PV Array and Battery size

[Grouch1980]

Hi all,
I came across 3 other questions:

1. If you have a solar PV array that is ground mounted, is there a max distance that the array can be located from a house? I'm aware of losses in the wiring if the distance is too far, just curious if anything in the NEC dictates a max distance between a house and PV array.

2. With a ground mounted array, is there a limit on the max wattage size of the array? since the array is now out in the open, theoretically now you can have as many solar modules as you want, and are not limited by the roof area if installed on the roof of the house. Understood that depending on how i connect to the utility service, can limit the size (connecting to the supply side or load side). Just wondering if there was a max wattage rating.

3. I've been looking through LG and Tesla batteries... their KWH ratings don't seem to make them last very long. Are there batteries out there that can last multiple days, to provide back up power for the critical loads panel? Assuming it's a lot of cloudy days, and utility power is out.

Thanks!

 

[Carultch]

1. No. The NEC doesn't have an upper limit to how far it can be from a house. Or more generally, from the building served by the array. If your solution can accommodate voltage drop, and all land involved is land that the same owner controls, it can be as far away as desired.

2. Not directly in the NEC. The interconnection rules will govern your maximum amperes (which are proportional to the Watts of the sum total AC capacity), with the best case scenario being a supply side interconnection that can fully utilize the rating of the service. You can also upgrade your entire service if desired to build a larger system.

If there is a limit to the total kilowatts, it is usually utility policies that would govern this. Either that, or incentive programs. Some utilities could require significantly more protective equipment for larger scale systems.

3. You are probably assuming the maximum possible consumption rate (units of kilowatts) when you see the kW-hr values and the corresponding runtime values. In reality, it is unlikely that your loads would really do this, since your loads aren't going to always draw the full capacity at once. And likely, not even draw the full capacity. You'll need to look at realistic load profiles of the critical loads, to determine if a given storage capacity is sufficient.

 

[Grouch1980]

(which are proportional to the Watts of the sum total AC capacity)

Thanks! I followed everything, except for this. The max amps of the PV array is proportional to the watts to the AC service?

 

[wwhitney]

Thanks! I followed everything, except for this. The max amps of the PV array is proportional to the watts to the AC service?

What he means is that your service's maximum interconnection will be determined in amps, and that will be proportional to the maximum AC wattage of inverters you may install (not DC rating of the panels). The constant of proportionality being the service voltage rating, for single phase.

E.g. 120/240V service, service conductor ampacity is 200A, maximum interconnected PV amps will be 160A or 200A depending on how you do it and the NEC year, and so the connected inverter rating(s) will be capped at 240V * 160A = 38.4 kW or 240V * 200A = 48 kW.

That's for a grid-tied system without any Power Control System (PCS). Under the 2020 NEC, with a PCS that limits the current going back to the grid, you could install more inverter power.

Cheers, Wayne

 

[Grouch1980]

What he means is that your service's maximum interconnection will be determined in amps, and that will be proportional to the maximum AC wattage of inverters you may install (not DC rating of the panels). The constant of proportionality being the service voltage rating, for single phase.

I got it. I thought 'AC capacity' was implying the service capacity. It's the inverter capacity. Understood now.

Thanks guys!

 

[ggunn]

I got it. I thought 'AC capacity' was implying the service capacity. It's the inverter capacity. Understood now.

Thanks guys!

Actually, it's both. By code you are limited to a PV system whose AC maximum current rating does not exceed the capacity of the service it is connected to. Your AHJ may have further limitations, but that's the biggest your system can be by code

 

[Grouch1980]

Actually, it's both. By code you are limited to a PV system whose AC maximum current rating does not exceed the capacity of the service it is connected to. Your AHJ may have further limitations, but that's the biggest your system can be by code

That I follow.

 

[gadfly56]

1. No. The NEC doesn't have an upper limit to how far it can be from a house. Or more generally, from the building served by the array. If your solution can accommodate voltage drop, and all land involved is land that the same owner controls, it can be as far away as desired.

2. Not directly in the NEC. The interconnection rules will govern your maximum amperes (which are proportional to the Watts of the sum total AC capacity), with the best case scenario being a supply side interconnection that can fully utilize the rating of the service. You can also upgrade your entire service if desired to build a larger system.

If there is a limit to the total kilowatts, it is usually utility policies that would govern this. Either that, or incentive programs. Some utilities could require significantly more protective equipment for larger scale systems.

3. You are probably assuming the maximum possible consumption rate (units of kilowatts) when you see the kW-hr values and the corresponding runtime values. In reality, it is unlikely that your loads would really do this, since your loads aren't going to always draw the full capacity at once. And likely, not even draw the full capacity. You'll need to look at realistic load profiles of the critical loads, to determine if a given storage capacity is sufficient.

Regarding 3., you can certainly get an idea from your current connected service billing, if this is an existing home. Based on our billing, we use about 30 kW-hr per day in the heating season, and 50 kW-hr per day in the cooling season. We'd need four (4) Powerwall II's at a cost of $7,500 each to cover one day's worth of electricity usage in the summer. That's $30,000, and personally a bit rich for my taste. To answer the OP more directly, you will not find any magic battery of the Powerwall type that will go for days. You could certainly use load shedding to extend your battery run time; we could use fans in the summer instead of AC, for example.

 

[pv_n00b]

Assuming this is a residential system with a NEM interconnection the size is usually limited by the utility to only produce the amount of energy used by the home annually. Usually, the owner gets little or nothing for over-production in the true up. As for batteries, you can add more batteries until you get to the storage you need or your budget is exceeded. Typically, people decide that it's not worth the money to have more than a day of backup, and that's for a reduced backup load on a backup panel.

 

[Joe.B]

I don't know how practical this idea actually is as I've never seen it in use, but for battery capacity check out flow batteries.

A basic description can be read here:

https://en.wikipedia.org/wiki/Flow_battery

 

[gadfly56]

I don't know how practical this idea actually is as I've never seen it in use, but for battery capacity check out flow batteries.

A basic description can be read here:

https://en.wikipedia.org/wiki/Flow_battery

I've seen info on these before. They have relatively low energy density. Instead of hanging on your garage wall like the Powerwall II, it would fill the garage bay. The classic flow battery runs on vanadium oxide, which is pricey. But if you have the space and money, why not?

 

[jaggedben]

It's worth noting that the way batteries can go for days and days is if they are recharged each day by the solar systems connected to them. This is of course highly dependent on the weather and season, but it's still a thing.

 

[Grouch1980]

Thanks everyone for the feedback.

 

[ggunn]

It's worth noting that the way batteries can go for days and days is if they are recharged each day by the solar systems connected to them. This is of course highly dependent on the weather and season, but it's still a thing.

And, obviously, the other factor is how many, how large, and for how long the loads are that you are feeding from the batteries.

 

[gadfly56]

And, obviously, the other factor is how many, how large, and for how long the loads are that you are feeding from the batteries.

...and how much cash you have to burn. Although I'm sure you could design an expandable system and add modules later.

 

[Grouch1980]

Is this something that you guys do on some projects, or see designs such as this?

http://www.dnr.louisiana.gov/assets/TAD/pdfs/STANDALONE_PV_Sizing_Guide.pdf

If you scroll to page 6, and look at the table on the bottom, they are selecting a battery cell of 478 amp-hours and hooking up 12 batteries together (combination of series and parallel connections). This was to give 7 days of autonomy.

I understand this is for a stand-alone system, with no connection to the grid. I'm just curious what's more common / industry standard when it comes to batteries for AC or DC coupled systems... purchase a pre-built battery from LG or Tesla, or do you sometimes see designs with custom made battery banks? I would assume pre-built batteries are the way to go, especially if it has to communicate with the multimode inverter.

 

[jaggedben]

...and how much cash you have to burn. Although I'm sure you could design an expandable system and add modules later.

Whereas a solar system may actually pay for itself and for the backup batteries, other backup options are closer to just burning cash.

 

[ggunn]

Is this something that you guys do on some projects, or see designs such as this?

http://www.dnr.louisiana.gov/assets/TAD/pdfs/STANDALONE_PV_Sizing_Guide.pdf

If you scroll to page 6, and look at the table on the bottom, they are selecting a battery cell of 478 amp-hours and hooking up 12 batteries together (combination of series and parallel connections). This was to give 7 days of autonomy.

That sounds like lead acid batteries; LA batteries are harder to maintain and easier to damage, and you can only use 50% of their total capacity. Plus they are big and heavy.

 

[Grouch1980]

That sounds like lead acid batteries; LA batteries are harder to maintain and easier to damage, and you can only use 50% of their total capacity. Plus they are big and heavy.

I'm trying to look it up on Google, I can't find anything on their battery selection.. it's the Exide 6E95-11... I guess it's lead acid.

So LG and Tesla batteries are Lithium Ion. Would lithium ion batteries be sold in a way where you can custom make a battery bank, such as in the example I gave in the link (for example, making a custom made bank that can offer multiple days of back up)? or is it industry standard to stick with the pre-made models that LG, Tesla, and other companies offer?

 

[ggunn]

I'm trying to look it up on Google, I can't find anything on their battery selection.. it's the Exide 6E95-11... I guess it's lead acid.

So LG and Tesla batteries are Lithium Ion. Would lithium ion batteries be sold in a way where you can custom make a battery bank, such as in the example I gave in the link (for example, making a custom made bank that can offer multiple days of back up)? or is it industry standard to stick with the pre-made models that LG, Tesla, and other companies offer?

I have heard that the individual Li ion batteries are about the size of AA batteries; if that's true then no, but I'm pretty sure that some companies offer rack mounted stackable units that you can combine.