Planning a Solar PV and Battery system

[Rob99]

So, I think I’ve got my head round panels although still need to decide on which ones and work out the ideal combination of size and output to max out the installed array. Looks like I might get between 8 and 9kWp on the roof which would be good.

I’ve now had an initial look at inverters. I have a pretty good grasp of most things, especially engineering or electrical related but at the moment I’m completely confused about the variety of inverters out there.
In simple terms a solar PV installation is a DC/AC split system with PV and batteries on the DC side and house consumption and Grid on the AC side. Analogous to East and West Berlin with the inverter being Checkpoint Charlie connecting the two sides.

Can I assume that the DNO and the rest of the outside world don’t give a hoot about the DC side and therefore you can have as much PV and battery storage as you like – fill yer boots!

On the other hand the DNO do want to know what you have on the AC side of the inverter as that is what you connect to the grid for export. Anything exporting up to 3.68kW gets automatic approval (G98 install and inform) and anything above needs some sort of additional application and approval (G99/G100) prior to install.

This seems nice and straightforward so far. However, when it comes to the inverter, how do you size it correctly? Presumably based on your PV array size but then it also has to be able to export what surplus you produce (or may discharge from your batteries).

So, lets say generation is 6kWp and your batteries are full, does this then all get output as AC power, some of which (say1kW) will be used and the remainder (5kW) exported. If your DNO approval (or restriction) is 3.68kW what happens to the “spare” 1.3kW?

Also, if your inverter has a max 3.68kW output due to DNO restriction, does it only output a total of 3.68kW to be split as usable power and export (if any left) regardless of the DC input.

This scenario doesn’t seem right to me so I think there must be a different explanation. For example, do inverters have separate outputs for export and for consumption? The export output being the one of concern to the DNO.

 

Unfortunately, lots of inverters don’t really make it very clear either.

I think I need another lie down………!!!
 

[SimC]

You could have an 8kW inverter with a 3.68 kW DNO export limit.

How any inverter achieves this is by having it's own metering device next to your utility meter (or along the route of the main live cable from the utility meter).

The inverter then has a live import/export power value that can be limited to whatever value is programmed in. The value is password protected so that only the installer and manufacturer can set or change it.

 

The inverter has power point trackers on the PV string inputs, that it can set for maximum/optimal output from the panels, or when the export limit is being reached the trackers adjust to non-optimal to reduce the electrical power output of the panels. The excess solar energy is then just dissipated as heat in the panels.

Solar panels are only around 20% efficient at generating electricity, so already dissipate 80% as heat meaning another few % due to the DNO export limit is not going to cause any damage.

 

Have you got a logical position for the inverter and batteries, bearing in mind there needs to be suitable cable routes for the DC feed from the panels, the ac power cable to a suitable consumer unit breaker, along with a data cable to the metering device?

Batteries are very heavy, so ideally on the ground floor. Depending on your building layout, a garage can be the best location, but only you know the layout.

[Marvin]

4 minutes ago, SimC said:

You could have an 8kW inverter with a 3.68 kW DNO export limit.

How any inverter achieves this is by having it's own metering device next to your utility meter (or along the route of the main live cable from the utility meter).

The inverter then has a live import/export power value that can be limited to whatever value is programmed in. The value is password protected so that only the installer and manufacturer can set or change it.

 

The inverter has power point trackers on the PV string inputs, that it can set for maximum/optimal output from the panels, or when the export limit is being reached the trackers adjust to non-optimal to reduce the electrical power output of the panels. The excess solar energy is then just dissipated as heat in the panels.

Solar panels are only around 20% efficient at generating electricity, so already dissipate 80% as heat meaning another few % due to the DNO export limit is not going to cause any damage.

 

Have you got a logical position for the inverter and batteries, bearing in mind there needs to be suitable cable routes for the DC feed from the panels, the ac power cable to a suitable consumer unit breaker, along with a data cable to the metering device?

Batteries are very heavy, so ideally on the ground floor. Depending on your building layout, a garage can be the best location, but only you know the layout.

Was informed by the DNO that they didn't care what I could limit the inverter to. If it was a 6kW inverter, that is what I had to have permission for!

 

Best check assumption with you DNO.

 

M.

[Marvin]

Our 6kW inverter can have almost 8kW of panels connected and the DNO don't care because the inverter will only allow a 6kW output.

 

I would check with the DNO. . 

 

M

[Marvin]

38 minutes ago, Rob99 said:

Can I assume that the DNO and the rest of the outside world don’t give a hoot about the DC side and therefore you can have as much PV and battery storage as you like – fill yer boots!

Only if the converted DC power goes through the same inverter as the panel DC.

 

If you have another inverter for the battery power which connects directly to the mains, then you need to add the maximum kWs output of that device and the PV inverter max kWs together and speak to the DNO.

 

M

[Marvin]

Hi @Rob99

 

You may wish to consider balancing your storage and your PV supply and your home demand.

 

We have a small amount of batteries for night use because we charge the car and heat the water and have ASHP.

 

So in the summer we have more than we can use when the sun is out, and not enough in the winter. We achieve about 4 to 1 summer to winter average daily output.

 

You could have a huge battery bank, but what are you going to use it on and remember that you loose about 18% converting from DC to AC.. 

 

During the summer nights we use about 2 to 3kWh and in the winter the PV is almost all used up on the ASHP, and car.. 

 

Good luck

 

M

 

 

 

[S2D2]

56 minutes ago, Rob99 said:

Also, if your inverter has a max 3.68kW output due to DNO restriction, does it only output a total of 3.68kW to be split as usable power and export (if any left) regardless of the DC input.

This scenario doesn’t seem right to me so I think there must be a different explanation. For example, do inverters have separate outputs for export and for consumption? The export output being the one of concern to the DNO.

I think this is the only question not answered above, I can only speak for Solax but I assume they're all the same (someone will be along to correct if not!). For G98 it's 3.68kW to the AC side, regardless of what your house steals before it gets to the meter. I was a bit disappointed by this as it doesn't matter to your DNO until it hits the meter but I assume they're not confident enough in the CT reading to pump more than 3.68kW into the AC and guarantee it's used down to 3.68kW before hitting the meter, which is understandable. Also cost cutting etc.

 

You can squeeze more out of the panels by leaving spare battery capacity, I've had to write a bit of control software to do this as it's not a standard feature. Then your max panel generation is 3.68 + max battery charge (2.5kW in my case). Note that max charge rate is both temperature and charge level dependent, so this gets complicated very quickly.

 

I believe there is now a fast track for up to 7.36kW export which didn't exist when I put my system in, I definitely would have gone this route if it did as managing the battery charge is just unnecessary complexity, even if automated.

[Dan F]

1 hour ago, Rob99 said:

the rest of the outside world don’t give a hoot about the DC side and therefore you can have as much PV and battery storage as you like – fill yer boots!

As long as you are not using an AC-coupled PV inverter, that is correct.

 

1 hour ago, Rob99 said:

Anything exporting up to 3.68kW gets automatic approval (G98 install and inform)

Only if the actual inverter rating (on the data plate) is <3.68kW can you use G98.  If the data plate is >3.68kW but you will limit output to 3.68kW then you need G99 fast-track SGI-1 instead.

 

1 hour ago, Rob99 said:

However, when it comes to the inverter, how do you size it correctly?

Based on the amount of PV you have and how quickly (kW) you want to be able to charge/discharge.   If the inverter is only 3kW, but you typically use 6kW cooking in the evening, then you would be importing half of your consumption from the grid (at the most expensive time if you are on an agile/flux tariff) even if you have a 100kWh battery that is full.

 

1 hour ago, Rob99 said:

So, lets say generation is 6kWp and your batteries are full, does this then all get output as AC power, some of which (say1kW) will be used and the remainder (5kW) exported. If your DNO approval (or restriction) is 3.68kW what happens to the “spare” 1.3kW?

If the inverter has this restriction in place, but PV is reducing more, then the MPPT(s) will throttle back.

 

1 hour ago, Rob99 said:

Also, if your inverter has a max 3.68kW output due to DNO restriction, does it only output a total of 3.68kW to be split as usable power and export (if any left) regardless of the DC input.

Might depend on the inverter.  With Victron at least, my understanding is that the limit applies to the excess after usage (the feed-in).  So if you are generating 6.68kW and using 3kW, then you'd not actually losing any potential generation.  This assumes your house is on AC-out-1 and not AC-in (so, the other side of the inverter).  If the house was hooked up to AC-in, then my guess would be that the max discharge from the battery (assuming not PV) would be 3.68kW including usage.

 

Assuming your inverter is >3.68kW,  I've seen people recommend starting with a G99 SGI-1 application (with the limit in place).  Then once you have that in the bag, apply for g99 SGI-3 fast-track (7.36kW limit), or a full G99 application (up to 50kW).

 

Edited April 26, 2023 by Dan F

[Rob99]

Thanks @SimC @Marvin @S2D2 & @Dan F

Some great comments, I'm away now until Friday evening but will try to read and digest everything by then 👍

[Dillsue]

Your inverter size will depend on the max output from your split array which you should have from crunching PVGIS data mentioned earlier. What's the max DC generation?

 

With a split 8/9kw array youll likely be looking at over 3.68kw AC output so a G99 application will be needed. If thats the case, then speak to your DNO to see what the max export theyll allow you. When you speak to them, don't quote the array size but use the calculated DC generation figure!!

 

When you choose your inverter check its on the ENA type test database. If your DNO wants export limitation make sure the inverter has a G100 certificate

[Dan F]

@Rob99 Another consideration (primarily in winter) is the max charger current of the inverter/charger.  This is separate from the inverter power and will define how much you can your battery during your off-peak.  Octopus Flux, for example, only has a 3hr cheap window (2-5 am).

 

For example, a SunSynk 3.6kW inverter has a 90A max charge rate, so approx 4.6kW, meaning in 3hrs you can fill 13.8kWh worth of batteries in 3 hours.  Some of the Solis hybrid inverters are closer to 60A though I think.

 

[Wil]

90A at 230V is 20.7kW isn’t it? Meaning in 3hrs you’d get 62.1kWh of charging at full rate?

 

[PhilT]

22 minutes ago, Wil said:

90A at 230V is 20.7kW isn’t it? Meaning in 3hrs you’d get 62.1kWh of charging at full rate?

 

Battery dc in the region of 50v

[Dan F]

1 hour ago, PhilT said:

Battery dc in the region of 50v

Yeah, depends if you use Pylontech (which only has 15 cells apparently) or something with 16 cells.  16 prismatic cells in series are 51.2v nominal.  In practice though, the charge voltage for LiFePO4 would be 55v on average, so it'd actually be slightly higher I think: 90A x 55v = 4.95kW. 

 

The Victron setup I'm planning has 3x70A, so about 33kWh in a 3-hour off-peak period, drawing 16A on each phase. I hadn't calculated this before, but this will work well with the 2x14.3kWh batteries on order.

[SteamyTea]

Charging of batteries, and thermal stores for that matter, is not linear.

So it may take 60 or 90A initially, but the power will decay exponentially as the batteries get closer to the maximum charge.  Thermal management may help a little, but that will use some of the available power.

[Dan F]

2 hours ago, SteamyTea said:

Charging of batteries, and thermal stores for that matter, is not linear.

So it may take 60 or 90A initially, but the power will decay exponentially as the batteries get closer to the maximum charge.  Thermal management may help a little, but that will use some of the available power.

 

I don't understand battery chemistry, but based on this graph charge current maintains stable up to 90% SoC, while voltage increases.  So, between 10-90% SoC power transfer actually increases slightly over time, doesn't it? Of course, once you get to 90% current drops off, but most people keep their batteries between 10 and 90%.

 

[SteamyTea]

6 hours ago, Dan F said:

but based on this graph charge current maintains stable up to 90% SoC, while voltage increases

That chart is not showing time though.

 

Edited April 28, 2023 by SteamyTea

[S2D2]

Here's one that does, a small Solax battery rated for 2.5kW charge, the reality is far more complicated:

 

[SteamyTea]

1 minute ago, S2D2 said:

the reality is far more complicated

Yes, reality is a killer.

[Nick Thomas]

Have some graphs of an LiFePO4 battery charging from 15-100%:

 

 

 

 

[SteamyTea]

15 minutes ago, Nick Thomas said:

Have some graphs of an LiFePO4 battery charging from 15-100%:

Isn't this just showing the effectiveness of the BMS and thermal management?

Probably find that the batteries could accept a much higher charge, but then degradation becomes a problem.

 

The main point I was trying (badly) to make is that you probably have to oversize the storage so that it can accept the capacity you need, safely, in the relatively short time you have to charge up.

[PhilT]

1 hour ago, SteamyTea said:

Isn't this just showing the effectiveness of the BMS and thermal management?

Probably find that the batteries could accept a much higher charge, but then degradation becomes a problem.

 

The main point I was trying (badly) to make is that you probably have to oversize the storage so that it can accept the capacity you need, safely, in the relatively short time you have to charge up.

Absolutely. The claimed "100% discharge" of the one I use, and its accompanying written guarantee, would be ludicrous if it didn't have massive hidden headroom built in

[Dan F]

1 hour ago, Nick Thomas said:

Have some graphs of an LiFePO4 battery charging from 15-100%:

 

This matches my understanding; constant current and marginally increasing power during charge (given an increase in voltage).  At least up to 90%.

 

[PhilT]

Large commercial storage batteries are very conservatively spec'd. For non critical applications such as radio control hobbies (airplanes etc.) the cell packs have "C" ratings as high as 120, which means they can safely discharge in 30 seconds!

[S2D2]

2 hours ago, Nick Thomas said:

Have some graphs of an LiFePO4 battery charging from 15-100%:

 

 

 

 

Interesting, the Solax is also LifePO4 but far more conservative with the current, which I just sort of assumed was normal. What capacity and temp is your battery for this charge?