A newbie is thinking about a small critical load with battery backup.

[StemSolar]

Hello

I am a newbie and thinking about adding battery to my existing solar system.
When I bought a house, it had thirty solar panels and a grid tie inverter. What a deal!
I live near urban area and hardly experience power outage but how knows. I realized a grid tie inverter didn't work when the grid was off. I started to watch videos to know how other people are doing.
My small plan is
1) Keeping existing solar configuration
2) Adding a hybrid inverter and a battery pack for small loads except dryer, wall ovens or air conditioners.
To achieve this I would buy a unit of MPP LVX6048 and one 48V 100Ah LiFePO4 battery pack.
I have total 7.2kw solar panels with five PV strings running down to the basement. Each string gives around 290V and 5A.
As PV voltage is medium to high side, LV6548 is not the option. Out of 5 PV strings, one will be used for my small backup or critical load. For the rest of PVs will still do the same job. Average generation per string is approximately 5kwh a day, which is enough for charging one battery pack of 5kwh. When the grid is down, the hybrid inverter will draw power from the battery to feed0 the critical load.
I am not sure if this is a feasible way. Any advice will be greatly appreciated.

 

[再生可能 エネルギー]

Which "Grid-Tie Inverter" do you have?

I would be surprised if there was not a setting allowing to charge a battery and to use this battery like an UPS.

May be I am missing something, I don't have too much solar experience either, but I don't see the need to use a secondary solar system?

It seems to me that you could you build a transfer switch to connect only your 'primary' load to your battery inverter, in case of power outage.
And otherwise, your 'primary' load would be connected to the main panel.

 

[timselectric]

I would be surprised if there was not a setting allowing to charge a battery and to use this battery like an UPS.

Most grid tied inverters, don't have battery connections.

I am not sure if this is a feasible way. Any advice will be greatly appreciated.

What you want to do is fine.

 

[StemSolar]

Which "Grid-Tie Inverter" do you have?

I would be surprised if there was not a setting allowing to charge a battery and to use this battery like an UPS.

May be I am missing something, I don't have too much solar experience either, but I don't see the need to use a secondary solar system?

It seems to me that you could you build a transfer switch to connect only your 'primary' load to your battery inverter, in case of power outage.
And otherwise, your 'primary' load would be connected to the main panel.

Thank you for your reply.
My existing grid-tie inverter is Fronius IG Plus 7.5.
It has worked fine but it only works when grid is on and doesn't have a battery charger.
I would keep this configuration but separate a small load from main panel with battery backup.
For a transfer switch, a hybrid inverter would to the switching automatically. When grid is down, it will draw power from the battery.
This is just my imagination.
If it were not very strange, I would try with small budget, like three to four grand.

 

[StemSolar]

Most grid tied inverters, don't have battery connections.


What you want to do is fine.

Thank you for encouraging me
Sooner or later I will try to build this.

 

[再生可能 エネルギー]

Thank you for your reply.
My existing grid-tie inverter is Fronius IG Plus 7.5.
It has worked fine but it only works when grid is on and doesn't have a battery charger.
I would keep this configuration but separate a small load from main panel with battery backup.
For a transfer switch, a hybrid inverter would to the switching automatically. When grid is down, it will draw power from the battery.
This is just my imagination.
If it were not very strange, I would try with small budget, like three to four grand.

I didn't realise that you could not upgrade your existing system.

I just wonder if it is really worthwile to have a Grid-Tied system?
How much do you get per kWh sent to the grid versus what you have to pay in the evening?

 

[TorC]

Looking at what you have, and given you've verified you have a grid tie only inverter, I would, as you plan, steal one string from your 5P6S to have a 4P6S for your grid tie. The string for your backup I would reconfigure as a 2P3S. Feed that to an SCC or AIO. I tend to favour the former because it's fewer parts in a box to fail and take your whole system out.

I agree with a hybrid inverter to feed any excess power back to the grid. If you're looking at the cheaper inverters be warned they often have a high idle consumption, which will eat into your power budget quite a bit. 50-60W is common. 50W*24h=1.2kWh/day+ just for the inverter. I have also seen advertised specs with a seller answered question about consumption that was about half what the manual in the box claimed. Observation says the manual was right. Inverters are a place to be very cautious about skimping on quality.

On choosing voltage, with what you've got I'd look seriously at a 48V inverter and battery because there will probably be a strong temptation to keep expanding your battery system over time and gain more independence from the grid. ETA: I see you were already planning 48V. Good choice, IMHO.

You're investing approximately nothing that will be lost if you later need more off-grid capable capacity, which is nice.

 

[StemSolar]

I didn't realise that you could not upgrade your existing system.

I just wonder if it is really worthwile to have a Grid-Tied system?
How much do you get per kWh sent to the grid versus what you have to pay in the evening?

For the upgrade, it means to replace the grid-tie inverter with a new hybrid one. I would but my current system has worked for a decade without any maintenance or problem, I just want to add small battery to fill the short gap from any grid outage,
According to my existing flat rate plan, I am paying the same rate regardless of peak-time. But you gave me a thought that I could change the plan to pay less. Eventually I will like to charge the battery by the solar and/or grid during off-peak, and sell the power during peak time.

 

[DATfool]

I had a grid tie system installed last year and now wish I had just invested $10k in batteries. I could charge batteries overnight at 7cents and save on the 26cent charge, summer rate "peak". SRP here in AZ pays me only 2cents for exported energy, so I can charge in the morning, off-peak, and the difference will pay for a mini-split AC (I hope, eventually) used in afternoon. I plan on to install my own solar panels and, thanks to Will Prowse's suggestions, see local San Tan Solar for used panels.

Money saved per month has not paid the financed charge for the grid tie system. I have only compared by previous year/month, but feel the screws being tighten by SRP... I plan for the grid tie to be back-up to my "DIY Solar Power"!

 

[timselectric]

Grid tied systems, just take people's money.

 

[DATfool]

Grid tied systems, just take people's money.

Agreed, but it does force me to monitor and reduce usage. I will slowly add my own system and maybe one day take SRP's money!

 

[timselectric]

I'm growing my system to have nothing to do with the power company. I'm halfway free, now.

 

[GXMnow]

Another option might be to AC couple your existing grid tie inverter to a battery inverter that supports this function. I have no idea what your budget is, but Outback has recently released their new Mojave AC coupled hybrid inverter system. So far, I have only found a few places selling it, and in each case, they include the 14.8 KWH battery with it. Knowing it comes with the battery, the $16,000 USD price tag is not that bad. Obviously, it is a lot more expensive that a DIY battery setup, but it is a complete engineered package.

There are certainly cheaper options available, depending on how much power and energy you wat available when in a grid fail condition.

I installed a Schneider XW-Pro hybrid inverter, AC coupled to my existing 4,800 watt Enphase microinverter system. Bu tI will admit up from, running it AC coupled only did present a few challenges. If you also get a Schneider MPPT and move over the single of panels, I think it would work very well for you. I was very close to adding a few DC coupled panels to get around the issues, but I finally got it all to work with a Programmable Logic Controller managing the settings in the Schneider inverter to control how much power goes in and out of the battery bank each day. If I was ONLY using it for backup power, this is not a concern. It will charge up and keep the battery charge, and inverter when there is a power failure. If you don't need to time shift sun time power to use in the evening, it works great for that. There are a few other Schneider XW users on this forum.

Another member on this forum is using the Outback Skybox. IF you search the forums, you should find several of their posts. It seems to be a decent middle ground for an AC coupled system, and it also includes a DC MPPT charge controller, if you do decide to move panels over to it.

These inverters that are actually designed to run AC coupled to grid tie inverters can force the grid tie inverter to keep making power when the grid is down. When the Schneider XW is in that mode, it works great. If the grid tie solar PV system is making less power than the loads need, the hybrid inverter uses battery energy to make up the difference. And if the solar is making more power than the loads need, the extra power flows backwards through the hybrid inverter and charges up the batteries. This is one reason the inverter has to be designed to handle AC coupling. Many inverters do not handle power flowing backwards. The second thing the hybrid inverter needs to deal with is when the batteries become full.

A solar charge controller can easily reduce the power it pulls from a solar panel. Most will just allow the voltage to climb up as they pull less current so it does not over charge the battery. The Grid Tie inverter in an AC coupled system does not even know there are batteries. They always just try to push as much power as possible back to the house and the grid. So AC coupled hybrid inverter have to "trick" the grid tie inverter to reduce it's output. Most new grid tie solar inverter are able to operate in different grid code modes. I an in California, so I use California Rule 21. One part of rule 21 is that the grid tie solar inverter must reduce it's output if the grid frequency rises above normal. The Hybrid inverter takes advantage of that by raising the inverter output frequency when it has too much solar power coming back in. If you have older inverters that don't support this feature, it can still work, just not as slick. The frequency will end up shifting far enough that the grid tie solar inverter will just se it as a bad grid, and it will turn off. In this case, the system has to run on just the battery, but not for too long. The battery voltage will start to drop, and the system will decide it needs the solar again. At that point, the frequency returns to normal. The grid tie inverter will then take 5 minutes to make sure the grid is good before it starts outputting power back into the system. If the batteries top out again, it will just cycle the inverter back off again.

With a good system, you can essentially run grid zero unless you get bad sun, and then it will seamlessly take any extra power you need from the grid. In the last 8 days, I only had to buy grid power for a couple hours during the off peak time.

 

[schmism]

Grid tied systems, just take people's money.

That's going to highly depend on your location as my state still mandates 1:1. Which is to say, if i use 10Kw at night, and give them 10Kw during the day they call it even.

Now granted if i use 10Kw at night and give them 11kw durring the day they only pay pennies for the extra 1kw however they also keep a running talley for 12 months. so you can "bank" in the summer and use it in the winter.

 

[timselectric]

That's going to highly depend on your location as my state still mandates 1:1. Which is to say, if i use 10Kw at night, and give them 10Kw during the day they call it even.

Now granted if i use 10Kw at night and give them 11kw durring the day they only pay pennies for the extra 1kw however they also keep a running talley for 12 months. so you can "bank" in the summer and use it in the winter.

But, that comes with a very expensive bill.
For a system that is not easily expandable.
At least, not easy on the bank account.

 

[GXMnow]

Even at 1:1 NET metering, you still lose a little because you pay the taxes on energy you import, but you don't get those back when you export. Then add in the "Time of Use" difference, and it is now under 50% credit here. I get about 15cents for each kilowatt exported from solar, but then have to pay 42 cents for each kilowatt I buy from 4 pm to 9 pm. That is a big reason why I added my battery bank. I now don't buy any power from 4 pm to 9 pm.

My grid tied solar inverters cranked out over 25 KWH today. I used about half of it, and stored the other half in my battery bank. My house will run on the battery bank alone until the sun comes up, but I am still a "Grid Tied" system. If solar production is low, or we use too much and the battery runs down, we just get whatever we need from the grid, seamlessly. So far this billing period, I am averaging buying 1 KHW a day. The solar is providing the rest. My bill for this month should be under $20. And with the battery, my backup loads and solar will keep working during a grid outage.

being able to "Bank" energy credit in the grid is going to be going away. Too many people are getting basic grid tie systems, and it is starting to be an issue with too much power while the sun is up. Many places are reducing what they credit for exported solar, and other places are not allowing export any more.

 

[StemSolar]

Looking at what you have, and given you've verified you have a grid tie only inverter, I would, as you plan, steal one string from your 5P6S to have a 4P6S for your grid tie. The string for your backup I would reconfigure as a 2P3S. Feed that to an SCC or AIO. I tend to favour the former because it's fewer parts in a box to fail and take your whole system out.

I agree with a hybrid inverter to feed any excess power back to the grid. If you're looking at the cheaper inverters be warned they often have a high idle consumption, which will eat into your power budget quite a bit. 50-60W is common. 50W*24h=1.2kWh/day+ just for the inverter. I have also seen advertised specs with a seller answered question about consumption that was about half what the manual in the box claimed. Observation says the manual was right. Inverters are a place to be very cautious about skimping on quality.

On choosing voltage, with what you've got I'd look seriously at a 48V inverter and battery because there will probably be a strong temptation to keep expanding your battery system over time and gain more independence from the grid. ETA: I see you were already planning 48V. Good choice, IMHO.

You're investing approximately nothing that will be lost if you later need more off-grid capable capacity, which is nice.

I haven't thought much about off grid solar but started to consider it seriously thanks to your advice. Under new administration energy prices soars and even worse Russia-Ukraine war broke. I usually paid 6.xx cents per kwh (another 6cents for distribution, tax and so on). Now the cheapest three year fixed plan already rose to 10-12cents for supply or generation itself. Thanks again.

 

[TorC]

You're welcome. The difference between off-grid and UPS starts to get a little grey, and has more to do with use philosophy than physical design. Main things is UPS is likely to be designed for minutes to hours of battery runtime with the batteries always charged, while partial off-grid tends to be designed for a 24-72+hrs of battery runtime and minimal input from generator or grid.