Inverters

uk1

ABrass said:

10kW of panels on a 5kW inverter? No, that's just silly.

You should be doing G99 and getting a larger Inverter.

Thanks.

Forgive me.  It’s been a long day.  It is of course in fact around 5.7kwh array as it currently stands. And the inverter being proposed is a 3.68 one.  I’ve corrected my post. 

Sorry.

Martyn1981

uk1 said:

Dolor said:

uk1 said:

Thanks for that …. I’m learning all the time.  I hadn’t realised that a 5k could be restricted in that way and still allow the rest to be used or stored. 

I may have introduced an element of confusion. If you have an AC battery (that is, a battery on the house side of the inverter) it is treated as a house load as is an EV. If your inverter is limited to an output of 3.68kW then that is its maximum output from your solar irrespective of the size of your array. If you had a 5.7kwh array on a bright sunny day you would just see a long straight output line at 3.68kW.

If the inverter is outputting 3.68kW and your house baseload is, say, 300W then 3.38kW is available to charge your battery. If your battery is at 100% and your baseload is 300W, then 3.38kW will be exported to the Grid. If you increase the home requirement to 5 kW by putting on the washing machine and kettle, then 3.68kW will come from your solar and 1.32kW will come from the Grid.

As I understand it, the situation is different for a DC battery on the array side of the inverter. The DNO is not interested in DC batteries as the total output will be restricted by the 3.68kW solar inverter. That is, the 3.68kW output could be a mix of solar and DC battery.

Thanks for clarifying, but I fear that I'm not as clever as I look because I thought just as I was begining to understand but I am now it seems going backwards.

My understanding was that I'm moving towards a system that looks like it is going to be around a 5.7kwh  array of PV panels on my roof feeding down into an inverter in my garage that will (a) convert the output of the arrrays irrespective of how many I buy because the total downfeed kwh will be reduced in the inverter to a maximum of 3.6kwh irrespective of what it then does with it.

It will then as it's first priority send it to (b) the house to use and then if any is left over as things go on and off from the house it will direct anything below 3.68 that is left over from the house to (c)  the set of batteries that are DC and store it at around 48v DC (which is then converted back to AC when there's nothing coming down from the array) and if any power is left after when the batteries are full and the house is using all it needs anything left  over will be sent up the grid and hopefully give me a few pence to buy some wine gums.   When there is nothing coming down from the array or the batteries I will be using the grid.  Have I misunderstood?

I'm told that in this scenario if I have a 3.68kwh inverter then it throttles back ALL of the power sent down from the PV array but only needs a G98 approval and the work can be installed and commissioned and everyone is happy.  Except me it seems who wants every ounce out of the array and is even thinking of more output. 

I am told however that if I get a 5kwh inverter that has the option to restrict the output to the grid to 3.68 then the DNO's say "No way Jose" you need a G99 or even a G100 (I know not which) and therefore I am being led to believe that if I am sensible I should accept a Solis 3.68 because the number of days in which my 5.7kwh array will exceed 3.68 although counter-intuitive is actually quite small but the larger overly rich array will be better for me because in winter when there is less of the bright stuff it will provide more to keep the lights on.  So it's a bit wasteful in summer and makes you smug in winter.  But I'd like more of the summer stuff to be fed to my batteries.

I have probably totally misunderstoof so has my grasp been a misgrasp?

Thanks again for your patience and sorry if I seem dim.  No pun intended. I have only been going to school for a week or so.

Hiya, I think Dolor's explanation is excellent.

Perhaps there is some confusion when we talk about DC or AC batteries. What we mean is which side of the inverter they are on, though the batteries themselves will all be DC.

So if the batts are on the AC side, effectively just a bolt in attachment to the house, that detects export, and then charges the batts (by converting AC to DC)  with that excess.

But for DC side batts, we mean they are before the inverter ..... inverts(?) (showing my ignorance now). So the inverter takes the DC from the panels, and converts as much as is needed to provide for household demand, but can direct the rest of the DC to the battery*, without DC to AC to DC losses. That means that depending on the rate at which the battery can charge*, you might be able to produce 5.7kW from the panels, with 3.68kW going to household demand and export, but ~2kW charging the batteries directly - PV DC being redirected by the inverter into the batteries .... so to speak!

*Obviously the battery itself will have a charge and discharge limit depending on its size, and as Dolor explained, the DNO won't be concerned as the inverter rating of 3.68kW would still be the limiting factor for DC side batteries.

Edit - Re-reading this, I may have made things worse! 

uk1

Thanks all,

In  digesting this all, it seems to me that my question distils into one simple decision on which I’d appreciate opinion.  

It seems to distil down to if I didn’t try to enrich the array and kept within the current 5.7kwh is it sensible for me to accept a 3.68kwh inverter?  If it does then I’m worrying about stuff I don’t need to and I can push on with it.  I guess the question is how much am I going to lose if there is opinion that it is a significant mismatch.  I’m right in thinking that inmy intended setup a 5kwh inverter requires the G89 approval?

Magnitio

uk1 said:

Thanks all,

In  digesting this all, it seems to me that my question distils into one simple decision on which I’d appreciate opinion.  

It seems to distil down to if I didn’t try to enrich the array and kept within the current 5.7kwh is it sensible for me to accept a 3.68kwh inverter?  If it does then I’m worrying about stuff I don’t need to and I can push on with it.  I guess the question is how much am I going to lose if there is opinion that it is a significant mismatch.  I’m right in thinking that inmy intended setup a 5kwh inverter requires the G89 approval?

There are two main advantages of the larger inverter: you won't get very much clipping of output i.e. where solar generated exceeds 5kW; you will be able to use up to 5kW in the house before drawing from the grid rather than being limited to 3.68kW. The benefits of the smaller inverter are: cost; ease of approval; slightly more efficient at lower power levels. If all of the panels are facing south, then I think you would find it frustrating for several months of the year that your output is being clipped. Even a 5kW inverter on 6.4kWp panels, there is some clipping e.g.

Although that was an exceptional day, you can gauge how much would have been lost with a 3.6kW inverter. You can use some of the excess to charge batteries, but they would very quickly be full.

The other issue of how often you use more than 3.6kW in the house is really a personal thing. Many people manage this by not putting on multiple loads at the same time. There's no way I could persuade my wife to consider this when cooking as she would just tell me to do it myself!

So, the easy solution is just to go with the 3.68kWh inverter, but I think you would regret it in the long term.

[Deleted User]

uk1 said:

Thanks all,

In  digesting this all, it seems to me that my question distils into one simple decision on which I’d appreciate opinion.  

It seems to distil down to if I didn’t try to enrich the array and kept within the current 5.7kwh is it sensible for me to accept a 3.68kwh inverter?  If it does then I’m worrying about stuff I don’t need to and I can push on with it.  I guess the question is how much am I going to lose if there is opinion that it is a significant mismatch.  I’m right in thinking that inmy intended setup a 5kwh inverter requires the G89 approval?

No. As I said at the beginning of this thread, get your installer to show you the output from the software that they have used to build a solar array and inverter system. This includes such things as array size; location; shading; roof slope etc. All my quotes included this level of detail. This will answer your question about potential losses.

If an installer is offering you a 3.86kW inverter it is possible that this is being done to keep things simple from his perspective: ie, throw it on the roof and notify the DNO afterwards. It is reported that some installers are only fixing their quotes for a month because of rising prices: they don’t want to wait weeks/months for DNO approval.

uk1

Thanks all …. very much appreciated.  

The quote showing the 3.68kwh inverter is from the contractor I am by far, most favouring for several reasons.  

A couple of further questions where I guess I’m second-guessing what his rationale might be apart from the G98 issue.  Might he believe that the slight efficiency gain over the whole year on a 3.68 inverter on a 5.7kwh array counteract or equal the loss on the best summer days.  Is that a viable opinion?

Secondly if I were insistent  on the 5k inverter is there much of an issue if I asked  for it to proceed on the basis of the 3.68kwh with the idea of an immediate g99 application and then when approved to replace the 3.68kwh with a 5k or larger inverter?  Perhaps even monitoring it for a year?   I know it’s a palaver but is that viable?

Thanks agtain.

Martyn1981

My thoughts are that you'd end up with a net loss, as the level of undersizing is too extreme.

Am I right in thinking this is a single orientation array, so all panels will peak at the same time? If spread over two orientations it wouldn't be so bad, my 5.58kWp total across E/W roofs, peaks at about 4.9kW briefly, but holds about 4.2kW in perfect summer conditions from around 11am to 2pm, so a cap of 3.68kW wouldn't bother me (too much), but for a single orientation it's too much, unless it's the only option.

I suspect the installer is looking to make the job simpler, but I may be being unfair. I'd also hope that the DNO might approve a higher limit, and something close to 5kW would be ideal.


Ooops, forgot the last part again. Regardless of permission, I think you'd need to go for a 5kW inverter anyway, which could be set to cap at 3.68kW. So you wouldn't need to change inverters later on if you applied for a higher limit.

Starting to lose myself now, but inverters will have a max DC input, so one large enough to cope with the array size will be needed ....... I think?

What was the specified inverter, as we can look at the data sheet to see what it can and can't cope with? I remember you mentioning Solaredge, and their HD Wave inverters can cope with some massive undersizing.

Edit - here's the datasheet for the Solaredge 3.68kW inverter (and others), and it maxes out at 5.7kWp of PV, but this doesn't change the fact that capping will be high.

uk1

Many thanks.  I’m still going around in circles a bit so hoping for clarity.  Yes it is a single south facing array. 

My understanding is that the larger 5k Solis inverter can be set using a ct clamp which is an export limiter but because it is a 5k inverter it needs G99.  Is that incorrect?

https://www.segen.co.uk/product/solis-5kw-s6-dual-mppt-single-phase-with-dc/

• Maximum string input current 14A
• Easy installation: Integrate with export power control function and DRM function.
• Max. Efficiency over 97.7%
• Wide input voltage range for better generation and low startup
• Dual MPPT design with precise MPPT algorithm
• Export Limitation built in – Requires CT clamp (sold separately)
• Integrated AFCI (DC arc-fault circuit protection)
• Warranty: 5 years (option to extend to 10 years.)
• Startup Voltage:  60V

Martyn1981

I think if the inverter is capped by the installer, then they can register the system as 3.68kW after install. But my confidence is waning slightly as I don't want to mislead you. Be interesting to hear what others say.

uk1

Many thanks again.  

Hopefully someone will confirm for me because that I believe would remove the issue.