let's talk about battery banks- I reckon the "they aren't worth it" thing is wrong

[PVfarmer]

(And saying "8 batteries for 15 panels is too much" without referencing Ah of the batteries shows you either aren't well versed in this or are not very good at communicating about the topic.)

So that's $4100 less already. By changing to lower/lesser items.
That's not actually showing that their pricing is "inflated" - just that you would choose a lower/lesser items.

You can get better batteries than those for $450/400A/6V, so that's $3600. But 8 batteries for 15 panels is too much- 4 would be plenty. So $1800.

I didn't mean amp minutes or amp days there. I figured you might have guessed, seeing as what else would it mean?
And the word "better"? Doesn't mean lesser. So that shows you aren't trying as hard as you could to read.

So a 3.8kW system in a 5 sun-hour region should have a 37.5kwh-75kwh battery bank.

And establishing battery bank size by saying "it should cost x or maybe 2x" without any info besides "the sun shines for 5 hours" is communicating?

3.9kW of PV might produce 20kWh a day. So if you were planning on using 5kWh of that at night...you'd want 4 batteries. 10kWh at night, you'd want 8. If you were in Alaska, you'd probably want 16. And more panels of course.
So I made the same mistake you keep making- I should have said "might be too much" about 8 batteries instead of saying "things are only this one way".

How many useable amp HOURS are you planning on storing in this 37.5kWh battery bank of yours? That's @ 48 volts, right?
You're the one not communicating properly. 37.5kWH of total storage drawn down 25% is a 9.4kWh bank...

 

[TommyO]

And the word "better"? Doesn't mean lesser.

You say you can get "better" batteries for $450 vs. the $500 price they have.
That may be true or may not be, but $450 vs. $500 doesn't mean to me that it's "inflated". 10% or more difference in price on similar (but not the same) items is pretty normal when you're shopping for things.

The reason I said lesser/lower is for 2 things.

1> You're saying it should be 4 batteries instead of 8. 4 IS less than 8.
(I'm assuming you mean same 6V,400Ah batteries, so doing a 24V system instead of 48V)

2> The other item you found significant savings on was the inverter/charge controller - which YOU said that instead of going with an "A" or "A-" brand, you'd go with a "B" level.
(I'd say outback is not an "A-" brand, but rather "A" - same as SMA or Solaredge. But A vs. A- doesn't really matter IMO, so for the sake of discussion I will consider it an "A-" brand as you do)

And establishing battery bank size by saying "it should cost x or maybe 2x" without any info besides "the sun shines for 5 hours" is communicating?

I didn't discuss cost.
I pointed out that other resources discussing battery pack size have the battery pack be 2x to 4x the size of the array times the number of sun-hours. Which would be at least double this kit's size - and you're saying that instead it should be halved.

3.9kW of PV might produce 20kWh a day. So if you were planning on using 5kWh of that at night...you'd want 4 batteries. 10kWh at night, you'd want 8.

So you're saying you'd take 10kWh out of a 48V, 400Ah battery pack.
And if it were 5kWh, you would run it on a 24V, 400Ah system.

Well, with that approach you're going to be replacing that battery pack in a lot less than 5 years - that's 50% DoD (more when you have a few cloudy days)

How many useable amp HOURS are you planning on storing in this 37.5kWh battery bank of yours? That's @ 48 volts, right?
You're the one not communicating properly. 37.5kWH of total storage drawn down 25% is a 9.4kWh bank...

No - I don't think it'd be called a '9.4kWh bank'.
I believe that the normal nomenclature is to refer to the size of a battery bank by the full label size, not the amount of it you use.

So it would be a 37.5kWh bank (48V, ~780Ah. So that probably means it would actually be a set of 24 of 2V/800Ah )
I would expect the DoD to be 25% (9.6kWh or less)
And my understanding is that for a battery bank of that size you'd want 3.8kW of panels (assuming 5sun-hours) Possibly you could have less if you're doing even less of a DoD.


All of this is rather beside the point though, since the discussion at hand is "Are batteries worth it".

You're saying you can get to $2-$2.50/kW for a similar battery system.
But in another post earlier you were estimating you could be at ~$1/kW for a grid-tie system.
And what's the benefit you get from that significant increase in equipment costs?
Obviously a great benefit if you don't have a POCO line anywhere near your house.
Otherwise? No economic benefit.


Since you probably didn't see my most recent post about the hypothetical situation you were discussing with wwhitney, I'll add it again here:
Assuming you can sell your energy to the POCO for $.15/kwh. And you buy it for $.25/kwh.

If it costs you an additional $.30/kwh for the batteries/equipment to store that energy would you still buy the batteries?
If it costs you an additional $.10/kwh?
If it costs you an additional $.07/kwh?

 

[PVfarmer]

1
You say you can get "better" batteries for $450 vs. the $500 price they have.
That may be true or may not be, but $450 vs. $500 doesn't mean to me that it's "inflated". 10% or more difference in price on similar (but not the same) items is pretty normal when you're shopping for things.
---
2
The reason I said lesser/lower is for 2 things.
---
3
I didn't discuss cost.
I pointed out that other resources discussing battery pack size have the battery pack be 2x to 4x the size of the array times the number of sun-hours. Which would be at least double this kit's size - and you're saying that instead it should be halved.
---
4
So you're saying you'd take 10kWh out of a 48V, 400Ah battery pack.
And if it were 5kWh, you would run it on a 24V, 400Ah system.
Well, with that approach you're going to be replacing that battery pack in a lot less than 5 years - that's 50% DoD (more when you have a few cloudy days)
---
5
No - I don't think it'd be called a '9.4kWh bank'.
I believe that the normal nomenclature is to refer to the size of a battery bank by the full label size, not the amount of it you use.
---
6
So it would be a 37.5kWh bank (48V, ~780Ah. So that probably means it would actually be a set of 24 of 2V/800Ah )
I would expect the DoD to be 25% (9.6kWh or less)
And my understanding is that for a battery bank of that size you'd want 3.8kW of panels (assuming 5sun-hours) Possibly you could have less if you're doing even less of a DoD.
---
7
All of this is rather beside the point though, since the discussion at hand is "Are batteries worth it".

You're saying you can get to $2-$2.50/kW for a similar battery system.
But in another post earlier you were estimating you could be at ~$1/kW for a grid-tie system.
And what's the benefit you get from that significant increase in equipment costs?
Obviously a great benefit if you don't have a POCO line anywhere near your house.
Otherwise? No economic benefit.
---
8
If it costs you an additional $.30/kwh for the batteries/equipment to store that energy would you still buy the batteries?
If it costs you an additional $.10/kwh?
If it costs you an additional $.07/kwh?

1
It is true. I wouldn't even buy those FR batteries, actually- where's the Peukert curve chart? And more importantly what does that chart tell us?
http://www.fullriver.com/products/dclist.htm

I'm not trying to dis that site, they are selling a package, it might take the average homeowner 100 hours of research to figure out what the best option is.
BUT they are selling the stuff at that price, I'm sure people pay for and use it and benefit from it.
However they're charging $4114 for those 8 batteries withOUT shipping. Say they come on the same pallet as the panels- you still have to add some shipping cost.
I know for a fact you can get what I have determined to be the best AGMs, 400(amp HOURS) out there for $450..that's WITH shipping!
450 * 8 = 3600.
If you paid either $4300 or $3600:
400Ah * 8 = 3200Ah
3200Ah *48V = 153.6kWh
153.6 * .25 = 38.4 kWh available

38.4 for $3600 = $93.75/kWh installed for batteries.
38.4 for $4300 = $111.68/kWh installed.

Exactly the kind of thing that DOES matter when you are trying to nail down profits. That's 17% difference, not your "estimation" of 10%.
---
2
I didn't "use less batteries".
I compared the prices on 4 or 8 of them.

So that's $2.22 a watt...60 cents/watt for panels and you're down to 2.11.
Stay with the 8 batteries and it's 2.50 a watt.

---
3
You didn't mention cost or the load being covered with the BESS, you just said "I read somewhere it should be 37.5kWH" without taking into account that every system is different.

---
4
you're going to be replacing that battery pack in a lot less than 5 years

But...have you even looked at a life cycle chart? How can you when FullRiver doesn't seem to have one available?
Red flag there!

---
5
No - I don't think it'd be called a '9.4kWh bank'.

You should check with Tesla on that one- their 7kWh Powerwall puts out 7kWh a cycle, so...? I really doubt that's 100% discharge, seeing as 100% is currently impossible.
I'd go with- it's a 48V, 400Ah bank that you are drawing 9.4kWh from.
Not- a 38.4kWh bank you are taking to 25% DOD and you get XXXX cycles- too convoluted.

All you need to know is how much it puts out a cycle, how many cycles, and price.
A 9.4kWh daily output system will last for 8 years and cost X- that's all the customer wants to know.
---
6
2 volt batteries do have advantages- you have one cell per instead of 3 cells in a 6V, so you can test/watch each cell individually, and if one 2V battery goes bad, you aren't recycling 2 perfectly good ones in a 6V.
Are there 800Ah 2V models? I thought they were usually 900Ah?

THE ADVANTAGES OF 2 V CELLS
http://solarprofessional.com/articl...h-capacity-battery-banks?v=disable_pagination
---
7
And my understanding is that for a battery bank of that size you'd want 3.8kW of panels (assuming 5sun-hours)...
All of this is rather beside the point though, since the discussion at hand is "Are batteries worth it".

Not sure how random assumptions tell us whether something is worth it.

Obviously a great benefit if you don't have a POCO line anywhere near your house.
Otherwise? No economic benefit.

Like that one- um, what's the cost from POCO? What does POCO pay for extra? You can't say "nope" without some specifics to illustrate that nope.

The price CAN be close to $1/kW for batteries- closer to $2/kW if you include the inverter in the BESS.

---
8
"Costs an additional"....additional to WHAT?:huh:

 

[PVfarmer]

I was just struck by a random thought, imagine that!

RE: BESSs and enclosures and code.
Another advantage is VRLAs exempt you from the nonconductive case requirement that comes into play when you have more than twenty-four 2V cells connected in series [690.71(D)].
http://www.codebookcity.com/codearticles/nec/necarticle690-10.htm

That is saying you could go over 48V with AGM batteries while...using a non-metal case correct? That opens a can of worms.
But which are the important worms?
400Ah at 96 volts certainly sounds impressive!!

But my question is:
Enclosures that raise the batteries off the basement floor- isn't that kind of counter-productive?
Wouldn't you want to use "passive conductive cooling", by putting the actual cells in direct contact with the 55-60 degree basement floor?
Instead of raising them up in a closed metal box where they'd run hotter and not last as long?
Am I just nitpicking there?
Seems like "non-conductive cases" would run hotter than metallic even, but...?

Could you conceivably cover the top and 4 sides of the bank while leaving the bottom in contact with the floor?
Wouldn't a small desktop computer fan (or maybe large) running cooler air through the enclosure help, no matter what it's made of?
If you haven't noticed by now, I'll entertain many types of thoughts!
You could water-cool the batteries by putting them on top of some PEX, and give your water heater a headstart... :happyyes:

 

[wwhitney]

PVfarmer, let me try one more time to show you that the LCOE of PV has nothing to do with whether batteries are economical.

Figuring out the extra cost to put energy into a battery and pull a kWh out is complicated, but let's say you've done that over the lifetime of the battery bank and come up with a $/kWh for storage. I'll call that LCOS. This is what you pay on top of the cost of the energy going into the battery.

The basic point is that if you are a net PV daytime generator, then whatever the POCO is paying/crediting you for your net generation is the cost to you for using that energy in some other way.

Say the POCO pays you $0.15/kWh for your net generation. Your neighbor comes along and says he'd like to use that energy instead of your selling it to the POCO, and offers you $0.10/kWh. You decline, because the POCO gives you more money. If your neighbor offers you $0.20/kWh to use that energy, you say "great", because that's more than the POCO will give you.

Notice that the LCOE of your PV didn't matter. Maybe you inherited the system from your dead uncle, and from your point of view the LCOE is 0. Maybe you grossly overpaid for some really low quality panels that will only last 5 years, so your LCOE on the PV is actually $0.50/kWh. It doesn't matter, those represent past costs that are fixed and can't be changed by what you do today. They don't enter into your decision process about what to do with the energy produced today.

Now, you have a PV system and want to know whether to add batteries to handle your nighttime usage. Suppose your nighttime cost is $0.15/kWh. Then there's no point at all, because you'd be giving up $0.15/kWh in daytime generation payments (your cost in this situation) to save $0.15/kWh in nighttime costs. It's a wash, there's no net savings, and any cost for the batteries is wasted money.

Suppose instead your nighttime usage costs $0.25/kWh. So if you can timeshift the energy from daytime to time, you give up $0.15/kWh in daytime credits, and save $0.25/kWh in nighttime costs. The net savings is $0.10/kWh, so there may be a case for batteries. If you can get a battery system with an LCOS of less than $0.10/kWh, great, go for it, you'll save money. If the LCOS of the batteries is bigger than $0.10/kWh, it's still not worth it.

In this decision process the LCOE of the PV had nothing to do with it. The decision is what do to with the energy you're generating, so you just compare the two different uses of that energy.

As a simple analogy, if you have a used car you don't need any more, and can sell it to two different buyers, what you paid for the car doesn't matter. The only thing that enters into your decision process is what the two different buyers are offering you.

Cheers,
Wayne

 

[ggunn]

whenever you are instantaneously generating more electricity that you are using, you get a POCO refundable credit of, say $0.15/kWh, and whenever you are consuming electricity from the POCO it costs you, say, $0.25/kWh.
Isn't that how ggunn said it works in Texas? Just with 10 and 16 cents not 15/25? BUT instead they charge you for your extra PV output?? Did I read that correctly? That's....kinda brutal. That must make the natural gas people smile...

To begin with, it's an Austin, Texas, thing, i.e., only in Austin.

The way it works is this: You have two meters - one on your PV system and one on the service entrance to your home. Austin Energy charges you your tiered charges for the sum of the two meter readings and then credits your account for your PV production at the flat $/kWh Value Of Solar they have determined. If you want to interconnect PV with Austin Energy's system, them's the rules. The point is not to make solar worth more to someone just because they consume more energy (that's what drives the tired pricing schedule).

 

[jaggedben]

Hypothetical.

P.S. While we're on the topic of batteries, I'd love to know when someone will make a relatively inexpensive turn-key AC-coupled battery inverter that talks to Enphase microinverters directly (not just via frequency shifting) to turn down production if it exceeds local demand when operating off grid. Just for backup purposes.

Fwiw, enphase announced its own battery inverters last year, based on the same micro inverter layout and even using the same trunk cable, apparently. Now whether it has the feature you have in mind, they haven't said. Not exactly clear what it's really supposed to do.

Also enphase powerline communications are not reliable enough for power control, and the current inverters have no features for it. So I don't have high hopes for your idea.

 

[jaggedben]

To begin with, it's an Austin, Texas, thing, i.e., only in Austin.

The way it works is this: You have two meters - one on your PV system and one on the service entrance to your home. Austin Energy charges you your tiered charges for the sum of the two meter readings and then credits your account for your PV production at the flat $/kWh Value Of Solar they have determined. If you want to interconnect PV with Austin Energy's system, them's the rules. The point is not to make solar worth more to someone just because they consume more energy (that's what drives the tired pricing schedule).

Why do they sum the meters? Why don't they just charge you one set of rates for consumption and credit you for production? Or is the solar meter behind the regular meter?

 

[ggunn]

Why do they sum the meters? Why don't they just charge you one set of rates for consumption and credit you for production? Or is the solar meter behind the regular meter?

Yes, the solar meter is behind the utility meter.

 

[PVfarmer]

P.S. While we're on the topic of batteries,

Good mornin' Wayne...(and everyone else)
While we're on batteries, could we maybe briefly discuss NET PRESENT VALUE???
By "discuss" I mean please someone explain it to me in layman's terms before I go nuts!
Where I am is, it seems like I should be able to tell a customer "tell your *accountant* that the NPV is so-and-so, and that's good because...."
and then...:dunce:
If you've ever heard the word "DERP", that's about all I've got.
Whatever happened to good old fashioned ROI?
If you spend $1 today, and it's worth $2 in 20 years that makes your ROI 100%, or $1, or you doubled your money... so then your NPV for that $1 "today" over the 20 years is $2?
If it's that simple...WHY BOTHER with the goofy equations for NPV?
Calculus = over complication, as far as I'm concerned!

So, to keep it simple, forget the BESS.
Say customer X procures a PV system for $10,000.
What other variables influence the NPV? Every "explanation" I find seems to be accountant >>accountant, I end up cross-eyed and give up.

From these 3 "starting points", what is the NPV for Cust. X and why does it matter?

A. Cust. X wins the lottery or gets a grant for $10,000
B. X gets a loan for the 10G at 5 years/5%...the interest on the loan has something to do with NPV-but what? There must be more to it than that.
C. X pays "out of pocket" meaning from a savings account, so is losing the interest on that, and that changes the NPV.....how?
---

Um...I see this as a making a mountain from a molehill?
C_t being...huh? "Net cash flow" is your lack of an electric bill, plus PBI (if any)...minus loan payments, if there are any...um?

The following is the formula for calculating NPV:

where:
C_t = net cash inflow during the period

C_o= initial investment
r = discount rate, and
t = number of time periods

Read more: http://www.investopedia.com/terms/n/npv.asp#ixzz3gLuz1bRa
---

And these next two, I feel like a dog chasing its tail, but not having fun.
Example: Same investment, but try it at 15%.

Money Out: $500
You invested $500 now, so PV = -$500.00
Money In: $570 next year:
PV = $570 / (1+0.15)¹ = $570 / 1.15 = = $495.65 (to nearest cent)
Work out the Net Amount:
Net Present Value = $495.65 - $500.00 = -$4.35
So, at 15% interest, that investment is worth -$4.35
It is a bad investment. But only because you are demanding it earn 15% (maybe you can get 15% somewhere else at similar risk).


Side Note: the interest rate that makes the NPV zero (in the previous example it would be around 14%) is called the Internal Rate of Return.
​

http://www.mathsisfun.com/money/net-present-value.html
---
Discount Rate Definition

What is the discount rate? The discount rate is the rate of return used in a discounted cash flow analysis to determine the present value of future cash flows.

In a discounted cash flow analysis, the sum of all future cash flows (C) over some holding period (N), is discounted back to the present using a rate of return (r). This rate of return (r) in the above formula is the discount rate.
http://www.propertymetrics.com/blog/2013/09/27/npv-discount-rate/

 

[TommyO]

400Ah * 8 = 3200Ah
3200Ah *48V = 153.6kWh
153.6 * .25 = 38.4 kWh availabale

No!
You do NOT multiply by 8 to get the Ah - the 8 batteries are in series to get to 48V.
8 of 6V,400Ah batteries makes 48V,400Ah bank.

38.4 for $3600 = $93.75/kWh installed for batteries.
38.4 for $4300 = $111.68/kWh installed.

Exactly the kind of thing that DOES matter when you are trying to nail down profits. That's 17% difference, not your "estimation" of 10%.

Well, when you arbitrarily change the price comparison from $3600 vs. $4114 to $3600 vs. $4300 yes it becomes 17% instead of the 12.5% that I rounded to 10%.

I didn't "use less batteries".
I compared the prices on 4 or 8 of them.

You quoted prices using 4 instead of 8 to get the $4100 savings.
That's using less batteries.

You didn't mention cost or the load being covered with the BESS, you just said "I read somewhere it should be 37.5kWH" without taking into account that every system is different.

I think you're confused - you're the one saying it should be smaller capacity for batteries without anything to support your statement.

But...have you even looked at a life cycle chart?

I have looked at them.
Do you think that a 50% DoD on AGM batteries is going to work for 5 years?

No - I don't think it'd be called a '9.4kWh bank'.

You should check with Tesla on that one- their 7kWh Powerwall puts out 7kWh a cycle, so...?

Do they call it a 7kwh battery bank? No.
They say it's a 7kwh Powerwall and they don't specify the battery bank within it.

Are there 800Ah 2V models?

Yes.

Like that one- um, what's the cost from POCO? What does POCO pay for extra? You can't say "nope" without some specifics to illustrate that nope.

Great - give some specifics to illustrate where there's an economic benefit for someone who is grid connected.

"Costs an additional"....additional to WHAT?:huh:

The additional expense to buy/install the battery pack and associated equipment.

Here - lets try again:

Assuming you can sell your energy to the POCO for $.15/kwh. And you buy it for $.25/kwh.

If it costs you an additional $.30/kwh for the batteries/equipment to store that energy would you still buy the batteries?
If it costs you an additional $.10/kwh?
If it costs you an additional $.07/kwh?


​

Here's a hint - The answer for the first question should be "No, I wouldn't buy a battery system in that case. It wouldn't be worth the extra cost."

 

[ggunn]

In an off grid system the size of a battery bank is determined by usage and the size of the PV is determined by the size of the battery bank.

 

[PVfarmer]

PVfarmer, let me try one more time to show you that the LCOE of PV has nothing to do with whether batteries are economical.

Figuring out the extra cost to put energy into a battery and pull a kWh out is complicated, but let's say you've done that over the lifetime of the battery bank and come up with a $/kWh for storage. I'll call that LCOS.
The basic point is that if you are a net PV daytime generator, then whatever the POCO is paying/crediting you for your net generation is the cost to you for using that energy in some other way.

And that's where out paths diverge!
What's the "other way"?

If cost from POCO = 15 cents...
The "cost" of your electricity when you are using exactly what the PV is putting out, at that one moment in time, is = to LCOE of your PV, correct?
You are waiting for the coffee maker to finish and are thinking "nice, this cost me 11.2 cents bc it's PV instead of 15 cents from the POCO".
(Side note: 15-11.2 = 3.8 cents- this affects your payback period.)

If you are putting out extra at one moment in time, your "cost" for putting it out is LCOE of PV minus POCO payment for it- it could start out as zero, and become negative (neg = you are profiting)
So if 11.2 and 15, you're making 3.8 cents from the get-go.

Now, you have a PV system and want to know whether to add batteries to handle your nighttime usage. Suppose your nighttime cost is $0.15/kWh. Then there's no point at all, because you'd be giving up $0.15/kWh in daytime generation payments (your cost in this situation)

I'm thinking you'd only be giving up 3.8 cents not 15.
If you can get a battery system with an LCOS of less than $0.10/kWh, great, go for it, you'll save money. If the LCOS of the batteries is bigger than $0.10/kWh, it's still not worth it.

And that makes me think that (if 3.8 + (LCOS of BESS) is less than the 15 POCO price) <-- is where you'd go for it.

 

[PVfarmer]

No!
You do NOT multiply by 8 to get the Ah - the 8 batteries are in series to get to 48V.
8 of 6V,400Ah batteries makes 48V,400Ah bank.

How'd ya get the 48V there? By going 6 x 8 = 48.

400Ah * 8 = 3200Ah
3200Ah *48V = 153.6kWh

is also

400Ah * 48V = 19.2kWh * 8 = 153.6

and the same as:

400Ah * 6V * 8 * 8 = 153.6

You put them together in series to make one 400Ah 48v bank.
There are still 8 batteries which are = to 24 2V cells there.
One 8 is electrical, the other is numerical.

 

[winnie]

I think that you are multiplying by 8 twice, which is incorrect.

If you are using 400Ah 6V batteries, then you have a battery with a nominal capacity of 2.4 kWh.

If you take 8 of these batteries and put them in series, then that gives you 6V * 8 = 48V at _400Ah_, for a nominal capacity of 2.4 * 8 = 19.2 kWH.

If you take the same batteries and put them in parallel, then that gives you 400Ah * 8 = 3200Ah, but only at _6V_. Again you have the same 3200Ah * 6V = 19.2 kWh.

You appear to be saying that if you take 8 batteries, each rated 6V and 400Ah, and put them together to get 48V 3200Ah; but this is _not_ how it works. When you put them in series you add voltage but keep the same Ah, and when you put them in parallel you add Ah but keep the same voltage. You _cannot_ put 8 2.4kWh batteries together to get a 153.6 kWh battery!

-Jon

 

[PVfarmer]

1 Well, when you arbitrarily change the price comparison

2 I think you're confused - you're the one saying it should be smaller capacity for batteries without anything to support your statement.

3 I have looked at them.
Do you think that a 50% DoD on AGM batteries is going to work for 5 years?

4 Do they call it a 7kwh battery bank? No.
They say it's a 7kwh Powerwall and they don't specify the battery bank within it.

5 Great - give some specifics to illustrate where there's an economic benefit for someone who is grid connected.

1 I didn't do that- you ignored all that stuff about shipping etc, basically. And you're ignoring that life cycles are directly proportional to overall price (LCOS!). See #3

2 You're saying "should be 2x to 4x" but you haven't said a single thing about: what's the kWh # for usage? Even after I pointed out how it is in fact the most important factor- how can you know how many cycles they'll last without knowing how much you are using them *including* of course the DOD. DOD being inversely proportional to life cycles and all.

3 So then which battery are you talking about? Some are 2x or more as good as others. More importantly, where's the chart for those FullRivers? I'm supposed to take your word that "yeah I looked"?
Are you even familiar with the late Mr. Peukert?
Here's what I go by, you must have missed it before, or...?:
http://www.lifelinebatteries.com/selfdischarge.php

What if you go with 15% and 4000 cycles? 4000/365 = 10.95 years. Hmm. Like I said before, 22% might be a sweet spot. So just using 25% like you want to might not be the best idea.
Saying "everyone in the world will take their batteries down 25% because I said so" is for sure the wrong way to go about it.

4 I seriously doubt they'd call it a "7kWh Powerwall" if it held 12. Or 5, or 6.6 for that matter.

• Technology Wall mounted, rechargeable lithium ion battery with liquid thermal control.
• Models 10 kWh $3,500 For backup applications 7 kWh $3,000 For daily cycle applications
• Warranty 10 years
• Efficiency 92% round-trip DC efficiency
• Power 2.0 kW continuous, 3.3 kW peak
• Voltage 350 – 450 volts
• Current 5.8 amp nominal, 8.6 amp peak output

First of all, "liquid thermal control" - or a bunch of AGMs sitting on PEX pipe with cool water running through! Conductive heat exchange is NOT rocket science. That was sort of a joke I guess.
Second, there's some weird "weekly thing" with the 10kWh model. It holds more, so you can only use it for backup power, BUT you have to cycle it weekly to keep it from...whatever bad thing happens to Li BESS if you don't use them.

Those sepcs mean: 10 year warranty = 3650 cycles, that's excellent.
It's a 400-ish volt battery...DC volts. That's the weird part!
That's why- 5.8A at 350V = 2030w, or 2.0kW continuous and maybe you get the 3.3kW peak at 383 volts?

Basically, it's pretending that it is some solar panels- the 350-450 range is to match the *required* DC to AC inverter. (Why else, other than being DC. would it require a DC to AC inverter separately?)
So 7kWh running a house at 120V, if you stay at the 2.0 continuous, it'll run for 3.5 hours and then shut itself off and go back to grid.
Make sense?

5 I did. Where are your specifics about how "batteries are never good for anyone anywhere".

 

[TommyO]

1 I didn't [arbitrarily change the price comparison]

I think it's clear that you did - but I am willing to believe you didn't understand you were doing it.

2 You're saying "should be 2x to 4x" but you haven't said a single thing about: what's the kWh # for usage? Even after I pointed out how it is in fact the most important factor- how can you know how many cycles they'll last without knowing how much you are using them *including* of course the DOD.

Yes I did - you even know I did because in your next paragraph you're saying I used 25% DoD.

What if you go with 15% and 4000 cycles? 4000/365 = 10.95 years. Hmm. Like I said before, 22% might be a sweet spot. So just using 25% like you want to might not be the best idea.
Saying "everyone in the world will take their batteries down 25% because I said so" is for sure the wrong way to go about it.

If you want 15% DoD, fine - we can go with that.
I used 25% because you did and that's fairly common choice.
If you want 50%, fine - we can do 50%. Just please pick one option and stick to it.
Because at times you've chosen 50% DoD but say the bank will last more than 10 years.
(And as you just pointed out to get 10 years you need it to be ~15%. Maybe a little more on some models and maybe a little less on other models.)

4 I seriously doubt they'd call it a "7kWh Powerwall" if it held 12. Or 5, or 6.6 for that matter.

They call it a "7kWh powerwall".
They do NOT call it a "7kWh battery bank".
The speculation I have seen is that the 7kWh and 10kWh Powerwall have the same battery bank internally - but because the 10kWh is only being cycled weekly it can have a higher DoD.

It's a 400-ish volt battery...DC volts. That's the weird part!

Why?

Seems like a good choice to me - ~350V makes it easier to create 240VAC than if you have lower voltage.
The downside is that you have to treat it differently than you would a 12V/24V/48V wires connected to typical battery bank. But since it's professionally installed and probably has the inverter mounted right next to it, that shouldn't be a problem.

5 I did. Where are your specifics about how "batteries are never good for anyone anywhere".

So which post is it where you say what situation batteries are useful for people in the US?
(and that's not a rhetorical question - I missed where you've explained when it's useful.)

Although you put it in quotes, I have not said what you are attributing to me.
I *have* said that in the US there are very few cases where they make economic sense. The cases where they make sense are when there isn't readily available POCO power.
As others have pointed out they really only make sense if price_when_selling + cost_to_store is less than price_when_buying. And that's pretty rare in the US. Anywhere that has net metering they won't make sense. (Batteries would have to be free). And anywhere with cheap power it won't make sense (because batteries have to be cheaper than price differential between selling to and buying from the POCO)

400Ah * 6V * 8 * 8 = 153.6

As has been pointed out - that is incorrect.
If you have 64 of 6V/400Ah batteries, then you have 153kWh
If you have 8 of those you only have 19.2kwH.

You can configure it as a:
48V,400Ah or
24V, 800Ah or
12V, 1600Ah or
6V, 3200Ah.
While those are possible, only the 48V, 400Ah is likely to be used by someone. The middle two have parallel/serial batteries, and most system designs avoid that if possible. And I don't know of an application where it'd make sense for a 6V, 3200Ah configuration.
All of those configurations are 19.2kWh. And just looking at the base unit (6V, 400Ah) you see it is 2.4kWh. So anything using 8 of those must be 19.2kWh

 

[wwhitney]

I originally sent this response via PM while the thread was locked:

Now, you have a PV system and want to know whether to add batteries to handle your nighttime usage. Suppose your nighttime cost is $0.15/kWh. Then there's no point at all, because you'd be giving up $0.15/kWh in daytime generation payments (your cost in this situation)

I'm thinking you'd only be giving up 3.8 cents not 15.

This right here is the problem. The POCO is going to send you a payment of $0.15/kWh for your excess generation, and charge you $0.15/kWh for your nighttime usage. Suppose in one billing cycle you generate 300 kWhs during the day and use 100 kWhs during the night. You get a payment of $45 for daytime generation, but you owe $15 for the nighttime usage, which nets out to $30 to you.

Now suppose you have some batteries and move 1 kWh from the day to the night. You generate 299 kWhs during the day, but use only used 99 kWhs at night. You get a payment of $44.85 for daytime generation, but you owe $14.85 in nighttime usage, which still nets out to $30 to you.

When you use 1 kWh during the day to charge your battery, instead of selling it to the POCO, you reduce your POCO credit by the full $0.15. That means charging the battery costs you $0.15/kWh.

The LCOE of the PV has nothing to do with it. Where the LCOE of PV comes into decisions is whether to install solar and how much solar to install. It doesn't affect decisions about installing batteries.

In fact, if you are on a TOU plan even without PV you always have the option to install batteries to do time of day arbitrage on the electricity rates. The only way PV interacts with batteries decision-wise is that (a) because PV generates during the day and not at night, you may effectively now be on a TOU plan and (b) the batteries now may provide the extra benefit of being a backup when the grid goes down, because the PV provides a way to recharge the batteries without the grid if you have the right kind of inverter.

Cheers, Wayne

 

[PVfarmer]

So which post is it where you say what situation batteries are useful for people in the US?
(and that's not a rhetorical question - I missed where you've explained when it's useful.)

You sure missed it alright. in the state where you live for one.

Substantial advances to energy storage technology is helping to make rooftop solar that much more attractive. Fittingly, the advances have emerged from the fuel-hungry auto industry. Innovations in hybrid and electric vehicles over the past decade have had to contend with the battery problem: how to create relatively small, but long-lasting, fast-charging, durable and scalably priced batteries for vehicular workhorses? It turns out that what works for fast cars may be perfect for power-hungry homes and offices.
The power-storage solution resolves a nagging issue with many renewable energy sources: variability of output. The sun sets, and the wind doesn’t always blow. Generated power usually flows right into the grid, but when you most need it, consumers have to draw down from some upstream power plant that burns coal or natural gas.
Cheaper, more convenient, high-capacity batteries allow renewable power to go completely off-grid. This development is radical enough that it could eventually disrupt utilities in both the US and Europe, says Byrd. “In the US, Hawaii and California represent the greatest opportunity for going off-grid due to their significantly higher electricity rates and strong solar conditions,” he says.
That’s the kind of solar revolution that just might redefine the whole idea of “energy independence.”
http://www.morganstanley.com/ideas/economics-of-solar-power-on-the-rise/

http://forms.greentechmedia.com/Ext... & Energy Storage Blue Paper July 29 2014.pdf

 

[PVfarmer]

When you use 1 kWh during the day to charge your battery, instead of selling it to the POCO, you reduce your POCO credit by the full $0.15. That means charging the battery costs you $0.15/kWh.

But that 1kWh is used at night, instead of buying it from the POCO, so...it cost you 15 cents but saves you POCO cost minus 15 cents. If POCO charges 20 cents, you saved 5.

But I still think if LCOE + LCOS < POCO price, why wouldn't you (get the BESS).
Or (same thing) LCOE < POCO price - LCOS.

LCOS being...($ for BESS *100 / lifetime kWh used from BESS) minus LCOE of PV.
If the LCOE of PV is 12 cents and ($ for BESS*100 / lifetime kWh) is 24 cents, the LCOS is 12 cents, as you've already done the LCOE for PV, so the input to the BESS isn't technically FREE, but more paid for previously.