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

[ggunn]

Seriously? You wanna start talking about WIND POWER now?

Absolutely not.

 

[PVfarmer]

In CA you get paid a small amount for the excess generation that you didn't consume in that fiscal year.
Other states could treat it differently.
But with net metering it is possible to get a small check back from the POCO for excess power generated.
Usually the payment per kwh is close to the PV LCOE, so you don't want to generate a whole bunch extra.

If you add a battery system, you now have additional costs to build the system but it doesn't reduce your bill with the POCO any.

So let me guess, this is just more PR?
I honestly don't understand this sentence at all:
Usually the payment per kwh is close to the PV LCOE, so you don't want to generate a whole bunch extra.

One of the report’s more interesting findings is that state incentives aren’t as critical as many in the industry make them out to be. That’s according to an analysis of solar-storage payback for a typical customer of California utility Pacific Gas & Electric, which showed the payback both with and without the state’s existing Self-Generation Incentive Program (SGIP), which provides up to half the cost of a typical 10- to 30-kilowatt commercial-scale system.

“Although incentives are an important value-adder, the economics for commercial customers can be acceptable even without them,” Manghani noted. “The returns for PG&E territory would be ~4 percent lower without the SGIP, but still within acceptable limits.”
Perhaps more important changes on the horizon are the potential state-by-state reforms to net energy metering (NEM), the mechanism by which solar customers in California, New York, Hawaii, Arizona, and most other states pay customers for their solar energy. Utility commissions in twenty-one states have discussed or are discussing NEM-related rate reforms, not all of which will have a direct impact on solar-plus-storage economics, according to Manghani. But there are some reforms that could have a positive impact on adding storage to solar.

Those include proposals to roll back net metering from retail to wholesale rates, adding fixed charges to residential customer bills, or introducing value-of-solar tariff structures that end up reducing the value that customers get for the kilowatt-hours of solar their home systems pump onto the grid. All of these measures could make storing solar energy more attractive, particularly if they include time-of-use tariffs with significant differences in peak vs. off-peak rates.
http://www.greentechmedia.com/artic...US-Solar-Storage-Market-Reaches-1B-in-3-Years

 

[TommyO]

I honestly don't understand this sentence at all

"Usually the payment per kwh is close to the PV LCOE, so you don't want to generate a whole bunch extra."

Ok.
Usually the payment is somewhere around $.04/kwh to $.06/kwh
The LCOE for PV installs is close to that as well (maybe $.05/kwh or maybe $.07/kwh)

SO
If increase the array to larger than needed, and generate an extra 1000kwh, then the owner gets paid $40 each year.
However the owner paid $1400 to buy the capacity to generate those extra 1000kwh.
That means the owner doesn't gets all his money back out of that investment. (but at least gets most of it)

Or at the other end of the pricing, it might be that the extra 1000kwh gives the owner an extra $60 each year.
And if he only paid $900 extra for that additional capacity he gets his money back in 15 years.

So upsizing the array to generate excess isn't going to be a great benefit, nor is it a huge detraction either. Most likely generating too much will slightly push out the PV owner's pay-back time on the investment.

 

[PVfarmer]

"Usually the payment per kwh is close to the PV LCOE, so you don't want to generate a whole bunch extra."

Ok.
Usually the payment is somewhere around $.04/kwh to $.06/kwh
The LCOE for PV installs is close to that as well (maybe $.05/kwh or maybe $.07/kwh)

SO
If increase the array to larger than needed, and generate an extra 1000kwh, then the owner gets paid $40 each year.
However the owner paid $1400 to buy the capacity to generate those extra 1000kwh.
That means the owner doesn't gets all his money back out of that investment. (but at least gets most of it)

Or at the other end of the pricing, it might be that the extra 1000kwh gives the owner an extra $60 each year.
And if he only paid $900 extra for that additional capacity he gets his money back in 15 years.

So upsizing the array to generate excess isn't going to be a great benefit, nor is it a huge detraction either. Most likely generating too much will slightly push out the PV owner's pay-back time on the investment.

Well here in my state the regular "payment" for net metering is 24.5 cents (limited total kWh) and if you do NOT net meter you can get 41 cents (for 15 years. no kWh limit).
How does that affect your calculations?

That means the owner doesn't gets all his money back out of that investment.

I think you'll find my (real) numbers have an effect!
So if you were being realistic instead of argumentative, you might say "BESS works in X states but not Y states".

 

[TommyO]

I think you'll find my (real) numbers have an effect!
So if you were being realistic instead of argumentative, you might say "BESS works in X states but not Y states".

So you want to look at RI - sure why not.

We'll need a bit more information in order for to do that.

Preferably with links to the information since I was not able to find anything about $.245/kwh on nationalgrid website

* price per kwh when selling to the POCO
* price per kwh when buying from the POCO
* how net metering is calculated in RI. (looks like it's a per-month net metering with actual $ being paid out each month)
* estimate for $/kwh for batteries and related equip. (I think Powerwall has been estimated to be about $.30/kwh - so that's probably the upper end of the range)
* estimate for $/kwh for PV generation (PV LCOE)
* How many kwh are generated
* How many kwh are consumed

 

[PVfarmer]

So you want to look at RI - sure why not.

We'll need a bit more information in order for to do that.

Preferably with links to the information since I was not able to find anything about $.245/kwh on nationalgrid website

* price per kwh when selling to the POCO
* price per kwh when buying from the POCO
* how net metering is calculated in RI. (looks like it's a per-month net metering with actual $ being paid out each month)
* estimate for $/kwh for batteries and related equip. (I think Powerwall has been estimated to be about $.30/kwh - so that's probably the upper end of the range)
* estimate for $/kwh for PV generation (PV LCOE)
* How many kwh are generated
* How many kwh are consumed

The selling to POCO prices changes depending on which program you're in- net metering gives you the WORST rate, which is equal to...

Rhode Island price for electric

21.07

<----sell and buy, net metered

http://www.eia.gov/electricity/monthly/epm_table_grapher.cfm?t=epmt_5_6_a

You get paid 24.5 cents thorough the other *better* program!
Here's a task if you are so interested- tell me the system kWh nameplate size limit with the 24.5 cent rate. (link at end of comment, you'll want to look at the REGP (Growth Program))

How NM is calculated is explained. Pretty simple- if you generate less than you use, you pay for what you use.
If you generate more than you use- YOU GET ZERO MONEY FOR IT.
That's a good deal? Really?

30 cents for a Powerwall is way off- it's 10 - 15 cents.

You already gave a PV LCOE for your state- how would it be different here?
Make it higher here than yours in Cali just for fun, call it 10 cents- NM is STILL no good!!

* How many kwh are generated
* How many kwh are consumed

As you can see, there is NO REAL REASON to generate more than you consume when NMing, unless you really like National Grid and want to pay too much for PV *and* give them free energy!
Get it?? I do NOT like net metering!! Not on my plate, not in my state!

• If the DG customer is a net importer (using more energy on-site than it generates) during the billing period, then National Grid will simply bill the DG customer for the net energy use (in kWh) during the billing period (which has already been reduced due to the on-site generation).
• If the DG customer is a net exporter (generating more energy on-site than it uses) during the billing period, then National Grid will: (1) bill the DG customer for zero kWh of energy usage and (2) credit the DG customer for the net energy exported to the grid during the billing period in the form of Net Metering Credits (NMC). Refer to the "Calculating Net Metering Credits" and "Allocating Net Metering Credits" sections below for more details.
• In all cases, DG customers will still be responsible to pay for any demand charge (measured in kW) and/or customer charges associated with the account. These charges will not be credited back to DG customers.
  
  Customers with Net Metering Services should NOT expect to receive a check or payment as compensation for accrued NMC.
  
  http://www.nationalgridus.com/narragansett/business/energyeff/4_net-mtr.asp

 

[TommyO]

The selling to POCO prices changes depending on which program you're in- net metering gives you the WORST rate, which is equal to...

Rhode Island price for electric

21.07

<----sell and buy, net metered

http://www.eia.gov/electricity/monthly/epm_table_grapher.cfm?t=epmt_5_6_a

You get paid 24.5 cents thorough the other *better* program!
Here's a task if you are so interested- tell me the system kWh nameplate size limit with the 24.5 cent rate. (link at end of comment, you'll want to look at the REGP (Growth Program))

OK - so do you want to use $.245/kwh for selling and $.21 for buying power?
Or $.21/kwh for both selling and buying?

If you generate more than you use- YOU GET ZERO MONEY FOR IT.

Here's what you quoted from their webpage.
"If the DG customer is a net exporter (generating more energy on-site than it uses) during the billing period, then National Grid will: (1) bill the DG customer for zero kWh of energy usage and (2) credit the DG customer for the net energy exported to the grid during the billing period in the form of Net Metering Credits (NMC). "
SO - you get "Net Metering Credits" - and ah - here's the section:"Any NMC that are not allocated to other accounts will accrue on the host account as a dollar-value credit. "
So I think it's saying that credits roll over to the next bill until they're used up. I don't see anything about them being limited in how long they stay as credits. Other states they'd basically evaporate at the end of each fiscal year.
So - do these credits roll over from one month to the next forever?
What is the (non-refundable) credited $/kwh? ($.21/kwh?)

30 cents for a Powerwall is way off- it's 10 - 15 cents.

If you want to use $.10/kwh, we can assume it's something other than a powerwall and is therefore cheaper, so that's fine.

Make it higher here than yours in Cali just for fun, call it 10 cents

Ok - so PV LCOE = $.10/kwh

* How many kwh are generated
* How many kwh are consumed

Please pick some numbers so we can do the analysis.
If you want, feel free to pick numbers for 2 or 3 scenarios.

 

[TommyO]

Please pick some numbers so we can do the analysis.
If you want, feel free to pick numbers for 2 or 3 scenarios.

Oh - I forgot - for the batteries option we also need to know how many of the kwh consumed go through the batteries first. (ie. how much of the kwh are consumed at night)

 

[PVfarmer]

OK - so do you want to use $.245/kwh for selling and $.21 for buying power?
Or $.21/kwh for both selling and buying?

So I think it's saying that credits roll over to the next bill until they're used up. I don't see anything about them being limited in how long they stay as credits. Other states they'd basically evaporate at the end of each fiscal year.
So - do these credits roll over from one month to the next forever?
What is the (non-refundable) credited $/kwh? ($.21/kwh?)

If you want to use $.10/kwh, we can assume it's something other than a powerwall and is therefore cheaper, so that's fine.

Please pick some numbers so we can do the analysis.
If you want, feel free to pick numbers for 2 or 3 scenarios.

Seems like I, if anyone, am being trolled here.

$.21 for buying power?

If you want to call the rate that people pay the POCO a new term called "buying power"....ok. Go ahead....not sure why, but free speech and all?
21 cents is what you pay to the POCO per kWh.
24.5 cents is what the POCO pays you for excess generation- BUT ONLY if you're over 25kW total/DCpanels. Under 25kW DC you get MORE!
It may be hard to believe, but this is the truth.

Isn't that nice of them to put the part about how you lose all your credit in a different section? The credits do in fact evaporate.
If you want any $ for excess generation, you have to do the other (and pretty much better) thing.
I mean, I see how transfer credit thing could work- some dude pays off part of the POCO bill from his machine shop using PV at his house, but...?
Again, not everybody does that.

Powerwalls don't cost 30 cents a kWh, they cost 10 or 15. Where'd you get 30 from? Use 15 then?

I gave ya plenty of numbers- why don't you use the ones for RI and see if you can find any numbers that prove any of these theories of yours about net metering and/or BESS?

 

[PVfarmer]

Oh - I forgot - for the batteries option we also need to know how many of the kwh consumed go through the batteries first. (ie. how much of the kwh are consumed at night)

10kWh per 24 hours. 5 a day 5 a night.
Then 4 a day and 6 a night.
Then 6 a day and 4 a night.

Good luck- you can try all the numbers you want- net metering loses. I've already done the numbers. But have fun! :happyyes:

 

[TommyO]

Ok - so 10kwh/day - we'll do a month since that's the billing cycle.

Scenario A - no PV
$.21/kwh * 300kwh = $63.

Scenario B - net metering, producing and consuming 300kwh/month
$.10/kwh * 300kwh = $30

Scenario C - sell the power at a higher rate (.245/kwh) and then buy the power you consume from POCO.
$.10/kwh * 300kwh - $.245*300kwh = -$43.50 [generating and selling the power]
$.21/kwh * 300kwh = $63 [buying the power you need]
Total: $19.50

Scenario D.1 - use batteries (4 kwh at night)
$.10/kwh * 300kwh = $30
$.10/kwh * 4 * 30 = $12 [cost for storage for the 30 nights]
Total: $42

Scenario D.2 - use batteries (5 kwh at night)
$.10/kwh * 300kwh = $30
$.10/kwh * 5 * 30 = $15
Total: $45

Scenario D.3 - use batteries (6 kwh at night)
$.10/kwh * 300kwh = $30
$.10/kwh * 6 * 30 = $18
Total: $48


Net metering isn't the winner here, since you have a program where you can sell the power for even more than you can buy it.

But buying batteries is obviously not better than just using net metering.
I gave batteries advantages even - I put the price per kwh at a low $.10/kwh. I also made it more favorable for batteries because I didn't add in the extra kwh that would need to be generated because they would be lost due to battery efficiency losses.

 

[ggunn]

10kWh per 24 hours. 5 a day 5 a night.
Then 4 a day and 6 a night.
Then 6 a day and 4 a night.

Good luck- you can try all the numbers you want- net metering loses. I've already done the numbers. But have fun! :happyyes:

So what is it, exactly, that you are trying to accomplish here? To convince me that batteries are always cheaper/more profitable than grid storage? To convince me to run out and buy everything I need to have a battery system in my house? Forget it. It ain't gonna happen. Your "math" is all sleight of hand and conjecture.

But if you are yourself convinced, put your money where your mouth is. Go out and buy a battery system and report back here how it is working out for you. Show me real numbers. I'll hang up and listen.

 

[PVfarmer]

Ok - so 10kwh/day - we'll do a month since that's the billing cycle.

Scenario A - no PV
$.21/kwh * 300kwh = $63.

Scenario B - net metering, producing and consuming 300kwh/month
$.10/kwh * 300kwh = $30

Scenario C - sell the power at a higher rate (.245/kwh) and then buy the power you consume from POCO.
$.10/kwh * 300kwh - $.245*300kwh = -$43.50 [generating and selling the power]
$.21/kwh * 300kwh = $63 [buying the power you need]
Total: $19.50

Scenario D.1 - use batteries (4 kwh at night)
$.10/kwh * 300kwh = $30
$.10/kwh * 4 * 30 = $12 [cost for storage for the 30 nights]
Total: $42

Scenario D.2 - use batteries (5 kwh at night)
$.10/kwh * 300kwh = $30
$.10/kwh * 5 * 30 = $15
Total: $45

Scenario D.3 - use batteries (6 kwh at night)
$.10/kwh * 300kwh = $30
$.10/kwh * 6 * 30 = $18
Total: $48


Net metering isn't the winner here, since you have a program where you can sell the power for even more than you can buy it.
But buying batteries is obviously not better than just using net metering.
I gave batteries advantages even - I put the price per kwh at a low $.10/kwh. I also made it more favorable for batteries because I didn't add in the extra kwh that would need to be generated because they would be lost due to battery efficiency losses.

Interesting! Now we're getting somewhere....but.

A- Yes
B- Yes
C- Almost? I get $20.25
You used 150kWh during the day @ 10 cents = $15. You sold 150kWh @ 24.5 cents = $36.75. At night you bought 150kWh @ 21 cents =$31.50
36.75 + 15 - 31.50= $20.25
Your way seems weird, because you didn't sell or buy ALL 300kWh. *Nor* did you buy all 300kWh!

D 1 2 and 3 - No. You did it again- you put the 10 cents with all 300kWh, and "undercharged" for night time by dropping it!
And of course dropped the fact that you covered 100% of the bill, thereby *saving* $$!
Really!
---
D 1- 4kWh a night with BESS
You used 180kWh during the day @ 10 cents = $18. You put 120kWh into BESS @ 20 cents = $24 (see how PV+BESS < POCO price there? Please?)
24+18= $42. You paid $42 for that month. You did NOT pay 63! You AVOIDED that cost- that is the whole reason for PV, BESS or not!
63-41 = You saved $21. Cost (42) minus savings (21) = you PAID 21! BESS wins!!
---
Ok, so LCOE for BESS = 10 cents is too low like you said-

D 1 again- same 4kWh - BESS=15

You used 180kWh a day = $18. You put 120 into BESS @ 25 cents = $30.
You paid $48 that month. Not 63. 63-48 = $15 in savings- $42-15 = You paid $27. BESS by a nose! (Net metering = $30)
---
D 1 again, same but BESS LCOE = 20 cents

180kWh day = $18.
120kWh night @ 30 cents= $36
36+18 = $54. That's only $9 saved from $63.
54-9 = $45!!
Whoops, BESS LOSES!
---
D 4- 6kWh a night - BESS LCOE =15 cents
120kWh day= $12
180kWh night (@ 25 cents)= $45
27 + 45 = $57.
Even worse!
---
D 4 - 6kWH a night BESS LCOE = 10 cents
120kWh day = $12.
180kWH night (@ 20 cents) = $36
36 + 12 = 48.
Sound familiar? BESS by (the same) nose again!!

Are we...understanding each other now??
You can't just guess-timate, will you at least give me that?

 

[PVfarmer]

So what is it, exactly, that you are trying to accomplish here?
To convince me that batteries are always cheaper/more profitable than grid storage?
To convince me to run out and buy everything I need to have a battery system in my house?

But if you are yourself convinced, put your money where your mouth is. Go out and buy a battery system and report back here how it is working out for you. Show me real numbers. I'll hang up and listen.

Well, forums are for discussions.
No and no.
Yes convinced, I haven't heard any convincing arguments against it (for anyone's situation). You of course can't get real numbers until it's up and running so, sure. I'll do that.
BESS involves a lot of complicated variables, and I've gotten some help here, from some folks...
Honestly, it would seem like getting a 5 year-ish BESS system today is the best idea. Like I said about smartphones...in 5 years BESS will probably be 5 cent/kWh.

 

[TommyO]

C- Almost? I get $20.25

Your way seems weird, because you didn't sell or buy ALL 300kWh. *Nor* did you buy all 300kWh!

Well if the POCO lets you do it, I'd sell all of it from one meter, and buy all the power on a separate meter- that would give the best financial solution.
But if the POCO doesn't allow that, then they don't allow that, and we'd have your result ($20.25)

D 1 2 and 3 - No. You did it again- you put the 10 cents with all 300kWh, and "undercharged" for night time by dropping it!
And of course dropped the fact that you covered 100% of the bill, thereby *saving* $$!
Really!

Well, that all 300kwh were generated by PV, so 300 * .10 would be correct.
The nighttime charges would be the 2nd line (storage) and part of the first line (generation).
I did not drop the fact that 100% of the bill is covered - there's no $ going to the POCO.

D 1- 4kWh a night with BESS
You used 180kWh during the day @ 10 cents = $18. You put 120kWh into BESS @ 20 cents = $24 (see how PV+BESS < POCO price there? Please?)

24+18= $42. You paid $42 for that month. You did NOT pay 63! You AVOIDED that cost- that is the whole reason for PV, BESS or not!

I agree that the answer for D-1 is $42.

63-41 = You saved $21. Cost (42) minus savings (21) = you PAID 21! BESS wins!!

So if you paid $21, then $21 must be the cost. So you saved $42!
$21 - $42 = -$21 = so you MADE $21! BESS Wins!!!1!111!!

In case you can't tell - that was sarcasm - You don't get to subtract the savings from your actual costs. Doing so is simply wrong.

D 4 - 6kWH a night BESS LCOE = 10 cents
120kWh day = $12.
180kWH night (@ 20 cents) = $36
36 + 12 = 48.
Sound familiar? BESS by (the same) nose again!!

$48 is more than $30 from Scenario B. Batteries lose compared to net metering.

Are we...understanding each other now??
You can't just guess-timate, will you at least give me that?

I don't know.
Do you understand yet that battery systems simply aren't a good solution in most places in the US?
We've been through 3 states so far, and the answer has been the same: Battery systems aren't financially useful for residential customers because of net metering.

 

[TommyO]

You of course can't get real numbers until it's up and running so, sure. I'll do that.

If it's for a residential location, you're better off putting that money into something else.
Since you have young kid(s), I'd strongly suggest using it for funding a 529.
ggunn may be happy to let you waste your money - but I think of your kid, who probably can use that money in 14 years to pay tuition.

Like I said about smartphones...in 5 years BESS will probably be 5 cent/kWh.

Unlikely IMO. I see batteries as a much more mature technology than most of the things that goes into cellphones.
But we'll see.

 

[PVfarmer]

Well if the POCO lets you do it, I'd sell all of it from one meter, and buy all the power on a separate meter- that would give the best financial solution.
But if the POCO doesn't allow that, then they don't allow that, and we'd have your result ($20.25)

I don't know.
Do you understand yet that battery systems simply aren't a good solution in most places in the US?
We've been through 3 states so far, and the answer has been the same: Battery systems aren't financially useful for residential customers because of net metering.

There's only one way do things. 20.25 is correct.
So is 30 cents for NM:

150 kWh day= $15 out during day
150 kWh night @ 21 cents = 31.5 out at night
31.5 + 15 = 46.5
63-46.5 = you saved 16.5 from the 63 cents you would have paid w/o.
46.5- 16.5 = 30 cents.

So how can my math be wrong for BESS systems but give the correct answer for net metering?

The answer is always the same from you- "I'm not listening"

If it's for a residential location, you're better off putting that money into something else.
Since you have young kid(s), I'd strongly suggest using it for funding a 529.
ggunn may be happy to let you waste your money - but I think of your kid, who probably can use that money in 14 years to pay tuition.

Unlikely IMO. I see batteries as a much more mature technology than most of the things that goes into cellphones.
But we'll see.

Honestly, thanks for the advice- but-
529- The plan will offer you several investment options from which to choose. Your account will go up or down in value based on the performance of the particular option you select.

Do you really trust Wall Street that much?
The sun is like a mutual fund- it goes up and down- but you don't lose money when it goes down! (Unless you're net metering- then you do lose a bit if your POCO price is higher than your LCOE/PV...)

Well, we see things differently, that's obvious- I see the new Powerwall and other things like is as the smartphones of the BESS arena.

 

[PVfarmer]

If it's for a residential location, you're better off putting that money into something else.
Since you have young kid(s), I'd strongly suggest using it for funding a 529.
ggunn may be happy to let you waste your money - but I think of your kid, who probably can use that money in 14 years to pay tuition.

This is just a basic system
System Advisor Model Report
Photovoltaic System
Residential
12 DC kW Nameplate
$2.50/W Installed Cost

exact same system and parameters, the first chart is with net metering, the second is without NM.
All I did was check and uncheck the NM box in the NREL/SAM program.
The installed price is cranked up pretty high.
Seems like the choice between NM-ing and not NM-ing is obvious, and that $6,000+ for batteries is a good idea overall?
Using realistic costs...sure helps with tuition!

Seriously- if you had the choice between having zero bill with PV / NM (no income), and zero bill WITH income...any amount, but especially a lot...what would you do?


Cumulative payback with expenses

1	-31302
2	-13615.6
3	-10688.7
4	-7648.93
5	-4492.38
6	-1214.84
7	2187.99
8	5720.57
9	9387.51
10	13193.6
11	17143.8
12	21243.2
13	25497.1
14	29911
15	34490.6
16	39241.8
17	44170.6
18	49283.4
19	54586.7
20	60087.3

---

1	-31302
2	-12255.4
3	-7943.89
4	-3494.91
5	1095.7
6	5832.22
7	10719.1
8	15760.9
9	20962.2
10	26328
11	31863.3
12	37573
13	43462.7
14	49537.6
15	55803.5
16	62266
17	68931.2
18	75805.2
19	82894.4
20	90205.2

 

[TommyO]

So how can my math be wrong for BESS systems but give the correct answer for net metering?

Because 2+2 =4 and 1+3 =4 also.
Just because you got the right answer doesn't mean you derived it correctly.

And in this case your explanation doesn't show the credits received for selling the power to the POCO, the cost for generating that power, and subtracts $16.50 for no good reason.

Looking at your explanation, I believe if you had used 4 kwh for what is used during night instead of 5 you would have come to the wrong answer.

The answer whether your consumption is all during daylight or all at night is still the same - $30.
It is 300kwh * $.10/kwh, the cost of your generation.


The correct way to figure it if you want to separate out 4kwh used at night.
180kwh * $.10/kwh = $18 = your generation costs for what's used during day.
120kwh * $.10/kwh = $12 = your generation costs for what's sold to the POCO.
120kwh * -$.21/kwh = -$25.20 = credits for selling to the POCO.
120kwh * $.21/kwh = $25.20 = debits/cost for buying from the POCO

Sum those terms:
$18 + $12 -$25.20 +$25.20 = $30

Those last two are always the same with net metering, so they cancel each other out. They don't impact the cost of your electricity.
In this case, your bill would be $0 with the meter showing the same value at the beginning and ending of the month. Your meter would go back and forth each day - going forward at night and spinning backwards during the day when you're generating.

Do you really trust Wall Street that much?
The sun is like a mutual fund...

Most 529 plans ARE mutual funds.

 

[TommyO]

Seriously- if you had the choice between having zero bill with PV / NM (no income), and zero bill WITH income...any amount, but especially a lot...what would you do?

Oviously take the income.
But that's a different question from whether batteries are worthwhile.
Add $6000 cost for batteries to the non-NM one and compare the results for doing with/without batteries.