PV Solar sizing with and without EV

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Ferdball

Well-known member
Joined
Sep 29, 2014
Messages
67
Location
San Diego
Okay, I'm confused.

I got a quote for solar from US Green for a 4.4kW system, which includes 16 LG panels. He said to add 1 panel if I were to add a Leaf, and 3 panels if I were to add a Tesla.

I use about 639 kWh per month.

I got another quote from Stellar Solar for a 4.8 kW system. This system and the one from US Green was about $20k (before fed discount). But then he said I need to add about 300kWh more for an EV. That added 50% to his calculations, which brings my system up to $30,000!

Question: If I plan to drive 12,000 miles per year on an EV, and I use 639 kWh monthly, what size should my PV system be?

11/2015 Update: I haven't finished a year yet, but my projections should be pretty close. My SDGE bill used to be about $1750 annually. It should end up being about $366 annually after solar, which includes 12,000 miles of EV charging for my Mercedes!

I had the 4.4kW system install in 2/2015, and am on SDGE's TOU2's plan.

https://docs.google.com/spreadsheets/d/1RQ77Q0qITvot0MlMR9xucdU5xjBeq1BjLK3Xoddn9HQ/edit?usp=sharing
 
Ferdball said:
Okay, I'm confused.

I got a quote for solar from US Green for a 4.4kW system, which includes 16 LG panels. He said to add 1 panel if I were to add a Leaf, and 3 panels if I were to add a Tesla.

I use about 639 kWh per month...

Question: If I plan to drive 12,000 miles per year on an EV, and I use 639 kWh monthly, what size should my PV system be?

Well, your solar guy is clueless. Obviously, the number of panels depends on a lot of things.

The amount of power that you will "pull from the wall" to operate your RAV4 EV is about 1 kWh per 2 to 2.5 miles.

So, 12,000 miles divided by 2.25 miles equals 5,333kWh per year divided by 12 months is 444kWh per month, assuming all the charging is done at home.

444kWh plus 639kWh = 1,083kWh per month.

That would require about 30 of the 228w panels that I have, assuming lots of sunny days at 33 degrees north latitude, with a south facing array, etc.

My 35 panel system averages 1,200kWh per month, cost $43,000 three years ago, and I need to add almost double to cover all my EV's.
 
Yeah. I was thinking that I would be getting free 'gas' for my EV if I went solar. Good thing I got multiple quotes. Now I have to recalculate my costs and benefits.
 
Good TOU rate and you can cut the system by 30% to 50%.
But still depends on usage as you will need to be a light user during on-peak rates to really take advantage.
 
smkettner said:
Good TOU rate and you can cut the system by 30% to 50%.
But still depends on usage as you will need to be a light user during on-peak rates to really take advantage.
I asked, but the guy from Stellar said that with net zero, it wouldn't matter. As long as I got the full capacity system...
 
Just to show the opposite end of the scale, I have a 4.25kW system, and charge TWO EVs almost entirely at home, (my daily driver, VOLT, driven electrically 87% of the time, and my wife's grocery getter, RAV4EV). Both cars are being driven a total of about 8000 miles per year. My house has central A/C, gas fired furnance and water heater. Everything else uses electricity. My wife and I are empty nesters, with two dogs.

According to PG&E, since my energy profile stabilized as described above, January 2014 was my worst billing month of the year at +304kWh. July 2013 was my best month at -328kWh. In contrast, July 2014 dropped to -255kWh probably because of the draught in Northern CA this year (no rain equates to dirty solar panels), and quite possibly more days of warmer weather (more sunny days equates to more use of A/C).

At my last "true up" annual statement (March 2014), I am a slightly net "surplus" energy customer, i.e. a Net Trueup Exporter of -182kWh. My net surplus compensation credit was valued at $8.00 (~4.4 cents/kWh). The "value" of my total energy credit for that year was $-275.89, so obviously there is no real benefit to over generating as a residential customer.

Pre both EVs, A/C and PV installations, I was averaging about +450kWh consumption per month. Now my average surplus is -15kWh per month.

It think it may be time to get a plug-in rechargeable battery powered motorcycle for fun on weekends. :mrgreen:
 
Last month I generated 464 kWh and purchased (net) 994 kWh for a total consumption of 1458 kWh.
I generated only 32% of what I consumed and my cost was $32. (3kW dc solar system)
Without solar and TOU my bill would have been $371.

So for producing 32% I cut my bill over 90%. If I produced 100% I would have a huge monetary credit that would have little value.
I don't think the salesman understands this. Or depending on your rates and usage it may not apply so much.
You may not need a full capacity system to reduce your bill to a very small amount.

Zeroing out your bill may not be the best return on the investment.
Salesman will always shoot for the largest system he can justify.

What utility are you on? Does it have TOU EV rates?
 
Ferdball said:
smkettner said:
Good TOU rate and you can cut the system by 30% to 50%.
But still depends on usage as you will need to be a light user during on-peak rates to really take advantage.
I asked, but the guy from Stellar said that with net zero, it wouldn't matter. As long as I got the full capacity system...
Dude is trying to sell you more than what you need. I get that some people want to be "net zero" but you're over-spending because your end of the year bill is going to be negative and that credit will be thrown out by the utility. If you over-produce they (PG&E for example) will only pay you about 4 cents/kWh over what you used. Not worth it. Tell them to size it for zero annual bill with your historical usage + 400kWh/month Off Peak for EV charging, using the EV rate schedule. If they don't know how to do that, find someone who does.

[Edit: 400kWh/day corrected to 400kWh/mo]
 
I used 660 kWh more than I made last month and still had a $11 credit on my bill, my solar made 563 kWh. My credit for the year so far is $130.
 
Ferdball said:
Sdge has a TOU with its lowest rate between midnight and 5am.
Stick with 4.8 max.

You will be selling a lot of electric at 48 cents in summer and buying at 16 & 22 cents. Car should charge fine at 16 cents.

Best to not use the air conditioner during on-peak hours so you can sell the max amount.
In summer you can cool the house a bit extra during off-peak and cruise through the day (6p).

http://regarchive.sdge.com/tm2/pdf/ELEC_ELEC-SCHEDS_EV-TOU-2.pdf
 
smkettner said:
Last month I generated 464 kWh and purchased (net) 994 kWh for a total consumption of 1458 kWh.
I generated only 32% of what I consumed and my cost was $32. (3kW dc solar system)
Without solar and TOU my bill would have been $371.

So for producing 32% I cut my bill over 90%. If I produced 100% I would have a huge monetary credit that would have little value.
I don't think the salesman understands this. Or depending on your rates and usage it may not apply so much.
You may not need a full capacity system to reduce your bill to a very small amount.

Zeroing out your bill may not be the best return on the investment.
Salesman will always shoot for the largest system he can justify.

What utility are you on? Does it have TOU EV rates?

I'm going to have to digest this. Any other recommendations are welcome.
 
Aside from the financial benefits, solar is GREAT for other reasons too. It's a statement against "blood for oil". Reduces greenhouse gases. Prolongs the life of a roof. Washes itself whenever it rains. Improves the resale value of real estate property, and still qualifies for a 30% federal tax credit. (Sorry, those last two are actually financial benefits.) Proves to all your neighbors that you are an advocate for renewable and sustainable energy. Goes hand in hand with owning an EV that gets some or perhaps all of its "fuel" from the SUN.
 
smkettner said:
...If I produced 100% I would have a huge monetary credit that would have little value.
I don't think the salesman understands this...
Okay, I don't either. If you're producing 100% of your actual usage, why would you have a huge monetary credit?

miimura said:
... I get that some people want to be "net zero" but you're over-spending because your end of the year bill is going to be negative and that credit will be thrown out by the utility...
Same. I don't get it. Can you help me understand this point?

So, apparently there's a sweet spot between my non-EV usage (639kWh) and with the additional EV usage (~939kWh). How do I calculate it?

BTW, I just came home with a B-Class. Sorry.
 
Ferdball said:
BTW, I just came home with a B-Class. Sorry.

Don't be! Tell us about it:
Decision why B-class over RAV4EV?
Acceleration comparison, if you drove both.
Pricing of B-class.
Range. Does it too have standard and extended charge modes?
EVSE selection?
Did you know that Tony Williams will eventually be offering a DC quick charge CHAdeMO connector for the B-Class? Might want to get on his list.
Anything interesting about it that you didn't know beforehand?
 
Ferdball said:
smkettner said:
...If I produced 100% I would have a huge monetary credit that would have little value.
I don't think the salesman understands this...
Okay, I don't either. If you're producing 100% of your actual usage, why would you have a huge monetary credit?

If you sell 100 kWh at 48 cents you get $48 credit, then use 100 kWh at 22 cents you have $22 in charges.
The net bill has a credit of $26 for zero net usage. (simplistic example)

You will not get a check at the end of the year. But you can save a bundle by installing about 30% less solar.

You may need to look at someone's actual bill that has TOU and solar to see how the credit works.
 
You definitely have to factor in TOU to see how much solar you need. Unfortunately, doing so is not so simple, because you not only have to factor in how much energy you consume now, but WHEN you consume it. Does your utility currently provide you with any such information?

To give another example of the effect of TOU, at the end of my last annual cycle I had consumed 4,900 kWh more than I had produced with my 3.84 kW system and two EVs, yet my total electricity charge for the year was -$70. Of course I didn't actually get a check for that - it just gets reset to 0 at the start of the next annual cycle. So my system is pretty well-sized for me, and perhaps I could've gone with one less panel, but then again you'll produce a little bit less every year and TOU rates could change at any time. But I hardly use any on-peak power (like running the A/C), so if you use more of that it'll make a BIG difference in your result.

If you have any more details about how much electricity you might use on-peak vs off-peak and so on, go ahead and post it and maybe we can help you do the math (but really a good solar installer should do this for you.)
 
I'm off to the Chargers game, so I'll be off grid for a while. But I'll post my impressions soon. I live pretty close to Tony, so maybe we could get a little met going on. We could get some better impressions that way.
 
Another way to explain this . . . as you accumulate surplus energy "credits", they have monetary value computed at the "RETAIL" price of electricity. However, your annual "true up" statement DEVALUES those same credits at the WHOLESALE price of electric.

As a very simple example, let's say "peak" TOU over generation credits accumulate at a value of 25 cents per kWh. After one year, your over generation results in 500kWh in credits, but they are now only worth ~4 cents per kWh, or $20. The result is you lost 84% of their original value. This translates to a HORRIBLE financial investment by any standard!

This is why over sizing a PV system to produce more than you actually need, although great for "bragging rights", is certainly financially unwise.

To further illustrate how poor of a financial situation this can be, let's say, each killowatt your PV system is sized to produce, costs $5000 as an initial investment. However, if throughout the year you are over producing because of just one killowatt. Your annualized rate of return from the extra $5000 invested, (based solely on over generation), amounts to 0.4% (20/5000 = 0.004). Your utility co. will write you a check for only $20 in "payment" for the surplus energy credits.

Thus, for each extra $5000 invested in the PV system to produce surplus energy you can sell back, you are only earning about the same amount (in annual interest) as keeping $5000 on deposit in a regular savings account.

Again, this is a very simplistic example, but not too far of a stretch from how easily "net energy management" can be skewed in the "wrong" direction.
 
Ferdball said:
I'm off to the Chargers game, so I'll be off grid for a while. But I'll post my impressions soon. I live pretty close to Tony, so maybe we could get a little met going on. We could get some better impressions that way.

Why don't you drive up to Cerritos for breakfast this Sat, Oct 11 ?

Plus, now that I know you have a MB B-Class ED, we need to drive it around my grueling San Diego loop for a little "range test".
 
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