Advice on battery backup for power outages

[EL34xyz]

Hi All,
I want to set up a battery backup system for power outages.
I have a generator set up on propane, but I would prefer not to run that and use propane.
I am looking at the Sol-Ark 15k: ($8250 and Free shipping during March)

I am not looking to set up solar yet, but I want that option for the future
I have about 40 years of electronics experience, so this sort of thing is not a problem for me to set up
I want a complete package instead of piecing out every component

Looking at my power bill, the average per day is 1341 kwh per day over a 12 month period
I have two buildings tied together and each one has it's own breaker panel
I have a nice spot right next to a breaker panel inside one of the buildings to set up a system

I run my business out of one of the buildings
I only need emergency power backup at this point

My question is, am I on the right track here with the Sol-Ark 15k?
Would two 48v batteries be good to get me started?
Or do I need more?

Like these batteries here?
Are these a good choice?
ARK 48 V 100Ah LiFeP04 Battery

ARK 48 V 100Ah LiFeP04 Battery - Practical Preppers

Email info@practicalpreppers.com if you have questions The completely modular battery system ARK Battery eBrochure ARK Battery Owner's Manual ARK + Sol-Ark Programming Guide

practicalpreppers.com

Thanks for any help.

 

[EL34xyz]

Oops, my power usage is 1341 kwh per month, not per day

 

[BentleyJ]

Oops, my power usage is 1341 kwh per month, not per day.

Your average usage is 45kWh per day. Two batteries is 10.2 kWh total, 9.2kWh usable at 90% DoD. About 5 hours of run time.
This is just rough napkin math and doesn't take into account inefficiencies or battery degradation over time.

Also, you did not mention Peak kW demand so we cannot comment on the Sol-Ark 15K. That rating is only valid when PV is available, its actually 12kW output on battery only. Back to the batteries, 12kW will require 250Amps from the batteries. That exceeds a 1C discharge rate and may exceed the internal BMS or breaker in the battery which is quite often 100A. I didn't look up the details of chg/dischg specs for this particular battery. Again, you probably need more batteries.

 

[Madco]

Agree with BentleyJ's math. The 45kWh daily average is a lot of energy, and that's going to require a large and costly solar energy backup system! So some (a lot of) load reduction is necessary, and that will require life style changes at least during the power outages. Probably better to do it now instead of waiting until the utility drops. Assuming the outages are infrequent and short, it will be much more cost effective to run the generator when you need it - unless you dramatically prune back that 45kWh average consumption.

 

[EL34xyz]

Thanks for the reply
My power requirements when the power grid is down are way lower than when the grid is up
I typically don't run things that are on the house panel like
The big heat pump, The oven, the electric hot water heater
I usually turn the breakers off for the large items
I usually only run the fridge, a small deep freeze, lights, TV in the house

I am more interested in the smaller building that has it's own breaker panel
My business is in this building and this is where the backup system would be
It has a Mr Cool Mini split some PC's and a few other low power devices
I have run this building plus food storage in the other building on my generator with no problem

I can add more batteries to the Sol-Ark 15k if needed
I was just curious if this system would get me started?
Thanks for the help

 

[EL34xyz]

Agree with BentleyJ's math. The 45kWh daily average is a lot of energy, and that's going to require a large and costly solar energy backup system! So some (a lot of) load reduction is necessary, and that will require life style changes at least during the power outages. Probably better to do it now instead of waiting until the utility drops. Assuming the outages are infrequent and short, it will be much more cost effective to run the generator when you need it - unless you dramatically prune back that 45kWh average consumption.

Yes, I do cut way back when the grid is down as I listed above
Also, the grid does not go down that often here, but it is a major pain when it does.
I am not so sure the power grid is going to be all that stable in the future and so I am preparing for that.
I can add more batteries if needed

My generator is just one of the 5500 watt Troy bilts that I converted to propane
I have to drag it out, plug it in to a panel via a 240vac plug, hook it up to a propane tank, cut off breakers in the house and the main breaker to the street
I would rather not use the generator if I don't have to

 

[Madco]

That info helps quite a bit. How big is the genset?

 

[dcg9381]

Your average usage is 45kWh per day. Two batteries is 10.2 kWh total, 9.2kWh usable at 90% DoD. About 5 hours of run time.
This is just rough napkin math and doesn't take into account inefficiencies or battery degradation over time.

Also, you did not mention Peak kW demand so we cannot comment on the Sol-Ark 15K. That rating is only valid when PV is available, its actually 12kW output on battery only. Back to the batteries, 12kW will require 250Amps from the batteries. That exceeds a 1C discharge rate and may exceed the internal BMS or breaker in the battery which is quite often 100A. I didn't look up the details of chg/dischg specs for this particular battery. Again, you probably need more batteries.

You really need to slap on a power monitor and determine peak draw. I have a 20kW propane generator and our normal house draw is 2-5kW. However, I can tell from the generator monitor that there are bigger spikes. For under $100, I put a monitor on the main and figured out that my largest spike in the last 3 months was 15kW. That's enough to sink your chosen inverter even if was backed by enough battery.

Total draw Math:
48V (battery) * <Required amps> = 15000 watts
To handle that spike, I need enough battery to do 312.5A.

Battery math:
Assuming I don't want to push my batteries harder than 75A, I need 5 of them. (312.5 A / 75A per batt) = 4.16 batteries. Round up to 5

You can "cut back" but these stuff switches on so fast you'll never know that the grid was down. You'll definitely fault and have to reset, well, if your spikes are like mine.

And integrating both solar, battery, AND generator, you'll need to pass all that through your inverters, as well as pass main power.. You're going to need two inverters.

Realistically, you're looking at over $25k in hardware to do this, automagically (auto-transfer).

If you want a more manual system - you can install a manual transfer switch. This will allow you to dump loads first, then switch on the inverter/battery/generator setup. Watch your N/G bond setup. You will NOT have a grid input, generator input only. You can size "down" to whatever you want that works. I'd be careful as SOME inverters can be sensitive to generator's "dirty" power.

 

[Madco]

Oops, you answered the genset size while I was pecking the last message. DCG9381 offers good advice. I was just going to run with replacing the 5500W genset with batteries and inverter(s). I am not familiar with the mini-split. Does MrCool heat as well as cool? Do you have any specs on him? Also, how many days of autonomy do you need?

 

[EL34xyz]

Oops, you answered the genset size while I was pecking the last message. DCG9381 offers good advice. I was just going to run with replacing the 5500W genset with batteries and inverter(s). I am not familiar with the mini-split. Does MrCool heat as well as cool? Do you have any specs on him? Also, how many days of autonomy do you need?

Mr Cool is a heat pump mini split and so it does heat and AC. But I run other heat sources in the winter because heat pumps struggle when the temps get down below 30F. That is in my shop/business where I need the power backup most.

In my house, I have a very large heat pump that is only 2 years old. I replaced a 22 year old unit.
I don't run that when running on a generator or my electric stove or electric water heater

My 5500 watt generator can run both buildings by not using the huge power draw items mentioned above

I supplement with propane heat in the winter when it's really cold.

 

[Madco]

OK, so if I haven't misread anything, it sounds like you are only interested in backing up the office and food storage areas. Those are conditioned with a heat pump based mini-split. The office has some computer equipment, probably running 24/7. Now some more assumptions: the mini-split's power consumption can be disregarded in the summer since the office can be cooled with box fans, if necessary. Winter is another story if it has heat strips. As dcg9381 mentioned, peak demand is a need to know because that figure will drive the size of the inverter(s). To size the system and eliminate as many unexpected problems as possible, you need to estimate power consumption, both summer and winter, as well as peak demand. For example, my home, water, and wastewater systems exert an average demand of about 500W. The peak demand is 13.1kW though. Without knowing the peak, any inverter selection is probably going to be severly undersized, leading to nuisance trips/overloads at inconvenient times-like when you are away for a week!

All that said, if you don't know the peak demands, you can add all the computer loads and the nameplate consumption (don't forget MrCool's heat strips if he has them) and that should get you pretty close. That nails down the inverter(s) sizing. From there, sum the energy consumption of your computer gear and determine the consumption of MrCool by looking at cooling/heating degree days and your level of insulation for your buildings. That total energy consumption along with days of autonomy will determine the number of batteries required.

It all sounds mind numbing but judicious planning and realistic assumptions now will help you to not end up with equipment that you can't use later as you expand!

 

[EL34xyz]

No heat strips are ever used in my heat pumps and Mr cool mini split.
I use propane heat as backup if the temps are too low for heat pumps

When the power goes out, I am able to run all the things I need on my 5500 watt generator including the Mr cool, computers, led lights, fridge, and a small stand alone freezer.
I am only looking for a backup battery unit so I do not have to drag out the generator and all that monkey business like propane hookups, cabling back to a panel, etc. When it snows, it is not fun to do all that outdoors.

A battery backup would be inside my office building and would be way easier to get up and running when the power goes off.
My 5500 watt generator can do all that, and it has saved my bacon that maybe 5-10 times in the last 15 years
My generator could be used as backup and for charging the battery bank if the power loss went on for a long time.

I am not doing any sort of off grid complete power system
I don't need to run high current appliances like electric ovens
I flip the breakers off on all those devices when running on a generator

I was thinking the sol-ark 15k would be able to handle the same smaller loads mentioned above for short power outages
Also, I could add more batteries to the system if I need to expand.

 

[dcg9381]

Now some more assumptions: the mini-split's power consumption can be disregarded in the summer since the office can be cooled with box fans, if necessary. Winter is another story if it has heat strips.

FYI - there are no mini-spits that I know of that use resistance heat. They are all heat pump and various versions may heat (or not have heat at all). Within those that do heat, there are substantial variations in heating efficiency that MUST be paid attention to if you're in a cool climate. There are great systems that will heat to very cold temps (Mitsu hyper heat). I have a Daikin system and below 20 degrees, it's output is probably 75% degraded.

 

[Madco]

If the 5.5kW generator runs everything you need then maybe look for a smaller inverter? The sol-ark is a great unit and would allow for future expansion but some would consider it a little pricey. In any case, you'll need 1 100Ah battery (lithium based) for every 3.8kW of powered load to keep the maximum discharge current below .75C. As you originally asked, go with two batteries to start.

 

[Madco]

One last thing, don't store your batteries at 100% SOC. Let them hang out at 60-70% until you know it is likely that you'll need them. For example, a major winter storm is forecast. Charge them fully before hand. If you need them, great. If you don't go ahead and run from them after the storm and discharge them back to 60-70% SOC.

 

[EL34xyz]

If the 5.5kW generator runs everything you need then maybe look for a smaller inverter? The sol-ark is a great unit and would allow for future expansion but some would consider it a little pricey. In any case, you'll need 1 100Ah battery (lithium based) for every 3.8kW of powered load to keep the maximum discharge current below .75C. As you originally asked, go with two batteries to start.

Thanks for the solid advice based on what my current needs are.

I only mentioned the 15k sol-ark because I may want to expand in the future.
I may want to add solar and stack up more batteries and handle bigger loads in the future
I know this from experience. Once I get into something, I always tend to expand.
Too many times I purchase a device and then turn around and get something better.
So now I like to just try and do it right the first time.

I am starting out slow for now and will see what my needs are down the road
Price is not an issue and the sol-ark source is only 46 miles from me.

 

[Madco]

Good luck and enjoy the fun!

 

[EL34xyz]

One last thing, don't store your batteries at 100% SOC. Let them hang out at 60-70% until you know it is likely that you'll need them. For example, a major winter storm is forecast. Charge them fully before hand. If you need them, great. If you don't go ahead and run from them after the storm and discharge them back to 60-70% SOC.

Good advice.
I probably will cut the power from my 2nd building every now and then and let the battery backup do it's thing
Just so I can get a feel for how the whole system is doing.

But one issue with the charging up in case of a storm

One of the worst power outages I have ever had was on a beautiful sunny day.
A vehicle had run into a power pole somewhere down the road
Also, I have seen a tree fall onto power lines and knock the power out.

The worst ever was a 3 day ice storm. This event could have been predicted

It's hard to figure out when disaster is going to strike

 

[Madco]

Yea, its just that storage at 100% is hard on lithium batteries. Lead acid may perform much better for you given the light loads and the fact that the system is in a truly backup mode. I have a backup system that just supports lighting, refrigeration, and a couple of critical outlets at my house. I have used 16 Trojan L16-EAC motive batteries (about 800Ah at the nominal discharge rate) for it since they sit at 100% charge at all times. Have had the same batteries for 12 years now with no signs of degradation. The main solar energy system on the other hand uses 800Ah of LiFePO4 and those guys are cycled a couple of times a week.

 

[Zwy]

Agree with BentleyJ's math. The 45kWh daily average is a lot of energy, and that's going to require a large and costly solar energy backup system! So some (a lot of) load reduction is necessary, and that will require life style changes at least during the power outages. Probably better to do it now instead of waiting until the utility drops. Assuming the outages are infrequent and short, it will be much more cost effective to run the generator when you need it - unless you dramatically prune back that 45kWh average consumption.

We used 20Kwh and under per day in my house before throwing the switch on the solar system.

Now use between 50Kwh and 60Kwh per day as I have been using resistive heaters until we install heat pumps this year. The power is there, just need a load to use it. One could go a little smaller on the battery but I wanted the ability to run essential loads for 3 days with poor sun and this bank will handle it.

Easily accomplished with 54Kwh battery bank and 8.42Kw PV array. Net cost after tax credit is under $25K. I don't consider that to be expensive.

One of the buildings in this thread is a business and as such the tax advantage is even greater than the 30% tax credit. The system can be depreciated out as a business expense under MACRS. From this article

Depreciation​

Since solar PV systems are considered assets of the business, depreciation deductions can be taken. Businesses may take a combination of bonus depreciation and Modified Accelerated Cost Recovery System (MACRS) to write off the solar PV system. The depreciable base is the total cost of the solar PV system less one-half of the credit amount.

For example, your client placed a $500,000 solar PV system in service in 2022. Since the ITC credit rate is 30%, 15% of $500,000 cost is $75,000, making the basis for depreciation $425,000.



Basically, the business gets 1/2 of the tax credit (15%) the first year plus writes off the remaining 85% under MACRS, usually 5 years. In the example above of $500,000, the business would get the 15% tax credit plus a $85,000 deduction in the first year with a $85,000 deduction for the 4 years after.