Battery backup that can take advantage of a large amount of solar panels

[nkchri2]

I am currently having a new solar system installed. I wasn't planning on DIY'ing anything myself, but the battery backup options just don't seem that great and are really expensive. I was considering getting the Enphase 10 system. We have a good size house, everything is electric, and we use a lot of power! I currently have a 14 kW system freshly installed on the roof. I was considering actually bumping it up to a 19.7 kW system. However, the local installer had told me about how the new Enphase batteries could hand electricity straight from the solar panels to the home without charging/discharging the battery. I thought this sounded great! We could basically last forever if the grid went down! I thought the panels could handle most of the load through the day and store some for the night. However, each Enphase module only has 4 microinverters... so the Enphase 10 can only handle 12 solar panels during a grid down situation. So if the grid goes down, we would only be making use of 12 out of the 58 panels for a 19.7 kW system. That seems like a gigantic waste. Why even have the battery backup if the production is going to be so limited? The solar panels are already cost effective and we got in on the good net metering agreement before it changed, but with that limited of production during a blackout, why spend 14k for an Enphase 10 when I could get a natural gas home backup for like 6k that would basically accomplish the same thing?

I'm not an electrical engineer so my knowledge is limited. Is there any kind of battery backup option that can actually use more of the solar panels during the day? Is it solely limited by the number of microinverters? Are any DIY route batteries capable of running heavy loads like large appliances or A/C?

 

[Hedges]

If your microinverters can do the frequency-watts option of Rule-21, they could interact with various battery inverters.
GXMnow uses them with Schneider XW. Should also work with Skybox and Sunny Island. Possibly also Sunny Boy Storage with Automatic Backup Unit.

I use SMA Sunny Island with Sunny Boy string inverters. These let grid-tie inverters pass their AC through to the grid, then when utility fails the act as a UPS to power the house from batteries. They control the grid-tie inverters and recharge batteries from the AC they make. All of the PV is available to run my house.

 

[nkchri2]

If your microinverters can do the frequency-watts option of Rule-21, they could interact with various battery inverters.
GXMnow uses them with Schneider XW. Should also work with Skybox and Sunny Island. Possibly also Sunny Boy Storage with Automatic Backup Unit.

I use SMA Sunny Island with Sunny Boy string inverters. These let grid-tie inverters pass their AC through to the grid, then when utility fails the act as a UPS to power the house from batteries. They control the grid-tie inverters and recharge batteries from the AC they make. All of the PV is available to run my house.

So do you have to have a microinverter for every panel? Like I said, my engineering knowledge is limited, so I'm still not quite understanding why those enphase batteries are limited to only 4 panels or what exactly is happening to allow the full system to be operational. You still also have to scale the battery to what you expect to be your max load correct? Are these charging the batteries and running the house load at the same time? What specific components am I looking for to set this up for a 19.7 kW system?

 

[Hedges]

When a load (like my A/C) turns off, battery inverter probably has to suck down all the watts produced from PV for a moment, until the GT PV inverter has been told to reduce its output. That could be why Enphase requires one battery for every 4 panels (if that's what they do).

I didn't use microinverters. With microinverters, power from a single panel is converted to AC, and all get connected in parallel.

I used high-voltage string inverters. Some of my original 2500W inverters had 24, 120W "12V" panels in series (24s1p), and some had 18, 165W panels in two parallel strings, 9s2p.

String inverters used to be more economical as well as slightly more efficient. Now that "Rapid Shutdown" isolation of individual panels (electrical safety for firemen on the roof) is required in many areas, they require a box per panel which negates most of the savings.

My SMA Sunny Island SI-6048 can each pass through 6.7kW 120V from GT inverters to the grid. To support 19.7kW at 240V, would require 4 of them, configured 2s2p (which is what I have.) To be more economical, you could use 2, configured 2s1p, with maximum 13.4kW of GT inverter on them. Leave the remaining 6.3kW of GT inverters connected direct to grid. (With two Enphase systems, perhaps some filtering is required to keep their communication separated, or perhaps not because isolated by relay inside Sunny Island when grid fails.)

This data sheet has a schematic, consider your microinverters where it shows Sunny Boy (assuming yours do frequency-watts)

https://files.sma.de/downloads/SI6048US-DS-en-27.pdf?_ga=2.154736062.765294067.1616255760-621369231.1589916091

A different battery inverter "Sunny Boy Storage" can do peak-shaving. It requires separate "Automatic Backup Unit" to do battery backup. I don't think it can be stacked for higher wattage. You would probably be limited to (expensive) compatible brand 400V batteries.

https://files.sma.de/downloads/SBSXX-US-DS-en-20.pdf?_ga=2.154235454.765294067.1616255760-621369231.1589916091

Some of the other battery inverters that support AC coupling can probably be stacked similarly to support the total wattage.

 

[nkchri2]

Well unfortunately I might just be up s**t creek without a paddle. Like I said this is being handled by a solar installer and isn't something that I thought was going to be an issue. The whole battery issue just seems like a disaster and I don't know why they would approach it that way. The Enphase Encharge 10 says it has twelve embedded IQ 8X-BAT Microinverters (which is why it can support 12 panels). However, the panels that are already installed on the roof are QCell 340 QPeak Duos, which have Enphase iQ7+ microinverters factory integrated with the panels. So I guess things are normally set up to be straight to grid with no storage, so then the storage route has to have additional microinverters to charge the battery? I'm assuming since the battery has microinverters built in there is no work around to get the panels in series before hitting the battery. They have already run through the micros so I would need something to series the current that is already AC into a DIY battery?

 

[Hedges]

Look over the Enphase website to see if Encharge battery is compatible with coupling to your Enphase PV microinverters.
Ask they installer if they would be able to do a future battery backup addition to the system.
Always easiest to stay with one brand designed to work together.

Sometimes microinverters are hardwired to a PV panel, but often they have MC4 connectors and can mate to any suitable PV panel. In the latter case, the PV panels could be disconnected and instead connected to any SCC or string inverter. However, individual microinverters avoids the need for "Rapid Shutdown" because volts and watts of each individual panel is under the limits. If panels are connected in series or in parallel with each other, some RSD solution may be required by local code.

It is possible that with 340W PV panels your microinverters are "overpaneled", clip output during peak production times to stay within their maximum wattage. That is a reasonable way to design a system, but if same PV panels are wired together with large enough inverter, would be able to harvest more energy during those times.

 

[nkchri2]

Look over the Enphase website to see if Encharge battery is compatible with coupling to your Enphase PV microinverters.
Ask they installer if they would be able to do a future battery backup addition to the system.
Always easiest to stay with one brand designed to work together.

Sometimes microinverters are hardwired to a PV panel, but often they have MC4 connectors and can mate to any suitable PV panel. In the latter case, the PV panels could be disconnected and instead connected to any SCC or string inverter. However, individual microinverters avoids the need for "Rapid Shutdown" because volts and watts of each individual panel is under the limits. If panels are connected in series or in parallel with each other, some RSD solution may be required by local code.

It is possible that with 340W PV panels your microinverters are "overpaneled", clip output during peak production times to stay within their maximum wattage. That is a reasonable way to design a system, but if same PV panels are wired together with large enough inverter, would be able to harvest more energy during those times.

This is the response from the mod on their company forum:

"Exceeding the limits set in this guide would result in poor customer experience during the grid's transition to back up in grid outage or sudden changes of PV or load in backup operation. If you have more PV micros than the limit defined in the ensemble planning guide for the number of Encharge units you would like to install at your site, part of your PV system can be connected to be active in both backup and grid-tied operations. The rest of the PV micros can be on when the system is connected to the grid."

Also, if it makes anything any easier, It's not super important for me to have automatic backup during a blackout. I work from home and we are home the majority of the time. I don't need any automatic system as I am just fine being able to flip a switch to disconnect us from grid and move to battery operation. Really the key here that I want is just to be able to make use of all my panels with no grid. Just venturing into this is the first I have heard of any of the rapid shutdown requirements for code. Is this eliminated if you don't have any kind of automatic switch and I can just manually disconnect from grid?

The date sheet for these panels says it has these connectors: Stäubli MC4, Hanwha Q CELLS HQC4; IP68. I'm assuming that means I could go with the string inverters?

 

[Hedges]

Just venturing into this is the first I have heard of any of the rapid shutdown requirements for code. Is this eliminated if you don't have any kind of automatic switch and I can just manually disconnect from grid?

Rapid shutdown is a fireman safety feature, so nothing to do with on-grid, off-grid, backup, etc.
It may or may not be code in your area at this time.
The purpose is that when a switch is thrown, all wires drop to less than 80V (and I think some wattage).
With microinverters, DC voltage is already within that limit, so just shutting down AC takes care of it.
With a high voltage series string of panels feeding a string inverter, the solution is a box per panel that disconnects all panels from each other when it doesn't receive a keep-alive signal.

The date sheet for these panels says it has these connectors: Stäubli MC4, Hanwha Q CELLS HQC4; IP68. I'm assuming that means I could go with the string inverters?

If the panels have MC4, they can be rewired to string inverters (or to RSD boxes if required, and those to string inverters or charge controllers)

 

[nkchri2]

Thanks for all the help, I really appreciate it! I think for right now I am just going to leave it grid tied with no battery. Really sounds like I just need a setup just like yours. I don't want a large battery, just for overnight basically, and just want all my panels to be producing through the day. I'm not sure if I feel up to trying to tackle it myself and as far as I'm aware I don't think the installer I have been working with so far would want to either. My uncle is an electrical engineer so he may be able to help me out, otherwise I just need to try and find someone with experience setting up such a system or experience with Sunny Islands. I'll also have to find out about rapid shutdown codes in my area.