Thinking about battery backup

Hello All,

I figured the best thing to cover first is my current system.

I have 27 LG315N1C-G4 panels for a total of 8505w. My Inverter is a SolarEdge HD-Wave SE7600H

On a good day i generate 50kwh with a daily usage of 20kwh. With the remaining 30kwh going back into the grid as a electric bill credit.

The primary purpose of the battery backup would be used for night time power (unless thats not a good idea) and for power outages. Which we dont get alot of, but when we do it can be out for 1-3 days.

So here are my questions
What would you all suggest for a good starting point for a storage solution (I was thinking 10-12kwh)?
Should i go 24v,36v, 48v?
Will my existing inverter work for what im trying to accomplish?
And lastly, what will i need (other than batteries) to purchase to get this accomplished?

Thank you all for any info and assistance you can provide

You will need a hybrid inverter and batteries. I would suggest 48 volts based on your power requirements. Your existing inverter can be connected to a hybrid inverter through a process called AC coupling.

I am most familiar with Outback Skybox and SolArk but there are others from Schneider, SMA and many others. The above are all DIY possible. There are also solutions like the Tesla Powerwall, Generac Pika, and a few others that are professionally installed.
In either case you may want to think about which circuits you want to back up because it gets more expensive to back up every circuit in your home. The specific items you will need to purchase will be driven by which system you chose and how much you want to back up. An energy audit is a good place to start. How much do you consume each year?

I would prefer to do it on my own, as the price of those "professional" jobs are crazy.

Our monthly average usage is about 600kwh.

For back up, ideally id like to do the fridge, chest freezer, water pump, heat pump water heater, outlets, and lights.

I know that sounds like alot, but during the day my solar gen should keep us more than covered and charge the batteries. So overnight would be the only "draw"on the batteries.

The biggest load may be the water pump. That and the heat pump water heater could be timed so they don't come on at the same time. The water pump may have the biggest startup surge that would drive the hybrid inverter size. Do you have any way to measure that load? Is it a deep well pump and or a pressure pump?

The pump is a Wayne 3/4hp Shallow well pump: Wayne 3/4hp
The Water heater is a Geospring: GeoSpring Water Heater

MASolarGuy said:

And lastly, what will i need (other than batteries) to purchase to get this accomplished?

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I got a Reliance 10-circuit transfer panel and I cannot tell you how useful it is if it you have utility. You just can't predict every scenario and this allows you to adjust your loads, which is important when running off a bank.

The only thing I'd love to improve on it is having a way to remotely switch it -- I physically have to go to the panel. And, I'd love to one day put intelligence into that switching to automate it. Automation would replace the Reliance panel with something like PLC or a processor that can toggle and AC A/B/OFF switch.

But, the Reliance panel is relative cheap and can be obtained via Home Depot.

So it sounds like all Ill need is a hybrid inverter and batteries. I plan to build the battery out of recycled 18650 cells harvested from 2-3yr old laptops.

You would also need a BMS for 16 cells. I presume you settled on 48 volts. Did you ever determine the load of your well pump including startup surge?

Ampster said:

You would also need a BMS for 16 cells. I presume you settled on 48 volts. Did you ever determine the load of your well pump including startup surge?

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I know this will sound super nooby, but where would i find that info?

Here is a sticker on the pump (set to 230v)

If you are running it at 230 volts it draws 6.2 Amps. I can't see anything that give the starting Amps. Sometimes it is described as LRA (Locked Rotor Amps) That will help you figure out how big of and Inverter you would need based on the surge capability of the inverter.
Once it is started it would take 1400 Watts to keep it going.
I also have a GeoSpring and in Heat Pump mode it takes 4000 Watts to run. I don't know the start up Amps but my 5kW inverter handles that easily. In other modes the Geospring takes a lot more Amps but no startup load.

Ampster said:

If you are running it at 230 volts it draws 6.2 Amps. I can't see anything that give the starting Amps. Sometimes it is described as LRA (Locked Rotor Amps) That will help you figure out how big of and Inverter you would need based on the surge capability of the inverter.
Once it is started it would take 1400 Watts to keep it going.
I also have a GeoSpring and in Heat Pump mode it takes 4000 Watts to run. I don't know the start up Amps but my 5kW inverter handles that easily. In other modes the Geospring takes a lot more Amps but no startup load.

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Ampster, thanks a ton for all your help. It is very much appreciated. What would you suggest for a 48v system to fit my needs (well pump, freezer, fridge, hotwater heater, wifi, and laptop (for work))?

Guesstimate 5x current draw for startup, 30A at 230V or 60A at 115V, but only for about 1/4 of a second.

You have options of manual or automatic transfer switch. Obviously automatic is great for the refrigerator if you are away from home. I have a $50 or $60 piece of sheetmetal that interlocks main and backfeed breaker in my main panel, and at the same time I turn off things I wouldn't power with a battery.

Batteries have to be much larger to power arbitrary loads at night. Much smaller if you only run fridge and pump during the day.
Traditionally 3x daily consumption/production for those extended times of darkness. Mine is 1/5 of daily production, barely keeps the lights (and several refrigerators) on at night, 1200W for 12 hours.

See if your inverter supports frequency-shift adjustment of power output. If so it would play nice with battery inverters like my SI 6048US, which raise frequency to request reduced output. That way my AGM batteries float along fully charged and PV output adjusts to match house load. Without that, the inverters bang off line, wait 5 minutes, come back on at full power, and repeat, so your batteries cycle constantly.

You also have the option of going batteryless with an inverter that generates AC directly from PV. Sunny Boy offers 2000W "secure power" outlet which will probably start your fridge but not large loads. So store water in a tank until power comes back on. It may not run your furnace blower, not sure.

I of course would recommend two of the SI 6048US that I have, if your inverter plays nice or you change to one that does. One of them and a 120/240V transformer would probably also work except more than 6.7 kW of PV exceeds the pass-through relay for grid-tie. SMA would recommend for 8000W of PV you get 800 Ah of battery at 48V (about $10,000 for AGM) but I have half that battery for twice that much PV. I think you could get away with 100 Ah to 200Ah, so starting at $1000.

MASolarGuy said:

What would you suggest for a 48v system to fit my needs (well pump, freezer, fridge, hotwater heater, wifi, and laptop (for work))?

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Thanks to @Hedges we have a pretty good idea of the startup of the well pump. You already have 8kW of GT solar so you need a hybrid inverter that can AC couple with your GT inverter when the grid is down.
As I mentioned earlier I own an Outback Skybox and it can reportedly AC couple with up to a 7.6 GT inverter which you have. The Skybox price has been reduced to around $3500 and it would be my choice. I know several people that love their SolArks. They are larger capacity and cost more. Both are all in one and you don't need anything else except to rewire some circuits to a critical loads panel to get the biggest benefit of load shifting when the grid is up.
Hedges also made some suggestions that may suit your purposes. Perhaps he can verify that they can AC couple because that is the key to being able to use your existing GT inverter when the grid is down. There are other brands that also have that capability.

Hedges said:

See if your inverter supports frequency-shift adjustment of power output. If so it would play nice with battery inverters like my SI 6048US, which raise frequency to request reduced output.

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He has a Solaredge HD Wave which is 7.6 kW. I have a smaller version and it AC couples with my Skybox well. These newer HD Wave inverters modulate with the frequency shift so one doesn't get the constant on off cycling of the GT inverter like the older models sometimes do if the loads shift during a grid outage. Do you know if the SI has any capacity limits on the size of the GT inverter it can AC couple with?

Ampster said:

He has a Solaredge HD Wave which is 7.6 kW. I have a smaller version and it AC couples with my Skybox well. These newer HD Wave inverters modulate with the frequency shift so one doesn't get the constant on off cycling of the GT inverter like the older models sometimes do if the loads shift during a grid outage. Do you know if the SI has any capacity limits on the size of the GT inverter it can AC couple with?

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SMA recommends a limit for off-grid Sunny Island installations of AC coupled grid-tie PV inverter wattage no more than 2x the Sunny Island capacity (6kW per SI). For wind power, only 1x. So one SI could manage 12 kW of PV. But, its relay can only pass through 6.7 kW (56A @ 120V).

Two SI in a 120/240V setup would be a robust system able to power far more than he needs. One SI and a transformer could probably support his needs if the existing PV panel strings are two different angles, so peak power never exceeds 6.7 kW. If it does, SI will open the relay and run with batteries for a while, I think. Unfortunate, but for the original European version at 230V single phase, pass-through power was twice as much.

Sunny Island 6048 can be found on eBay between $2000 and $2500, new in the box. MSRP was $5000.
It may need a bit more support stuff than some others. Certainly breakers or fuses on input and output. It should have a separate breaker panel (or at least breaker/fuse) going to the SolarEdge and a "load shed" relay before the protected loads. Reason is, if battery drains due to loads, the PV inverter can't run and get the battery charged. So loads are disconnected before then, and battery is used to give a local grid to the grid-tie inverter while waiting for the sun. That isn't needed if you have at least some DC-coupled PV which will start to charge battery back up.

Are you saying the SI doesn't have a low voltage disconnect or load shedding setting? I think my Skybox has all that stuff built in but it is a newer design aimed at the market of GT inverters that now want batteries.

It has all that. But it only has two AC connections: Grid/Generator in and an output. It has an internal relay to connect input to output if grid voltage/frequency is stable and in spec. But that is the only internal power relay. It has two signalling relays to control external power relays.

AC coupled inverters (e.g. Sunny Boy) are on the output. It charges batteries (one DC connection) from its output. If the relay is closed, that includes power from the grid or generator.

But what can it do if your house drains the battery to 70% DoD during the night? If it keeps producing AC, the batteries drain to zero. If it turns off the AC, then the AC coupled inverters never see AC so they never deliver power. A relay (SPDT) switches at some state of charge, default 70% DoD. That can switch AC or DC, and a 48V PTC protected supply is on-board. In my case that goes to a 48VDC coil, 3PDT relay. With 48V delivered, the protected loads (e.g. house) are on Sunny Island's output. After switching at 70% DoD, the loads are disconnected. Sunny Island's output keeps producing AC, with no load on it, and in the morning AC coupled inverters wake up and recharge the battery.

Each Sunny Island has two signalling relays, and each relay in a system (of 1 to four Sunny Island) can be programmed to one of 18 functions. These include a couple DoD signals, battery fan, and others.

So you can shed or add loads at various depths of discharge, with external relays or control circuits, commanded by Sunny Island. The suitable relay was hard to come by, though, and I bought a couple 100A 48V relays from a European vendor.

Appears to be the same for Skybox. AC coupled PV goes on the same terminals as protected loads. AC coupling can only start working in the morning if Skybox is producing AC, which means using battery power to supply loads until there is enough PV to do that and charge as well.

Difference between Skybox and Sunny Island is Skybox has a PV input. For Sunny Island, a separate MPPT charger is required for DC coupled charging. Also, only a single Skybox not stacked Skyboxes can be used with AC coupled grid-tie inverters "at this time."


The thing to do would be move some of the PV panels over to Skybox PV input. That way a battery drained at night to the point of shutdown would still get recharged. If you go for two stacked Skybox then move all PV over.

Skybox spec is 5000 VA up to 45 degrees C, 120/240V split phase, 24A max continuous output current. Don't know the surge capability but it is a transformerless design. Probably capable of starting that pump, but could be marginal with just one Skybox, and additional household loads might keep it from starting. Outback may have given it a decent surge current, don't see that listed, but by searching I found another forum where such performance was described as poor. You need to know if it can start your motors, maybe contact manufacturer.

Hedges said:

Appears to be the same for Skybox. AC coupled PV goes on the same terminals as protected loads. AC coupling can only start working in the morning if Skybox is producing AC, which means using battery power to supply loads until there is enough PV to do that and charge as well.

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I have solar connected to the Skybox so I have never have experienced that issue. I assume when you say protected loads you are referring to the critical loads panel? It has several low voltage thresholds that I presume accomplish what your external relay does.
Do you have a link to that bad review? There is an Outback forum that has a Skybox section but I have not seen a post where performance was described as poor.

Not so much a bad review, just a comment about poor motor-starting surge performance (which I have heard is generally the case for transformerless inverters). See posting #143 and the included posting it comments on "... have commented on the Skybox being pretty poor with surging in an offgrid mode ":


Oh, what the hell am I saying? I'm referring you to your own posting, which is responding to another's!

The question for OP would be whether it can start his 3/4 HP load. Maybe you could try that with yours; you posted you're only running lighter loads.
Besides my 4x SI 6048 system I have a single SI 5048 on a cart with 100 Ah 48V battery. I could try starting a 2 HP compressor and a 1.5 HP sprinkler pump with that.

I see the SkyBox has AUX relay: 10 A @ 240 Vac, 5 A @ 30 Vdc which might serve to connect or shed moderate size loads. A larger relay externally would be needed to do that for full load. But with DC coupling (PV panels wired to SkyBox) nothing more is needed to recover from discharged batteries once the sun comes up.

What I have on the output of 4x SI is a breaker panel for multiple GT inverters. Coming off that is a 100A relay which then goes to everything to be backed up (my house main panel, but through a manual transfer switch.) Low power communication gear could be connected to the first panel and always powered. Refrigerator, A/C, etc. are on the second panel where they can be "load shed" in case of low battery.