Backup Battery vs Backup Generator

[Jeff Thomasson]

I'm new to this site I recently inquired about a Tesla backup battery to power my whole house when we have power outages due to rolling power outages and it can be out from 4 hours or more.
We have 35 panels on our house but even during the day with out battery backup we have no power !
The Battery Backup for 1 battery pack was over 12k and would not be enough to power the entire house so to do so multiple would be required but how long will the battery last.

I have not inquired about a backup generator yet but i have seen the Generac generators advertised any advice will be appreciated.

 

[newbostonconst]

You need a hybrid inverter and then separate out what you want to run off it and put those circuits in what is called a critical loads panel. (it is just a sub panel run off the inverter). Then calculate out the loads you have and to size the battery and the inverter accordingly.

You will have to see what hybrid invertor will work with the 12k of solar. I listed Alte below because they can help you choose components and have a nice list of most of the options in one place. I have had good luck working with them.

This is something an electrician should do....It is complex for DIY.

Hybrid Inverter | Hybrid Solar Inverter | altE

Hybrid inverters, mostly used in grid-tie solar systems, can provide backup power when the electric grid fails. Call 877-878-4060 to size your system today.

www.altestore.com

 

[newbostonconst]

O' and welcome on board....

 

[Hedges]

We have 35 panels on our house but even during the day with out battery backup we have no power !

I have not inquired about a backup generator yet but i have seen the Generac generators advertised any advice will be appreciated.

What brand and model PV inverter?
Several brands have their own battery inverter which the PV inverter will play nice with.
There are some mix-and-match arrangements which would work well.

Some may claim a battery inverter with frequency shift (technique to request reduction in power production) can work with any grid-tie inverter but it isn't great, causes battery to cycle repeatedly as PV inverter goes offline for 5 minutes then returns at full power. Best to pair battery inverter with PV grid-tie inverter that share a method of power regulation.

Generators - only if you gotta have power and maybe want to run everything. Or run lots of loads at night. PV with a small amount of battery can run your air conditioner during the day. It takes lots of expensive battery to run heavy loads at night, and power failure rather than completely off-grid means you only use what you paid for occasionally. Battery vs. generator vs. battery inverter automatically starting generator is a more complicated and expensive topic. As you have noticed ... but if you can right-size your critical loads then very small battery can work.

 

[Jeff Thomasson]

What brand and model PV inverter?
Several brands have their own battery inverter which the PV inverter will play nice with.
There are some mix-and-match arrangements which would work well.

Some may claim a battery inverter with frequency shift (technique to request reduction in power production) can work with any grid-tie inverter but it isn't great, causes battery to cycle repeatedly as PV inverter goes offline for 5 minutes then returns at full power. Best to pair battery inverter with PV grid-tie inverter that share a method of power regulation.

Generators - only if you gotta have power and maybe want to run everything. Or run lots of loads at night. PV with a small amount of battery can run your air conditioner during the day. It takes lots of expensive battery to run heavy loads at night, and power failure rather than completely off-grid means you only use what you paid for occasionally. Battery vs. generator vs. battery inverter automatically starting generator is a more complicated and expensive topic. As you have noticed ... but if you can right-size your critical loads then very small battery can work.

I am looking to be able to run AC and keep everything on if possible even at night. Even though it doesn't happen that often it is not uncommon for our power company to have rolling power outages on the hottest days & nights because of the wild fires in California. Just like yesterday it was 113°F at the hottest part of the day and even over 100°F after dark late into the night they were threatening possible power outages.
we have lost power as much as 5 or 6 hours and in the dark.

I know the Generac could power my whole house and as long as it has gas and it could be hooked up to our natural gas that we have on our house already.

My understanding of the Battery Pack is that it would supply energy every night to a point where it would then go back to the grid till charged again the next day is that correct ?

 

[Supervstech]

I am looking to be able to run AC and keep everything on if possible even at night. Even though it doesn't happen that often it is not uncommon for our power company to have rolling power outages on the hottest days & nights because of the wild fires in California. Just like yesterday it was 113°F at the hottest part of the day and even over 100°F after dark late into the night they were threatening possible power outages.
we have lost power as much as 5 or 6 hours and in the dark.

I know the Generac could power my whole house and as long as it has gas and it could be hooked up to our natural gas that we have on our house already.

My understanding of the Battery Pack is that it would supply energy every night to a point where it would then go back to the grid till charged again the next day is that correct ?

Charg/use parameters should be adjustable for most configs.
Best advice is check with the company that makes your inverters.

 

[Hedges]

I am looking to be able to run AC and keep everything on if possible even at night. Even though it doesn't happen that often it is not uncommon for our power company to have rolling power outages on the hottest days & nights because of the wild fires in California. Just like yesterday it was 113°F at the hottest part of the day and even over 100°F after dark late into the night they were threatening possible power outages.
we have lost power as much as 5 or 6 hours and in the dark.

I know the Generac could power my whole house and as long as it has gas and it could be hooked up to our natural gas that we have on our house already.

My understanding of the Battery Pack is that it would supply energy every night to a point where it would then go back to the grid till charged again the next day is that correct ?

Batteries cost more than grid power, so don't try to use them to save money while the grid is up.
Running an A/C at night with a natural gas generator could be the most economical.
If you want to run a 3000W A/C for 10 hours at night, that would draw 30 kWh from batteries (assuming it never shuts off).
Since this is occasional power failure, you could use 40 kWh of AGM battery, drained to 75% DoD. Might last 400 cycles.
Cost is about $10,000 for SunXtender batteries like I have (mine is 20 kWh costing $5000.) FLA would be cheaper.
Lithium is an option for longer life (not needed for occasional power failures) but costs more (except possibly DIY).

You need to determine exactly how many kWh you want to supply at night to size the system.
Is your area dry enough to use a swamp cooler?
Could you build an ice house, freeze water during the day with PV running a freezer and use that to cool house at night?

 

[newbostonconst]

Just a note....When I had small batteries and ran them all the way down there wasn't enough power to bring the inverter back online to start charging off solar again in the morning.

 

[dmholmes]

I am looking to be able to run AC and keep everything on if possible even at night. Even though it doesn't happen that often it is not uncommon for our power company to have rolling power outages on the hottest days & nights because of the wild fires in California. Just like yesterday it was 113°F at the hottest part of the day and even over 100°F after dark late into the night they were threatening possible power outages.
we have lost power as much as 5 or 6 hours and in the dark.

Would a smaller window unit AC suffice for these occasions? Much less power required for a 5,000 BTU AC.

 

[Hedges]

Just a note....When I had small batteries and ran them all the way down there wasn't enough power to bring the inverter back online to start charging off solar again in the morning.

Exactly. That can happen during the day as well, if loads exceed PV. With only AC coupled PV, you need a load-shed relay so battery inverter can disconnect loads. Some DC coupled PV is another way, but then only that much PV is available to recharge to the point of reconnecting. With Sunny Island and AGM batteries, my load-shed disconnect occurs at 70% DoD and reconnect at 50% DoD. Better to also separate less important loads and shed them at a higher SoC. I plan to do that for refrigerators, A/C, laundry, yard lights, pool equipment. That way I can run all loads, not just critical loads, off PV when enough power is produced.

 

[GXMnow]

I am also in So Cal and have added storage to my system. We have not had many "Public Safety Power Shutoff" events here, but they keep threatening them and I want to be ready before it becomes bad. I did choose a larger 6,800 watt inverter that could run my A/C if I need to but I currently don't plan on moving the A/C to my critical loads panel. I only have 17 KWH of lithium battery and just under 4,000 watts of AC coupled Enphase solar. With the base programming in the Schneider XW-Pro battery inverter, it "Should" be able to do the backup mode quite well. I have not done a power cut test yet as I have not moved all my critical loads over yet. When my solar was installed, they moved all the breakers around, and didn't bother to correct the labels. So I have a circuit hunt on my hands. If we have a power cut, I can move them fairly quick as it will be easy to find while the power is off.

The main problem with the software on AC coupled setups is if you run the battery too low, they have to shut off the inverter, and if that happens, then the solar can't function. For AC coupled to work, the battery needs to form the grid. There are a few different ways to handle this.

The best way is to have enough battery that it should not be a problem. If you want to run whole home A/C, that is a ton of battery. But for my limited "essential loads" I think I will be fine.

The next step is having a load dump system. I may add this later to mine. Basically, if the battery falls below a certain threshold it will cut power to some less essential loads. The idea being, that the remaining battery should run the small load left and have some left to fire up the solar the next day. Obviously, the first thing to go would be the whole house A/C.

My initial system is just going to have the battery state of charge cut set up at about 40% remaining. This will shut off the inverter and I will lose power if I use too much. But I will have plenty left for when I need to get running again. Bu to do that, I will need to go in with a battery powered laptop and change the cutoff setting lower to bring the inverter back online and get the solar running to charge it back up. This is not ideal and a dead laptop battery could make it a serious pain. The PLC that I am programming for managing the inverter settings will be able to do this change as well, so I will program that to a button by the inverter, and the PLC will be able to run off the battery bank.

As much as people want to go on running everything when there is a power failure, I just don't think it is cost effective now. Batteries with enough storage and a powerful enough inverter do get very expensive. I can see it working with an auto start generator, but that adds another layer of comlpexity as well. I do have a generator, but it is a manual changeover and start. If I do have an issue with my battery getting too low, I can fire it up and charge quite fast with 5,000 watts. But I have to have my solar isolated when the generator is on. They can't feed into each other. But at that power level, I could get enough into the battery in 20 minutes to then fire up the inverter and let the solar take over again.

Every situation is unique, and what people think is "Essential" to keep running is different for different people. To design a system for your needs, you really need to take account of what you need to run and how much power and energy that will take to decide how much needs to be battery, how much solar it will take to keep it alive, and how much generator if there is not enough sun.

To the original poster...
12 KW is a lot of solar. I don't know of any single battery inverter that could take all of that in AC coupled mode. 2 or 3 units ganged might work. Or split it and do two separate systems. A larger one that can do the A/C etc., but then a smaller setup for the truly critical loads. What inverter(s) are being used in the original solar power setup with 35 panels? What is your normal daily production? What is your usage from the utility? To backup the whole house, you are basically making an off grid system with the grid as a backup. Try using the off grid design calculator at the AltE Store.

Designing an Off-Grid Solar System - 6 Steps | altE Solar Blog

Interested in designing an off-grid solar system? altE's Solar Queen has 6 steps to get you started.

www.altestore.com

 

[Hedges]

12 kW of PV could pass through two Sunny Island (120V, 56A relay).
A single Sunny Island with 120/240V transformer could manage 12 kW of PV on its output (throttles back power production by frequency shift, sucks down about 6 kW max to battery if battery large enough).
I originally commissioned my system as 2S1P SI with 100 Ah, 48V battery then made it 2S2P SI with 400 Ah.

My PV inverters are on a separate panel after Sunny Island.
I have interlocked manual breakers to backfeed main panel, like you would with a generator.
I can run A/C all day direct off PV, but at night have to manually minimize loads because battery can only supply about 1200W average.

Using a generator with PV would work with this system, because SI would disconnect generator to prevent backfeed.

PV is cheap, batteries are expensive. Installing excess PV and running appliances primarily off that with batteries only for surge is one way to go.

 

[tomvee]

I am installing a main panel lockout plate to separate the Main breaker from a new 30A 240V breaker connected to a 4 wire 30A 240V male generator plug.(L14-30) The generator plug will be powered from 12V - 110V 3000W inverter. The inverter has 2 female 110V 15A outlets. Can I connect the 2 hot leads of the 110V plugs to the 2 hot leads of the L14-30. I would also connect the common grounds but leave the neurals disconnected. Would this power the two sides of my main panel? I would not be using any 240v breakers in the panel ie oven, AC etc
thank you

 

[Hedges]

I am installing a main panel lockout plate to separate the Main breaker from a new 30A 240V breaker connected to a 4 wire 30A 240V male generator plug.(L14-30) The generator plug will be powered from 12V - 110V 3000W inverter. The inverter has 2 female 110V 15A outlets. Can I connect the 2 hot leads of the 110V plugs to the 2 hot leads of the L14-30. I would also connect the common grounds but leave the neurals disconnected. Would this power the two sides of my main panel? thank you

It needs neutral connected.
Check the two 110V outlets, see if you have 220V across the two.

There is a hazard in using two 110V plugs this way. If one 110V plug from main panel L14-30 goes to inverter but other doesn't, the one that isn't plugged in can have 110V on exposed pins (circuit completed by a 220V appliance in breaker panel.)
Better to add a 220V outlet on your inverter.

Probably, your inverter doesn't put out 220V, both 110V outlets are in phase. So you would need a 110/220V transformer.
Or just use one 110V outlet to power your main panel.
Powering both legs of your panel with the same 110V could also work, but potentially would cause a neutral wire to carry > 20A, and that isn't fused so could be overloaded.
110V/220V transformer is the best way to go. And wired to suitable point in inverter, since 15A outlet shouldn't carry 3000W

 

[tomvee]

Hedges, thank you ; I realize that this would make a suicide cord if neutrals are connected ; so you are correct that this is not safe but I saw on youtube where this worked without the neutrals & it seemed safe. ..ie only hot & ground to each side of L14-30...
wanted to get another opinion; thx

 

[Hedges]

Hedges, thank you ; I realize that this would make a suicide cord if neutrals are connected ; so you are correct that this is not safe but I saw on youtube where this worked without the neutrals & it seemed safe. ..ie only hot & ground to each side of L14-30...
wanted to get another opinion; thx

It's only the hots that make it a suicide cord. (unless its a cheap inverter where hot and neutral are both hot.)
The two hots are probably both same voltage, two outlets from the same supply/phase because only one phase. So that wouldn't power anything.
"ground" might have been part of the circuit. But if one of those cheap inverters, hot would only be 60 Vrms from ground.
If someone had a 120/240V inverter and connected both hots but not the neutral, it would work badly, putting 240V across two circuits in series. If identical loads, it might work. I did this as a kid (by opening the neutral) and put refrigerator in series with a lightbulb. Refrigerator won the tug-of-war.

Find a way to hook it up so it only presents 120V to 120V circuits or supplies 120/240V split phase, doesn't get overloaded, and doesn't have a "no ground allowed" neutral unless you put it through an isolation transformer.

I hope that is a pure sine wave inverter, the "modified sine wave" (which I call modified square wave) work with some loads but not others.

 

[Supervstech]

Hedges, thank you ; I realize that this would make a suicide cord if neutrals are connected ; so you are correct that this is not safe but I saw on youtube where this worked without the neutrals & it seemed safe. ..ie only hot & ground to each side of L14-30...
wanted to get another opinion; thx

Neutrals are required for power to function on 120v.

 

[GXMnow]

Some cheap inverters do not have a real neutral. To save cost, the only produce about 130 volts DC above the negative battery input, and then they switch both prongs of the outlet so it reverses the polarity 120 times a second to make the 60 hz wave. But the "hot" just has half the wave, and the "neutral" has the other half of the wave. Not a real neutral. If it was connected to a ground bonded panel it would be shorting out one of the half bridge outputs. My crap 700 watt one works like that. It is okay for many small loads that can handle 100-240 universal input, direct to a power transformer systems, and wall packs and such, but don't use it for any high power gear or delicate electronics. I had an old VCR that had an input filter with some kind of capacitor network on the line cord input, and it started to smoke. The VCR still worked fine, but the noise filter was toast.

Any "hard wire" inverter should have the neutral at the chassis ground potential, even if it is isolated, it should handle being tied. And the DC input should be isolated fully from the AC outputs and also be safe to have the negative input tied to chassis ground without damage. This can be done with both HF and LF inverters that are well made and designed right.