Need advice on backup battery focused system with solar potential

[LearningNewThings]

I am looking for advice on building a system focused on battery backup in the near term, and potentially augment with a few portable solar panels in the long term to deploy temporarily in the yard as needed. The goal is to get to 3-7days of backup power for critical things, including 2 fridges (maybe just 1 of them if the power consumption is too high), a freezer, a gas powered tankless water heater, modem/router, and some mobile device and laptop charging.

Proposed hardware:
-6 server rack rack batteries (probably eg4 LL or LifePower4)
-closed sever rack (need to keep kids out)
-an inverter (not sure which one, but I'd rather buy one than two, recommendations please)
-transfer switch (not sure which, is this function ever built into an inverter? what is a good automatic transfer switch?)
-what else am I missing? do I need an additional subpanel?
-permit? as far as I can tell no permit is required for just batteries?

The goal is for this to stay charged most the time and to kick on automatically if the power goes out and use the backup power on the critical switches. I had been thinking of a split phase inverter like the MPP solar LV6048 on the off chance that I upgrade and try to power an AC unit or something like that with solar panels, but do I really need 240V? On the other hand, if I deploy solar panels, my county (in Georgia) seems to suggest I would need to get the setup inspected, but would that apply for just temporary deployments? Not sure. If my inverter is not UL listed (LV6048 is not UL listed), could the inspection fail or is the UL usually only important if I want to feed power back to the grid (not interested in that)?

Does my setup sound reasonable? Is there a different inverter you would recommend? The MPP one is not in stock anyways. I see plenty of growatt in stock but wondering if there is a reason for that.

Also, any risks that I should be aware of? Does the bms in a server rack prevent the batteries from doing anything crazy like catching on fire? What is the risk of an inverter damaging any of my appliances?

 

[MichaelK]

Let's start out by adding up values from the list you are providing. Things like refrigerators and freezers will vary a lot based on size. I'll plug in my numbers, but if yours are different just plug in yours

1 refrigerator running 24/7: 1200Wh per day
1 freezer running 24/7: 600Wh
30W computer router running 24/7: 720Wh
devices/laptop 100W running 3 hr: 300Wh
inverter running 24/7: 1000-2400Wh

So, that works out to be 3820Wh to 5220Wh per DAY. That is a lot of power to be pulling out of a battery with no means of replacement. AC on top of that? Forget that completely.

For three days of use, you'd need ~15kWh of power, which would be a 30kWh lead-acid battery, or a 20kWh Li battery. Going with a 48V system voltage, that corresponds to a 625Ah or 417Ah battery respectively. More than double that for a week.

With solar recharging each day, this is MUCH more manageable. If you go to just 2 days of backup, the numbers look much better battery-wise.

Assuming you get only 2sunhours of light on your poorest day (DEC21), you are likely to make this much with about 1800W of panels. Call that six 300W panels. Less if you put them on rotating mounts.

You want air-conditioning too? Add another 1000W of solar just to keep one little 8000BTU window unit running.

If you care about your appliances make sure you buy Sine-Wave only. I would not even consider purchasing non-UL listed equipment. Some here would poo-poo my suggestions as elitist, but I've built a bullet-proof system that has stayed on continously for years. I did to a full system shutdown back in Sept 2020, but that was just to install ceramic tile in the utility room where the batteries are.

 

[joshgray]

My first suggestion is to buy an energy monitor like the Emporia Vue and use the clamps on every circuit you are even slightly considering. As you continue planning watch your trends to get an idea of consumption, given that this is only one part of the year though, That will give you real numbers to work from.

 

[Ampster]

I have an Emporia and it has been very helpful to understand my numbers. I have 42 kWhs of batteries that I think will allow me to get through 3 days without sun. I also have 9 kW of Solar that allows me to be fairly grid independent when the sun is shinning. These number are to give the OP a sense for the scale of at system. It could be a lot smaller if you only want to keep a few lights on and the refrigerator and freezer.

 

[LearningNewThings]

Thanks for the thoughts everyone. Let's assume that I have 5-10kWh/day consumption, and I plan to have 6 batteries (total around 30kWh). Also, lets say that in the future I plan to add 5-10 solar panels to deploy only temporarily if the power goes out for more than a day or two. The county may require inspection if I add the panels, so I assume everything should be UL listed to be on the safe side.

In this scenario, what inverter would you recommend? I live Georgia (USA). I have been looking at split phase 120/240V inverters with around 6kw output, like the MPP LV6048, but its not UL listed. I was thinking I want an inverter that is 240V capable incase I decide to later upgrade the system (maybe more panels, or permanent panels) to try to power an AC, an electric car, or other big appliances. Would the 6kw type inverters be enough if I upgraded, or would I likely have to buy a second inverter anyways if wanted to power more/bigger appliances. The main thing is I don't want to damage any of my appliances (e.g., an old subzero fridge that costs $12k to replace), is there any risk of that happening with some of the more budget friendly inverters like MPP or growatt? Since my main use case is emergency back up, it's hard to justify a really expensive inverter.

 

[Ampster]

I am happy with my Outback Skybox.

 

[MichaelK]

Thanks for the thoughts everyone. Let's assume that I have 5-10kWh/day consumption, and I plan to have 6 batteries (total around 30kWh). Also, lets say that in the future I plan to add 5-10 solar panels to deploy only temporarily if the power goes out for more than a day or two. The county may require inspection if I add the panels, so I assume everything should be UL listed to be on the safe side.

What you are describing is just about exactly what I have now, so I'll outline what I've got. It's all UL listed

Schneider XW+6848 120/240V inverter
Midnight200 charge controller
12 Rolls 4 SC 17P batteries, wired in series for 48V
15 300W 72-cell solar panels wired 3S5P
Midnight power center to that holds all the breakers controlling everything.

I also have two auxiliary 4-panel arrays consisting of four 60-cell panels wired in series to make the same voltage as my 3-panel arrays. I just divert power from my workshop in either early morning or late afternoon if I need more power to run my well-pump, or air-conditioner.

 

[Shimmy]

How many kWh per month do you currently use? You are buying a lot of batteries, so it can work but verifying your loads is important. From my research, I like the Schneider XW-Pro 6848.

Without solar, the inverter and batteries will use as much as 1kWh/day, increasing your total usage.

 

[LearningNewThings]

Average kWh per month - about 1600
I range from about 30-60 kWh a day except for summer months which is more like 50-100kWh/day. Its a bigger house - 3 hvac units, 2 kitchens.

At least in the beginning though, I am really only hoping to power 10kWh at the most per day for back up purposes. Less if I can get by with it. So that means no AC. I have 2 fridges and a freezer. One of those fridges is a beast (its a 26 yr old subzero fridge) and might consume the same as the other two units combined. So I might cut power to that one if I suspect I need power longer. The longest I've ever seen power out around here is 3 days, but seems prudent to prepare for longer.

Seems like the Schneider XW-Pro 6848 is about 2x the cost of the growatt and mpp inverters. Any ideas on what makes the difference? Is it worth it? Is it worth it if I am only using the system occasionally?

 

[LearningNewThings]

I guess one thing is the Schneider has an automatic transfer switch built in but no MPPT charge controller? Does that sound right? And the MPP/growatt inverters have a built in charge controller but no auto transfer switch?

 

[Shimmy]

Average kWh per month - about 1600
I range from about 30-60 kWh a day except for summer months which is more like 50-100kWh/day. Its a bigger house - 3 hvac units, 2 kitchens.

Sounds like about 50% more than mine. I wouldn't do less than a 40kWh battery for my needs, but I expect more backup.

At least in the beginning though, I am really only hoping to power 10kWh at the most per day for back up purposes. Less if I can get by with it. So that means no AC. I have 2 fridges and a freezer. One of those fridges is a beast (its a 26 yr old subzero fridge) and might consume the same as the other two units combined. So I might cut power to that one if I suspect I need power longer. The longest I've ever seen power out around here is 3 days, but seems prudent to prepare for longer.

For my house, I have a subzero (~4kWh/day, a small fridge (<500Wh/day), and a very large chest freezer (<800Wh/day). Functionally, I would not cut the main fridge; it is a high-impact change. Depends on exactly what you see the needs as. For me, the Wife Approval Factor is too important to cut that (or the wine fridge).

Long outages that are rare are best dealt with by a small generator. The inverter helps make that much easier, since you can just run chargers off the generator and keep it to ~8h/day running.

Seems like the Schneider XW-Pro 6848 is about 2x the cost of the growatt and mpp inverters. Any ideas on what makes the difference? Is it worth it? Is it worth it if I am only using the system occasionally?

It is more expensive, and cheaper alternatives can be attractive. You need to understand exactly how you will connect the system to determine if the premium is worth it. I was initially thinking to just do an isolated system with a cheap off-grid inverter for backup and certain "base" loads only to maximize self-consumption of my solar. Schneider started to be more attractive in making the solution more integrated and flexible for my needs.

 

[Shimmy]

I guess one thing is the Schneider has an automatic transfer switch built in but no MPPT charge controller? Does that sound right? And the MPP/growatt inverters have a built in charge controller but no auto transfer switch?

Correct; the limitation is there is a single power converter though, so it is either charging the batteries from solar/utility, or discharging. That has some functional limitations, especially if you have A/C coupled solar.