Simple solution for home battery backup and existing solar install?

[gvl]

I have a 9kW array with SolarEdge inverter that was put in service about 10 years ago. Are there off-the-shelf solutions that allow to charge a battery from the inverter (AC coupling I think is the term) in case of a power outage? I realize I will need to install a transfer switch between the inverter and the main panel/grid to be able to charge the battery.

Ideally I envision a poor-man's setup that consists of a backup power inlet feeding the main panel and a mechanical interlock installed on the main breaker. In case of an outage I flip the main breaker OFF/connect the battery to the backup power inlet/turn on the battery and flip the backup power breaker ON, this fools the solar inverter into thinking the grid is connected, it starts to produce power that powers the home and charges the battery. The battery naturally buffers any reductions in solar output.

What are my options?

[organic farmer]

In my system, the battery bank is 48vdc [the amperage is not material to this discussion]. I found a 52 volt power supply online, it may only put out 20 amps but that is enough for a slow charge.

I checked the fluid levels in all batteries, Then started charging. Every day that is was charging I checked the fluid levels again. Eventually after a week the battery bank was fully charged once again.

[gvl]

Just to share my findings, I can do all that with a Schneider XW Pro inverter/charger and 14khw or so worth of batteries. It is being discontinued however, while discounts are good long-term support is a concern. Not convinced I will proceed, but it's tempting.

[solardreamer]

Keep in mind, you will need 2 XW Pro's and 18kWh battery minimum to safely AC couple with your 9kW SolarEdge grid-tied inverter system. Check the Schneider docs for AC coupling.

[gvl]

Yeah, that sounds about right. The 2nd XW Pro is the killer. I rarely see more than 6.5kW output these days from my aging and dirty solar array. What the worst can happen if I use 1 XW Pro? Protection kicks in and it turns off?

[solardreamer]

Most likely protection shutoff is the result. However, the inverter MOSFET transistors may suffer some degradation for each occurrence. So, if this done repeatedly the transistors could suddenly fail at some point. Also, keep in mind that 6.5kW is an average and there could be sudden PV power spikes (I often see this with the cloud edging effect) up to PV system limit (9kW) that can potentially damage the inverter.

[jzchen]

Sorry if I hop on this thread. I can try and create a new thread if desired.

First of all thank you for unbanning me. We did end up with a 9.16 kW SolarEdge grid tied system here in Southern California with SCE, and currently CPA as our power aggregator. It was installed in 2013 and within the last year or two passed the initial payment of $36k based on the monitoring and 2013 electricity rate....

Unfortunately the $11.xx monthly grid-tie fee has finally (after warnings from SCE) been upped to $24.xx. This is a big hike. My parents actually get paid for excess electricity production, even though we have added an EV to our stable....

Any thoughts on what I might go with brand/model and size would be greatly appreciated! Thanks in advance!

[wesley123]

You’re on the right track, but your “poor-man’s” setup won’t really work because SolarEdge inverters shut down during outages (anti-islanding) and won’t restart just from a backfed battery. The proper way is an AC-coupled system with a battery inverter (like Powerwall or similar) that creates a stable mini-grid so your solar can run and charge the battery safely. You’ll also need a transfer switch or backup panel to isolate loads. If you want to explore the right setup for your system, you can reach out to Rocksolar’s technical team—they work across Canada and the USA and can guide you even if you’re just looking for advice.

[jzchen]

I guess my question/thoughts are trying to go "off-grid". The interconnect fee is high now and eats into what they pay for the excess electricity production of our system.

The question in my mind becomes how does a battery off-grid system deal with excess production? How much backup should I get for winter season or low sunlight season more specifically...

[SunEagle]

I guess my question/thoughts are trying to go "off-grid". The interconnect fee is high now and eats into what they pay for the excess electricity production of our system.

The question in my mind becomes how does a battery off-grid system deal with excess production? How much backup should I get for winter season or low sunlight season more specifically...

The decision to go off grid can change as your needs change. I know some of the guys on here live up in New England and have decided to go off grid. But they didn't have the correct amount of battery to provide all their needs the first year and maybe can help you. So you may have to add both solar panels and batteries to your system.

[jzchen]

Thank you both for the insights and warnings! I'm really not happy with the interconnect hike/jump. I just checked and we received $160.77 for our excess last NEM 12 mo cycle. With the hike it likely means nothing will be paid next cycle (rough math in my mind).

It seems like very careful planning is necessary, and/or adjustment in EV charging habits...

Is the earlier roughly double the kWh sizing of battery (so 18/19 kWh) a reasonable amount? (My ZIP code is 91007).

Maybe this is just too much of a challenge, or not cost effective? It would be a bummer to just shut off production when the batteries are full...

Thanks again for any insight!