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 Post subject: PV Solar sizing with and without EV
PostPosted: Sat Oct 04, 2014 4:54 pm 
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Joined: Mon Sep 29, 2014 11:14 am
Posts: 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/ ... sp=sharing

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Last edited by Ferdball on Wed Nov 11, 2015 3:51 am, edited 1 time in total.

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 Post subject: Re: PV Solar sizing with and without EV
PostPosted: Sat Oct 04, 2014 5:19 pm 
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Joined: Wed Nov 14, 2012 9:50 pm
Posts: 3868
Location: San Diego
Ferdball wrote:
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.


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 Post subject: Re: PV Solar sizing with and without EV
PostPosted: Sat Oct 04, 2014 6:48 pm 
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Joined: Mon Sep 29, 2014 11:14 am
Posts: 67
Location: San Diego
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.

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 Post subject: Re: PV Solar sizing with and without EV
PostPosted: Sat Oct 04, 2014 6:49 pm 
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Joined: Mon Feb 24, 2014 8:51 pm
Posts: 528
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.


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 Post subject: Re: PV Solar sizing with and without EV
PostPosted: Sat Oct 04, 2014 8:34 pm 
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Location: San Diego
smkettner wrote:
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...

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 Post subject: Re: PV Solar sizing with and without EV
PostPosted: Sat Oct 04, 2014 9:08 pm 
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Joined: Wed Nov 07, 2012 6:50 am
Posts: 1213
Location: San Jose, CA
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:

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 Post subject: Re: PV Solar sizing with and without EV
PostPosted: Sat Oct 04, 2014 9:25 pm 
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Joined: Mon Feb 24, 2014 8:51 pm
Posts: 528
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?


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 Post subject: Re: PV Solar sizing with and without EV
PostPosted: Sat Oct 04, 2014 9:31 pm 
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Joined: Thu Jan 17, 2013 11:13 pm
Posts: 1606
Location: Los Altos, CA
Ferdball wrote:
smkettner wrote:
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]

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Last edited by miimura on Sun Oct 05, 2014 1:31 am, edited 1 time in total.

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 Post subject: Re: PV Solar sizing with and without EV
PostPosted: Sat Oct 04, 2014 10:03 pm 
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Joined: Mon Sep 29, 2014 11:14 am
Posts: 67
Location: San Diego
Sdge has a TOU with its lowest rate between midnight and 5am.

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 Post subject: Re: PV Solar sizing with and without EV
PostPosted: Sun Oct 05, 2014 12:42 am 
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Joined: Sun Mar 03, 2013 8:18 am
Posts: 208
Location: SoCal
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.

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