Converting grid tied to DC

[diWhy]

My question is, what would it take to convert my system from grid tied to DC?
This post is me thinking out loud but asking for input to know if I am on the right track or missing some things.
In another question, I'm taking on the task of testing all of my panels and inverters (all listed in my signature).

The task is going to be a difficult one because of the way this system is mounted so I'm trying to figure out what my options are so I can limit the amount of times I'll have to get to each panel/inverter.

I use around 2KW 24/7 for a computer development environment that is connected to a UPS. My thought is that I might actually get more return from the system by using it to charge batteries and to run the dev env rather than getting the crummy retail rate I get from the power company.
I also like the idea of being able to actually use the system during grid outages.

Therefore, I could convert half of the system for example to DC, leaving the rest connected as it is.

No matter what I do, I still need to try and plan this. Let's say I went 100% batteries, no more grid tied at all.

Each panel is marketed as an AC one because there is an inverter mounted onto them. If I take the inverters out of the mix, then I now have panels that output DC.

I have 42 panels which are connected together in this way, 2 x 10 panel strings and 2 x 11 panel strings.
These lead to four breakers just before the power company's second meter for my solar.

What I'm thinking is that I would rewire the system so that each sting is a series that outputs a total of X.
At the junction box, I would have four DC outputs once re-wired.
At this point, I suppose it depends on a number of things.
I could get four smaller inverters that tie together to give one final output.
I could get two smaller inverters that tie together to give one final output.
I could get one inverter that has multiple inputs

I've seen there are all kinds of options for inverters such as some that have everything built in, charge controller, inverter etc.
Or I might get separate devices.

The main thing I'm trying to better understand is dealing with the DC outputs to keep things safe.

If you were in my position, what would you be looking at?

 

[Samsonite801]

If it were me, I would just build a separate system alongside, like an all-in-one inverter (with parallel stackable capability), and add batteries, tie to your critical loads panel (or main panel), and use your grid AC input on it for charging batteries. And then start transitioning DC panels over to its PV inputs as you go (not sure if your solar panels have integrated micro-inverters or if you can remove them and use the panels for DC, or if you'd need new panels).

If your plan is to decomm the old equipment eventually, then do you still want the ability for the new system to backfeed to grid? If so you would need an all-in-one that is UL listed and supports grid-tie, like a Sol-Ark or something similar...

 

[Ampster]

If you were in my position.......

If I were in your situation I would consider AC coupling .
Depending on how new the micro inverters are, the most cost effective solution might be to use a hybrid inverter with batteries to AC couple with your existing GT system..Do you have a NEM agreement?

 

[diWhy]

Hi, thanks for the input.

I would actually use a separate panel instead of connecting to the main one.

>And then start transitioning DC panels over to its PV inputs as you go (not sure if your solar panels have integrated micro-inverters
>or if you can remove them and use the panels for DC, or if you'd need new panels).

Ah, so basically, just add some new hardware and slowly re-wire panels over to that.
Yes, these are what I have. The panels are called AC but they simply have the inverter mounted onto the frames.
Disconnecting those gives me DC from the panel.

>If your plan is to decomm the old equipment eventually, then do you still want the ability for the new system to backfeed to
>grid? If so you would need an all-in-one that is UL listed and supports grid-tie, like a Sol-Ark or something similar...

I'm thinking of taking advantage of both really. At this point, I don't need 10KW so I would probably keep 5KW going to the grid and re-route the other half to my own panel.

>If I were in your situation I would consider AC coupling. Depending on how new the micro inverters are, the most cost effective
>solution might be to use a hybrid inverter with batteries to AC couple with your existing GT system..

If the research I've done had me learning anything, I think AC coupled means nothing changes other than the AC output of the system goes into a hybrid inverter, then on to the grid. The hybrid allows a clean transfer, disconnecting the grid during outages so it can use my batteries.
I hope that's what it means otherwise, I'll have to watch a lot of videos again .

>Do you have a NEM agreement?

We do. We get some crummy retail rate back for our generated power.

The main thing I'm not clear on is how the panels would get wired if I remove the inverters.
Each panel has an H4 Helios connector going to the inverter. Can I assume I could get new cabling that just connects all of the panels in series for a final output into the new inverter?

I have four strings as mentioned, 2x10 and 2x11 so if I went with half the system, I would have 1 string of 10 panels and 1 of 11 panels.
I'm not sure if all of those would be connected in series or if I need to get either a single inverter with dual inputs or two inverters that can be tied together?

Just trying to understand the hardware side of all this.

 

[Samsonite801]

Hi, thanks for the input.

I would actually use a separate panel instead of connecting to the main one.

>And then start transitioning DC panels over to its PV inputs as you go (not sure if your solar panels have integrated micro-inverters
>or if you can remove them and use the panels for DC, or if you'd need new panels).

Ah, so basically, just add some new hardware and slowly re-wire panels over to that.
Yes, these are what I have. The panels are called AC but they simply have the inverter mounted onto the frames.
Disconnecting those gives me DC from the panel.

>If your plan is to decomm the old equipment eventually, then do you still want the ability for the new system to backfeed to
>grid? If so you would need an all-in-one that is UL listed and supports grid-tie, like a Sol-Ark or something similar...

I'm thinking of taking advantage of both really. At this point, I don't need 10KW so I would probably keep 5KW going to the grid and re-route the other half to my own panel.

>If I were in your situation I would consider AC coupling. Depending on how new the micro inverters are, the most cost effective
>solution might be to use a hybrid inverter with batteries to AC couple with your existing GT system..

If the research I've done had me learning anything, I think AC coupled means nothing changes other than the AC output of the system goes into a hybrid inverter, then on to the grid. The hybrid allows a clean transfer, disconnecting the grid during outages so it can use my batteries.
I hope that's what it means otherwise, I'll have to watch a lot of videos again .

>Do you have a NEM agreement?

We do. We get some crummy retail rate back for our generated power.

The main thing I'm not clear on is how the panels would get wired if I remove the inverters.
Each panel has an H4 Helios connector going to the inverter. Can I assume I could get new cabling that just connects all of the panels in series for a final output into the new inverter?

I have four strings as mentioned, 2x10 and 2x11 so if I went with half the system, I would have 1 string of 10 panels and 1 of 11 panels.
I'm not sure if all of those would be connected in series or if I need to get either a single inverter with dual inputs or two inverters that can be tied together?

Just trying to understand the hardware side of all this.

Yeah, this is where you need to start paying attention to that white sticker on the panel. See the Voc and the Isc?

Nominal Voltage, Voc, Vmp, Isc | Solar Panel Specifications

Solar Panel Specifications like Nominal Voltage, Voc, Vmp, Isc, and Imp are important to check before the installation of solar panels

www.electronicsforu.com

And then start adding up numbers using this calculator:

Solar panel maximum voltage calculator - How to work out the max voltage

To get the maximum solar panel voltage you should expect from your solar panel, use our solar panel maximum voltage calculator.

gold-coast-solar-power-solutions.com.au

You need to take this into account when selecting an inverter, in relation to the PV input max volts and amps, etc, so you can determine suitable wiring scheme of series/parallel (to get the highest PV volts without going over, and not going over maximum Isc amps the inputs can tolerate)... Basically, the best combo of volts/amps to get the max watts the input is rated for without going over limits.

For example, a Sol-Ark 12k has a max PV input voltage of 500v, so you can wire more of them in series than I could with my LV6548, which only has a 250v max PV input voltage. Basically, on my LV6548, I can do 4s2p which is 8 of my 400w panels based on my Voc and Isc numbers...

 

[sunshine]

The main thing I'm not clear on is how the panels would get wired if I remove the inverters.

A wrong decision was originally made to have these inverters. Removing them now probably only compounds that first mistake.
You also need to analyse the costs a bit better in regards to actual battery costs at your 2kwhr requirement.
Maybe you would be better served by looking around for a better retail rate that suits your needs.
The current energy pessimism will only be temporary unlike the graves Putin has dug!

You are certainly right that DC is the way to go. Unfortunate that solar hadn't been invented in time to back Thomas Edison!

 

[diWhy]

The installer left me hanging with the system all messed up so I've decided to take it on myself.

Can you expand on how it was the wrong decision? Do you mean by the installer or myself buying a grid tied system? At the time, I didn't know the difference and the seller never bothered to tell me otherwise, I'd have gone for a hybrid or something with batteries for sure, not solely grid tied.

I have no other options for what I get for the power, it's the only power company around. Do you mean there are other ways of selling the power? Like a third party or something that acts as a third party? I mean, it's connected to the only power company in the area.

 

[diWhy]

Can anyone continue with this thread so I don't have to post a new question.
I'm interested in the AC coupled method if all it means is putting something in series with the whole output.
On the other hand, I'd really like to have some of those panels supplying DC but that would mean having to re-wire.
Re-wiring isn't too much of an issue if I can find the right hardware.

Since the panels are currently connected in series, I assume I would have to connect each panel individually to some sort of combiner. From there, combine all of the four strings together or use an inverter that has multiple inputs to connect the strings to.

 

[Ampster]

Can anyone continue with this thread so I don't have to post a new question.
I'm interested in the AC coupled method if all it means is putting something in series with the whole output.

It is your thread so I am happy to answer your AC coupling questions. Answer my earlier question about whether you have a Net Energy Metering agreement with your utility?
What is the capacity of your existing system in kWs?
How much battery capacity do you want to add in kWhs?
Knowing these answers I may be able to help with your goal of having some backup and being able to use your existing system without having to rewire everything.

 

[diWhy]

>Do you have a NEM agreement?

We do. We get some crummy retail rate back for our generated power.

I thought I did answer that and included lots of information about my setup.
As for batteries, I don't know yet, it depends on what my options would be.
Since it's mainly for emergencies, then I don't need to power the entire house, only critical things like freezer, refrigerator, lights, pretty basic things.
Right now, we have more than enough power using a 7KW generator when the power goes down for extended periods but I'd like to have more than just a geny.

Here is another breakdown which I hope helps.

I currently have a grid tied 10.5KW system using 42 panels.
It is broken down into 4 strings (2x 10 panels and 2x 11 panels).

The panels are Mage / Powetec Plus, 250 / 6 PL US AC with built in (mounted on each panel) AC inverter.
The panels put out between 18V and 37V.

The inverters put out 240V AC.
AC Max Cont output current, 0.992A
AC Max Cont output power, 238W
AMPHENOL Helios H4

Since the panels come with the inverters pre mounted, they don't give any info on DC output.

However, I am able to find specs on the inverters so that gives me all I need.

The inverters are Pantheon II.
INPUT (DC) DATA P250LV-208/240
Recommended STC Module Input Power Rating 235 W – 280 W
Maximum Continuous Input Power 250 W
Maximum Input Voltage 48V
MPPT Voltage Range 18V-37V
Maximum Power Voltage Range 25V-37V
Maximum DC Short Circuit Current 15A
Maximum Input Current 10A

Based on the inverter requirements, it's how I know the panels are putting out between 18V-37V DC.
I tested some and those were in the 32V range when I did.

Now, my unknowns are as follow.

Am I correct to assume that I disconnect the inverters from the panels then connect each of those panels to a DC/DC combiner.
Right now, panels are connected in series and since I have shade issues, I'll need to use parallel.

I assume each panel will get connected to a combiner to get one final output of 48VDC.
I assume I would keep strings to 10-11 panels,

I'm thinking of using a 6KW-8KW inverter but that might be a little overkill? Is it?
I'd like to convert one string initially but I might want to add another later.

This means I'll try to find either an inverter that will allow me to add more panels or one that allows multiple inverters to be connected together.
I've not seen anything that would allow me to connect 20+ panels. Maybe a panel combiner of some sort?

I might be using the term 'string' incorrectly in terms of my new setup.
In some cases, a string is panels connected in series and in combiners, they seem to imply that each panel is a string.

The DC section/s will not be connected to the grid but to one or more 48VDC inverter/s that will either charge my battery bank directly or provide a clean 120VAC or 240VAC output that I can use to charge existing UPS's I have for example.

Again, the panels have standard AMPHENOL Helios H4 connectors.
There is tons of used solar hardware on eBay and on Craigslist.

Wondering if I am overlooking something? And what I would need to start with one string so 10-11 panels converted to DC.

 

[Ampster]

I thought I did answer that and included lots of information about my setup.

Yes you did.

As for batteries, I don't know yet, it depends on what my options would be.
Since it's mainly for emergencies, then I don't need to power the entire house, only critical things like freezer, refrigerator, lights, pretty basic things.
Right now, we have more than enough power using a 7KW generator when the power goes down for extended periods but I'd like to have more than just a geny.

Then you need to do some calculations about how much you want or how long you want to go before using the generator. Your options are flexible and could be phased in over time.

Now, my unknowns are as follow.

Am I correct to assume that I disconnect the inverters from the panels then connect each of those panels to a DC/DC combiner.
Right now, panels are connected in series and since I have shade issues, I'll need to use parallel.

No you could use the majority of those panels with their micro inverters to put out AC as they do now. If you took ten or eleven of those micros off and made a DC string to power a hybrid inverter you would have some spare micros for the future. When the grid goes down the hybrid would be the grid forming device that would keep the micros functioning to power the AC loads and charge the batteries through AC coupling. The DC panels would also cover some of the loads and assist in charging the batteries. An all in one hybrid would save you from buying a separate charge controller and give the flexibility mentioned above. During and outage when the sun is shining you would have a lot of power and when the sun goes down the batteries could carry your loads and the generator could fill in. One of the features you may want is a generator input with auto start so you would not have to wake up in the middle of the night and start the generator.

I assume each panel will get connected to a combiner to get one final output of 48VDC.
I assume I would keep strings to 10-11 panels,

I think those details will be determined by the hybrid and its ability for multiple strings. Many hybrids operate more efficiently with higher voltage strings much greater than 48 volts DC. Some as high as 600 volts.

I'm thinking of using a 6KW-8KW inverter but that might be a little overkill? Is it?

It all depends on how much power you need overnight. The beauty of AC coupling is that you can use the power of the AC micros and get by with a smaller inverter running off batteries over night.

I'd like to convert one string initially but I might want to add another later.

This means I'll try to find either an inverter that will allow me to add more panels or one that allows multiple inverters to be connected together.
I've not seen anything that would allow me to connect 20+ panels. Maybe a panel combiner of some sort?

Look at the DC specs of some hybrid to answer that question. My Outback Skybox can operate in a DC range of 200-600 volts which gives a lot of flexibility in stringing panels. I currently use 11 use Sunpower 305 Watt panels.

The DC section/s will not be connected to the grid but to one or more 48VDC inverter/s that will either charge my battery bank directly or provide a clean 120VAC or 240VAC output that I can use to charge existing UPS's I have for example.

The optimal system would be connected to the grid but one of the features you may want is the ability to use the grid as last resort because of your unfavorable NEM agreement. I would leave that open. I use my Skybox in Self Consumption mode AC coupled to 8kW of Enphase micros. I really only use the grid to charge my EVs and in winter when my solar production is low and my energy consumption is higher because my heating is via heat pumps.

Again, the panels have standard AMPHENOL Helios H4 connectors.
There is tons of used solar hardware on eBay and on Craigslist.

Wondering if I am overlooking something? And what I would need to start with one string so 10-11 panels converted to DC.

I would assume you can find connectors to make all this work. I bought some used panels once and upgraded all the connectors to be consistent with the latest connectors on my micros.
The only thing you are overlooking is to have a good estimate of your overnight loads in kWhs which will be useful in sizing a hybrid inverter and how many kWhs in batteries you want to purchase. I assume your solar production from 10 kW of AC and DC coupled solar panels are sufficient to cover daytime loads and charging battery consumption of overnight loads. Then the only issue is how much reserve you want for cloudy days. You already have a generator which makes the battery size less critical depending on how often you want to run your generator.

 

[diWhy]

Just wanted to acknowledge your great reply and will need a little time to absorb this .

The reason I don't have a set plan for what I'd like in terms of battery reserve is because I figure at least a 1000 watts overnight would be useful. A couple thousand watts overnight until the sun came back would be more than enough to run essential things in a real emergency.
A real emergency to me also means NG would stop flowing at some point too and I'd be relying 100% on solar.

I did do some calculations but wasn't able to find my notes so will look for those so I can reply properly.

 

[Ampster]

In the he winter we have less than ten hours of useable sunshine so 100O Watts for 14: to 16:hours is 14-16 kiloWatt hours if that helps

 

[diWhy]

I've not found my notes but I think I was somewhere in the 20KW to 30KW area at the very top end IF I wanted to run as much stuff as possible.
However, that was my thinking about running a 2kw/3kw hardware rack on batteries overnight. My thought was that I'm making such a little return on what I get from the power company that I am better off to simply use that power any time I can.

My thoughts are a bit different now because of drought and all that's happening in the world. I feel like it's possible that essential services could be disrupted so I'd like to harness what I have and since it's a grid tied system, I can't right now.

Getting even 1KW of usable power overnight/extended would be very usable if all went to hell.
Getting 2KW or 3KW of overnight would be more than we need but might allow us to help neighbors that want stuff recharged etc.

My hope/plan right now is to convert some of the system in a cost effective way that allows me to get those 10-11 panels usable and be able to add more as time goes by. I'm somewhat budget restricted thanks to some other projects which is why I'd like to do this in a modular/expandable way.

 

[Ampster]

Okay, to prevent misunderstanding use the correct units. KWhs is the measurment of energy over time, ie one kiloWatthour (kWh) is the equivalent of one kiloWatt (kW) over one hour. You don't want someone to assume you need a 20 to 30 kW inverter, because that would be very expensive. The easy way to remember is kWhs are how our utilities bill us for energy, and Watts or kWs is the capacity of solar panels and inverters.

 

[diWhy]

Sorry, I thought my mention of 6KW inverter/s would define that.

>In the he winter we have less than ten hours of useable sunshine so 100O Watts for 14: to 16:hours is 14-16 kiloWatt hours

Perfect. So, the minimum I am after is 14-16KWhs. That said, I'd like to be able to add as I can afford.

 

[Ampster]

My hope/plan right now is to convert some of the system in a cost effective way that allows me to get those 10-11 panels usable and be able to add more as time goes by. I'm somewhat budget restricted thanks to some other projects which is why I'd like to do this in a modular/expandable way.

The advantage of AC coupling is you can leave your existing system intact or mostly intact and leverage what a hybrid inverter can do with batteries. That kind of integration can be cost effective rather than having two isolated systems. Also, never discount the advantage of having the grid as backup for cloudy days. Spend the money on hybrid features that will be useful in the future. One of the features that I like best about my hybrid is that it is grid interactive. In my case I am on time of use rates so I can use the hybrid for peak shaving, to avoid having to buy high priced energy.
Many of the mid priced hybrids have a limit of around 7kW of AC coupled grid tied that they can effectively couple with, and that is why I suggested removing the micros from one string and putting those panels in series to power the hybrid . Many hybrids can run with batteries alone but they are more efficient with a small string of solar panels. If you buy the new crop of 48 volt rack mount batteries you can keep adding them as your needs grow or become more clear.

 

[Ampster]

That said, I'd like to be able to add as I can afford.

I did a DIY battery pack that started out to be 14 kWh, then expanded it to 28 kWh and finally 42 kWh. It was 48 volts and that was just the increments 16 each of the 280 Ahr cells I used. There are also a number of Rack mount 48 volt packs that are smaller but contain BMS and are simply plug and play. I am not sure what the maximum number of those that can be combined. With and inverter of 6kW I would definitely consider a 48 volt system which I think you have already concluded.

 

[diWhy]

So here's where I think I am. Sorry it takes so long to get back to this. I'm not getting any email notifications and it's winter so solar is a while away.
First, what I know;

The panels are Mage / Powetec Plus, 250 / 6 PL US AC. The specs are based on the microinverter pre-installed on them so obviously, the specs are in fact the AC output.

AC output, 240V, AC Max Cont output current, 0.992A, AC Max Cont output power, 238W.

The inverters are Pantheon II. Not really sure how to figure out how much current the panels are actually putting out.
INPUT (DC) DATA P250LV-208/240
Recommended STC Module Input Power Rating 235 W – 280 W
Maximum Continuous Input Power 250 W
Maximum Input Voltage 48V
MPPT Voltage Range 18V-37V
Maximum Power Voltage Range 25V-37V
Maximum DC Short Circuit Current 15A (short circuit current?)
Maximum Input Current 10A

If I go by the amount of current the inverters output, then I'd go with the 0.992A as some sort of guesstimate.

If that's the case, then a rough idea of how I could break things down if I used all 42 panels follows.
I'm using all 42 panels just for curiosity. I'll probably leave 10-20 to the grid as I think I'll have more than enough with half of this being converted to DC. I have shading issues so would not want too large of strings.

2 panel strings:

• 21 strings in total
• Each string would have a maximum continuous output power of 238W * 2 = 476W
• Total output voltage would be 18V-37V DC (same as for one panel)
• Total output current would be approximately 0.992A * 2 = 1.984A

3 panel strings:

• 14 strings in total
• Each string would have a maximum continuous output power of 238W * 3 = 714W
• Total output voltage would be 18V-37V DC (same as for one panel)
• Total output current would be approximately 0.992A * 3 = 2.976A

4 panel strings:

• 11 strings in total
• Each string would have a maximum continuous output power of 238W * 4 = 952W
• Total output voltage would be 18V-37V DC (same as for one panel)
• Total output current would be approximately 0.992A * 4 = 3.968A

5 panel strings:

• 8 strings in total
• Each string would have a maximum continuous output power of 238W * 5 = 1190W
• Total output voltage would be 18V-37V DC (same as for one panel)
• Total output current would be approximately 0.992A * 5 = 4.96A

6 panel strings:

• 7 strings in total
• Each string would have a maximum continuous output power of 238W * 6 = 1428W
• Total output voltage would be 18V-37V DC (same as for one panel)
• Total output current would be approximately 0.992A * 6 = 5.952A

Am I thinking about this correctly now?

 

[TorC]

When you get the chance, it would be helpful to see if you can find a panel model number to look up exact specs or see if you can stick a phone or something under and grab a decipherable pic of the panel spec label to be sure.

That said, a series string DC keeps amps the same. Lacking panel specs, I'll use round possible numbers of 30V Mpp, 8A Mpp:

2 panel series: 60V, 8A
4 panel series: 120V, 8A

2 panel series, 2 strings parallel (2S2P): 60V, 16A
4 panel series, 2 strings parallel (4S2P): 120V, 16A

More than two strings parallel requires some additional fusing or breakers, but can be done if appropriate for your SCC input limits. I'm setting up mine with breakers for PV disconnect even though the primary setup has only two strings on an SCC, in part so I could rearrange if an SCC failed and put three strings on a couple of them.