

Hello! I'm installing a 3.7kwh solar array for a small property, and I'm wondering about whether to add battery storage now or wait to see how the system runs first. Which one I decide on will determine whether it's a DC coupled battery and hybrid inverter (installing at the same time as the solar array) or AC coupled battery (retrofitted to the array).

From reading up about AC Vs DC coupled, people seem to love one or the other, but I'm struggling to see much difference. 

 

- AC is less efficient, but only by a few percent so not worth worrying about

- A hybrid inverter might become incompatible with future batteries as manufacturers bring out new ranges, whereas AC coupled batteries have their own inverter and so any brand can be added to any standard inverter at a later date

 

Is this correct? I read something about one not being able to feed back to the grid, but I don't think that's right. If those are the only differences, I might be better off waiting to see how the system runs then adding AC coupled later if required.

 

A couple of other questions:

1) I've been offered a SolaX inverter. Does anyone have any experience with these? The online reviews look pretty awful and I'd prefer paying a little more for a reliable brand.

2) Should I oversize the inverter to anticipate future battery installation? I've been quoted for a 3.3kw inverter, but eg underfloor heating would probably pull double that by itself and so a 3.3kw inverter would limit the benefit of a large battery system in the future

3) If I did get a battery, what size would be good? I've been recommended 5.12kw, but it seems to me this would hardly touch the sides with winter energy usage, even in a small one bedroom house.

 

Thank you for any replies and advice!

[Nick Thomas]

Hey hey. A few thoughts - I'm sure others will be along shortly.

 

One hybrid inverter is probably cheaper than an inverter for the panels, and a separate inverter for the batteries, depending on which brands you go for.

 

Solax is junk, for me. Mine was insecure out of the box and not possible to secure to my satisfaction, even after a chat with solax engineers. I have an X1 Boost sat in a box in the garage as a result.

 

For grid export, if you have two separate inverters then you're meant to sum their capacities when it comes to DNO approval. IME, installers of AC-coupled systems aren't that interested in dealing with DNO approval, so just disable grid export. Presumably that changes if you splash out on powerwalls or whatever, but even if you found one that permits it, you'd still be looking at G99 for a simple install when a hybrid inverter could go in under G98.

 

If you get an inverter that can do more than 3.68kW out, it'd need G99 anyway - or maybe G100 (export limitation - I don't know whether that would allow you push, say, 6kW when the house demand in 3kW, or not).

 

It used to be the case that the batteries were only VAT-free if you got them installed at the same time as the solar, but they're now VAT-free regardless, which makes AC-coupled slightly less bad.

 

My hybrid inverter comes with an EPS circuit which can run from the batteries, and in my head, the batteries will continue to charge from the solar in the event of a power cut. I don't know if this actually works, and it's a pretty marginal benefit, but I like the idea of it ^^.

[SteamyTea]

Worth looking into the minimum load that triggers the battery/inverter.

This is often 200W, which will not run small loads like lighting and laptops.

 

Mt view is that batteries are still too expensive, much better to store excess energy in DHW.

 

2 hours ago, Hammertime said:

installing a 3.7kwh solar array

It is kW, which is power (actually kW_p where the subscription is peak)

kWh is energy i.e. the size of the batteries, or how much energy the PV system generates over a set period of time i.e. 4000 kWh.year^-1.

[sharpener]

6 hours ago, Hammertime said:

I've been quoted for a 3.3kw inverter, but eg underfloor heating would probably pull double that by itself and so a 3.3kw inverter would limit the benefit of a large battery system in the future

 

Are you talking about electric UFH here?

 

6 hours ago, Hammertime said:

If I did get a battery, what size would be good? I've been recommended 5.12kw, but it seems to me this would hardly touch the sides with winter energy usage, even in a small one bedroom house.

 

Yes, 5kWh (I assume that is what you mean) is on the low side for a small house even without electric heating. Depends on whether you have an electric cooker, if so 7.5 kWh probably the min size to get you through the evening until Economy 7 kicks in. But in winter any savings would be dwarfed by the consumption of the UFH as you say.

 

A thoughtful discussion started by someone who has a sound theoretical approach here. I presume you have also read the other recent threads on this forum?

 

 

 

 

Edited May 27 by sharpener

[JamesPa]

11 hours ago, sharpener said:

 

17 hours ago, Hammertime said:

If I did get a battery, what size would be good? I've been recommended 5.12kw, but it seems to me this would hardly touch the sides with winter energy usage, even in a small one bedroom house.

 

 

You need to bear in mind that

 

in winter you arent going to generate enough from Solar PV to cover your energy use

in summer you will, unless you have an electric car or aircon, generate more solar PV than you can use in a day.  As its pretty impossible to do long term storage cost effectively, the excess will be exported. 

 

You also need to think a bit carefully about what you want to do.  There are two possible uses for batteries namely (a) to store excess generation from solar PV and (b) with a ToU tarrif, to import when energy is cheap in order to reduce your average energy cost. 

 

You need to decide which of these you are doing because it affects the choice of equipment and the battery size.

 

If you want to take advantage of ToU tarrifs then the maximum useful battery size is set by your energy consumption outside the cheap rate period.  Make an estimate of that and you have an upper limit for the useful battery size.  If you have ASHP heating at say 6kW, a typical 4MWh/year electricity consumption excluding heating, and the cheap rate lasts for 6 hours out of 24, then that's about 6/24*(4000/365 + 6*24/3)  (3 being the ASHP COP) = 45kWh.  On most heating days however your heating will be operating at half max capacity or less so a more realistic figure is 6/24*(4000/365+3*24/3) = 34kWh = 26kWh.  Both are probably larger than you can realistically do.  There are also cheaper ways to do part of the energy storage namely in your DHW (about 10kWh can be stored in a 200l cylinder) via a diverter/timed heating or in the slab if your house is on a nice insulated concrete base.  The numbers are only examples, you need to substitute your own figures and your own situation. 

 

If the sole objective is to store excess PV then you need to think about how much you can use 'real time' and how much you have to time shift.  This is dependent again on your actual circumstances and there are two many variables to provide a meaningful example without more info.  I did do one quick calculation for one particular scenario.  5kWh is in the right ballpark however, depending on your DHW usage, using a solar diverter to store the usable excess may be much more cost effective!  If you have (or plan to get) an electric car and do miles on a regular basis, as opposed to irregularly, thats a very large battery sitting outside which you can also use.

 

I realise none of this gives 'an answer' and thats because its very dependent on your situation.  The basic message though is that, if you just put in 5kWh you probably wont go too far wrong, however there may be more cost effective measures you can take.  Please bear in mind that the environmental credentials of battery storage (as opposed to simply exporting to the grid) are at best questionable, so whilst PV and ASHPs can be justified on environmental grounds in addition to any cost justification, battery storage cant easily be justified on environmental grounds so needs to 'pay its way', at least in my book.  For me I can't make the numbers stack up, I have an electric car and solar diverter so already can store energy.  It's possible that, once I get planning consent for an ashp (which is proving challenging), time shifting import to take advantage of ToU tariffs will tip the balance.

Edited May 28 by JamesPa

[sharpener]

40 minutes ago, JamesPa said:

You need to decide which of these you are doing because it affects the choice of equipment and the battery size.

 

Not that they are mutually exclusive!

 

With a new solar installation, a diverter for the hot water is much cheaper, simpler and more cost-effective so do that first.

 

It's probably not economic to install enough battery to time-shift all your heating load in winter esp with non-HP electric heating.

 

However if you do have an HP you can get Octopus Cosy which has two cheap periods a day (I don't know if they can tell whether you actually have one or not). so you can cycle the battery twice a day and 13kWh would then be enough using @JamesPa's calcs. (I have 10.5 but am also planning an HP plus a thermal store charged twice a day as well).

 

Others have reported though that their Agile or Intelligent Go tariffs can work out even cheaper.

 

[The first fraction should I think be 18/24 not 6/24, but that is what you have actually used anyway?]

 

 

 

 

[JamesPa]

37 minutes ago, sharpener said:

[The first fraction should I think be 18/24 not 6/24, but that is what you have actually used anyway?]

Yes and yes

 

38 minutes ago, sharpener said:

Not that they are mutually exclusive

Agreed

[Hammertime]

Thank you all so much for your great advice.
@Nick Thomas - I hadn't realised I'd need to combine the capacities of the main inverter and the inverter on the AC coupled batteries for G98/99 approval. This is also relevant to my question of whether to oversize the inverter; just for preventing those times when the inverter becomes a bottleneck (eg all appliances on, or a future large battery charging at limited off-peak times), it would create a far more complicated application process.
I'll ask the quoting company about EPS with either DC and AC coupled batteries. As you say, maybe marginal benefit, but in the countryside where power cuts do happen, it would be useful to both power a battery from the solar array and power the property from the battery.
I will take on board your SolaX experience and ask for a different brand. Are there any you recommend? GivEnergy seem decent (and the company has the Octopus tie-in), and I've read Fronius are too.
@sharpener - yes, it would be electric ufh. I don't have gas (100% electric), and space is probably too tight for an ashp cylinder. I've been reading the other forum threads with interest.
@JamesPa- After reading your example usage breakdown I think, for my size of solar array, I need to shift my thinking. I'd been viewing battery storage as a mixture between off peak grid storage and topping up with my own (potentially meagre) trickle of solar, and trying to cover a large percentage of my energy usage like that. But actually maybe I need to view any solar production as an added bonus for now and think more widely about timeshifting energy purchase. Battery storage is only one option, and maybe not the go-to option either. I like the idea of storing excess solar in DHW, but I'm already considering removing the immersion cylinder to save space, and replace it with an instantaneous water heater, because space is at a premium. I'm also probably not getting an ashp for the same reason. Instead, I wonder if I could view the electric ufh not as an expensive luxury, but as a thermal store. It's not on-demand heating, so if done correctly, I wonder if it would be possible to heat it from the grid only at the lowest off-peak rates (directly, rather than via a battery), with the aim of maintaining a low comfortable temperature, and then top up the room temperature when needed with a log burner or a boost of more instantaneous heat such as IR panels. This would then time-shift a large percentage of my heating, without having to buy a battery. And if left at a low-ish temperature, I could leave it to sort itself out year-round. 
For all other electricity usage, I can then use my own production to give a little helping hand, with any excess sold to the grid; and in the future if desired, I could get a battery to store it instead, with the 5-7kwh mentioned by @sharpener and @JamesPa being an achievable size given that heating would be covered with the off-peak ufh and only requiring a top-up.
I don't know if all of that's technically possible or if I'm being wishful; I will also need to speak to underfloor heating companies to discuss minimum insulation and concrete thickness etc. Also, I'd have to ensure it was possible to tie all this into a Hive-style app. Each item I read about comes with its own app, but it wouldn't be great having five different apps on your phone independently running different pieces of the puzzle.
One other thought lurking at the back of my mind is that all the timeshifting of energy usage is entirely dependent on the electricity companies offering decent off peak rates, and it's in their power to take it away if it suits them as more people get electric cars or timeshift their energy purchase.
 

[Nick Thomas]

2 minutes ago, Hammertime said:

I will take on board your SolaX experience and ask for a different brand. Are there any you recommend?

 

I've got an LXP inverter now: https://luxpowertek.com/hybrid-inverter-lxp3-6k. I'm quite happy with how I've managed to set it up, and it wasn't *awful* out of the box, but I don't have any basis for comparison with, say, GivEnergy. In practice you might find your options are limited by the installers available to you.

[JamesPa]

You are mixing a couple of things.  You talk of an 'ashp cylinder' by which I assume you mean dhw cylinder.  Yes an ashp can heat a dhw cylinder and that is the normal arrangement.  But whatever alternative you were planning to use (what?) for dhw with electric ufh will also work with an ashp as a separate system, just as it would be with ufh.  So in your mind think about the two separately.

 

If you are going to batch heat to store energy, the only real control you have of your house temperature is how much energy you store.  Whatever you store will leak out into the house, just like a storage heaters.  So Hives etc may be pointless, multiple ones definitely so, and there won't be much instant control the aim is to get it right with little real time feedback.  What you ideally need is something that looks ahead at the forecast and tops up accordingly.  Others will know if this is available.  Failing that a weather compensated control system with a long time constant.  Several on the ashp forum do batch heating so it might be worth posting a batch heating question there as well as on the ufh forum.

 

Electric ufh will be three times as expensive to run as an ashp.  If your house is passive or nearly so that's fine, otherwise be prepared for big bills.

 

Just a few pointers, obviously you are early on in the process so time to think.

Edited May 28 by JamesPa

[sharpener]

32 minutes ago, Hammertime said:

Instead, I wonder if I could view the electric ufh not as an expensive luxury, but as a thermal store.

 

32 minutes ago, Hammertime said:

I don't know if all of that's technically possible or if I'm being wishful; I will also need to speak to underfloor heating companies to discuss minimum insulation and concrete thickness etc.

 

If the house exists already then retrofitting a sufficient thickness of concrete truly sounds like a labour of Hercules.

 

1 minute ago, JamesPa said:

What you ideally need is something that looks ahead at the forecast and tops up accordingly.

 

Homely claim to do this, will integrate with some but not all HPs. Various types of home automation exist also but would not be something that would work straight out of the box. Victron inverters have Dynamic ESS s/w which will adjust battery charging according to the weather forecast. But not control electric UFH, you would need sthg else for that so you are in the realms of Node Red scripting to join it all up.

 

It's all beginning to sound like overkill for a "small one bedroom house".

 

1 hour ago, Hammertime said:

I like the idea of storing excess solar in DHW, but I'm already considering removing the immersion cylinder to save space, and replace it with an instantaneous water heater, because space is at a premium.

 

That's definitely a retrograde step as it will store maybe 10kWh, will easily integrate with the solar PV and is a sunk cost. Can you move it into the loft space?

 

 

51 minutes ago, Hammertime said:

As you say, maybe marginal benefit, but in the countryside where power cuts do happen, it would be useful to both power a battery from the solar array and power the property from the battery.

 

Make sure what is being offered here. Many hybrid inverters come with an (extra cost) EPS output but it is only good for 2 x 13A emergency sockets not the whole house.

 

If you get an AC-coupled PV inverter it may or may not continue to generate in a blackout depending on how it is wired, as it needs to see another source of AC power to synchronise to, either the grid or a sine-wave inverter.