Please help in solar installation sizing!

Need help from all you seasoned EV and solar folks.
We are in SDGE(San Diego) territory and buying electricity from the grid is like an ever increasing penalty. Household of two (about to have our first child), both of us work so peak time usage is pretty much nil except on weekends - this is subject to some changes once the kid arrives.
Average usage across the last 12 months = 454 KwH/month. We have a model X on reservation which would spike up the monthly kwH usage by another 440 kwH (assumption: 15000 miles/yr X 0.350 KwH/Mile).
Pre-EV annual usage = 5500 KwH
Post-EV annual usage estimate = 10800 kwH
We do get grandfathered into netmetering, and have the option to go with TOU/TOU-2 rates (lowest rate 0.17 c/kwH, still costs more than solar cost/kwh)
So, give all the options above, should we be factoring in 100% household consumption + 100% EV consumption OR 100% household consumption + X% EV where X is the value I need your expert advice on.. some have said factor in 60% EV usage, some say put on the biggest system to cover all of your current and foreseeable usage. Please advise! Many thanks.

Odenator said:

This is a tough question to answer. There are many variables such as budget and roof configuration which may limit the size of your installation. Then there is the lease vs own argument. Do you never want to send a check to SDGE or are you looking at a reasonable ROI after a few years? Last I checked, the price on solar was around $4/watt. This can vary depending upon where the panels are manufactured, the efficiency of the panels (200-230 watts) and the type of inverter (microinverter vs string inverter) and the number of inverters in the system. a 5kW system is around 25 panels and a 10kW system is almost 50 panels. Get three bids form reputable solar installers and make sure they explain what and why with their bids. Good luck!

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I should have listed in more caveats and assumptions in my question.
- I intend to pay cash down, that won't be a problem and IMHO it makes financial sense (best bag for the buck).
- House is ~3500 sq/ft with enough space on the South (~16 panels) and West roofs(~24 panels). No shade issues from houses/trees.
- Considering 300+ watts per panel (LG300/LG315 likely) with Solaredge power optimizers and Solaredge inverter. This also means I would need fewer panels.

gimmi80 said:

Too may variables involved to recommend a specific size: shading, roof size, cost of installation in your area, local incentive.
Just looking at the kWh a system between 5kW and 10kW should cover your needs.
There are some great tax credit at the moment, are you able to take advantage of them in full? If not, leasing might be a better option and SunCity will size the system for you.
If you can take advantage of all the tax credits and money is not an issue, I would say go with the largest system you can fit on your south facing roof (possibly east) skipping shaded areas.
I had three company prepare plan for me, one large nationwide, two local. the large nationwide was totally out of budget ($44k) the two locals were doable ($25k).
the first one offered me a smaller system (16 pan) with higher efficiency, all-black panels (for the esthetic), only on south portion of roof, 6kW
the second offered me a larger system (29 pan) with normal efficiency, less attractive esthetically, south + east portion of roof, 8.2kW
I went with the larger. the east portion of the roof will not produce 100% of what's estimated, but it's going to produce at least some energy.
Get some estimate from reputable local companies. Personally I would recommend you size your system to 110% of your need (actual+tesla)

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Great input, thanks! I have taken quotes from 8 companies including Solarcity who were the worst in terms of followup (never called me back after the first design review) and were the most expensive, plus won't allow me any choice in panels and design. Makes me disappointed as a customer and investor. I will likely go with one of the two local businesses.
Yes I want to fully utilize tax credit, are there any caveats/gotchas on getting the full 30% (household of two with AGI > 250-300k).

TSLA Pilot said:

Strongly recommend a visit to SolarCity.com's website. Not only are they part of the Musk World, but you'll be all set for a Powerwall in the years ahead, if it's not available at time of installation. This will be huge going forward.
IMPORTANT TIP: there is a far, far cheaper option that SCTY is downplaying, but it's what we used with our lease--the ONE payment option. With one payment your savings are huge as you're not leasing the panels with interest over 20 years. In our case, it's many thousands of dollars.
Contact me with a PM and I'll give you more details (and half the SCTY referral), but our system was as follows and this was in 2013--I'd guess they've come down even further in price by now?
31 panels
4 mounting surfaces/angles
2 inverters
7.5kW system size
$8k total cost in ONE payment (repeat--NO MORE PAYMENTS)
20-year lease
Paid for itself in about 18 months, and we drive two MS's now, effectively for free . . . .
What's not to like?!?

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Solarcity experience was disappointing (please see comment above), but I will still look up this option. They are at least 25% more expensive than the median quote I got, and won't allow me to choose the panels/inverter/setup config.

wws said:

For San Diego, use a average solar insolation value of around 5. (See http://www.gaisma.com/en/location/san-diego-california.html) With an estimate of 10800kwh/yr of usage, and assuming a decent south facing array, that means to cover all usage you would need 10800/365/5 = about a 6kw system. Note that this would zero out your usage in terms of kwh.
However from a fiscal point of view, you will not need a system quite that big, as billing is in dollars. The goal then becomes minimizing cost at the annual True-Up. First, utilities have a minimum monthly charge just for grid connection. Here in PG&E land, it was just raised to $10/month. So you won't get your annual electric cost lower than $120/yr regardless of how large (within reason) your solar array is. Second, the rates change between summer and winter months. With net metering, summer time generation is more valuable than winter time generation. Since you can generate more than you use in the summer, this allows some arbitraging of the kwhs. Third, with TOU rates, again you can bank higher priced prime time over-generation against lower-priced night-time usage. Taken as a whole, you may be very happy with a 5kw system instead of a 6 kw system. This is where details matter and a good solar installers advice will be valuable.
In order to get a check from the utility for over production, one has to generate an excess of kwh in _all_ 12 months of the True-Up period. With a December insolation average of 2.64, that would mean you would need a (550+450)/30/2.64 = 12.6 kw system to handle the worst month. Undergenerate by even one kwh, and no income for the entire year. Even if you make it, they only pay like $0.04/kwh. So in conclusion you would need at least 2x the system to get a small income from over-generation compared to simply minimizing your annual costs. For most people, it isn't worth it.
Finally, it is usually best to purchase your system outright, rather than a lease or a PPA. Then it becomes an investment instead of an expense. Use a home equity loan if you have to.

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Best advice so far. This is precisely in line with my question on sizing. I want a system which makes the MOST financial sense over lets set 10-15 yrs. I don't really want a check back from SDGE but instead hit the sweet spot when it come to investing on the PV system vs. paying for electricity (net cost yearly). This may very well entail not sizing the system to meet 100% of use and leverage some tier 1 pricing in combination with TOU. Would love more insights for brainstorming on this! $120/yr is OK, and I really want to see how TOU could come into play here, given that both of us in the household are at work 8-10 hrs a day (peak rates) and can charge the X(on reservation) and delay appliance cycles to late night. More discussions/advise on this please.... thanks!!

EVSteve said:

Optimizers Optimizers Optimisers.. I'll say it one more time OPTIMIZERS. Lots of these companies will attempt to push you towards micro-inverters because it makes their job so simple and they can hit you for the replacement labor when they fail. They WILL fail. String line setups limit your installation's power output and kill efficiency. Best of both worlds is to use power optimizers (Solar Edge). I recall hearing they're being integrated to function perfectly with the PowerWall but don't quote me.

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Absolutely Sir! thats the way to go, PV design that I am inclined towards - LG300/315 panels + Solaredge power optimizers + Solaredge inverter.

TSLA Pilot said:

Strongly recommend a visit to SolarCity.com's website. Not only are they part of the Musk World, but you'll be all set for a Powerwall in the years ahead, if it's not available at time of installation. This will be huge going forward.

IMPORTANT TIP: there is a far, far cheaper option that SCTY is downplaying, but it's what we used with our lease--the ONE payment option. With one payment your savings are huge as you're not leasing the panels with interest over 20 years. In our case, it's many thousands of dollars.

Contact me with a PM and I'll give you more details (and half the SCTY referral), but our system was as follows and this was in 2013--I'd guess they've come down even further in price by now?

31 panels
4 mounting surfaces/angles
2 inverters
7.5kW system size
$8k total cost in ONE payment (repeat--NO MORE PAYMENTS)
20-year lease

Paid for itself in about 18 months, and we drive two MS's now, effectively for free . . . .

What's not to like?!?

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Sent you a PM yesterday, your pricing sounds insanely good, too good to be true. I am astonished that you could get a 7.5kW system for 8k :love:
Could you please share some more details? I did get quotes and site survey from solarcity and the pricing for buyout was HUGE !!!! So I totally ruled them out, and they never followed up after the first call to explain the initial design, on which I had a bunch of questions. They also have a MyPower option these days. In short for a PV generating 6000 kwH/yr (3.98 kW) they were asking for pre-payment (and done) of $12.5k, and for buying it outright they quoted me $21k. Its insanely expensive compared to competition (and some 3x expensive compared to your system?). What am I missing?

ggr said:

I live in San Diego, and we installed a 3.3kW system back about 7 years ago (just before getting the Roadster). We used Solar City and got a discount from them. More recently I looked into increasing the size of the array, again asked Solar City, but I wasn't very happy with the service and we ended up not doing it. Anyway, on the EV-TOU2 plan, with two adults, two Teslas and two four-legged kids, both working from home, our total bill turns out to be a whopping couple of hundred per YEAR. You might want to aim a little higher, 4kW should pretty much zero your bill. Actually it depends somewhat on where, in Sandyeggo, you live. We're close to the coast and get June Gloom for three months of the year. If we were a bit further inland our 3.3kW system might just zero us out. Note that you never get any money back from them, so oversizing the system doesn't pay back.

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Thank you. I am not very clear on a 3.3kW (should generate about 5000 kwH /yr approx in coastal areas) system almost offsetting a household + 2 Tesla usage , care to explain please?
From some back of the napkin calculations, I foresee my annual household+EV usage (1 Model X) hitting 10,800 kwh. I am further inland so the kwH=kw X 365 X 4.66, that brings my requirement of 10,800 KwH/year to 6.34 kW PV system. Now then, TOU/TOU2 would certainly pull that number down further considering both me and my SO are out at work all day and can postpone majority of electricity usage to non-peak hours.. but how to come up with an equation for that is what I am currently working on. I also see that "SDGE Green Button Download My data" provides very detailed (hourly) usage history. Any hints/pointers are most welcome!

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Thanks @EVger. A 9kW is massive, congratulations on going solar and possibly setting up a platform to be independent of the grid.
Good advice on getting in touch with the builder's roofing company, I did not think of that. As for netmetering rules here in San Diego SDGE territory, the broader understanding/assumption/consensus is that you get grandfathered in on the NEM1.0 rules for 20 yrs. But yeah, I can understand one cannot completely rely on that.

ggr said:

Well, we don't commute so the cars don't consume much. We use gas for cooking, hot water, clothes drying. We are near the coast, so our air conditioning hasn't run for over three years. I guess we're just frugal with our power.

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Makes sense, thanks!

One more issue I would seek opinions on is:-
a) Go with Solarcity - better guarantee, support (hopefully) but no choice on panels/inverter/optimizers. Expensive (20% higher than lowest small business owner quote)
b) Local small business owner - full flexibility in designing my own system from panel choice to optimizers and inverters and even layout (to some practical extent). But extremely likely this business owner will fade away in few years as the money evaporates from solar business as Fed rebates go away and Netmetering rules change.

Folks may I ask how many times have you needed to get your panels serviced after the initial install and the first few adjustments post installation. The reason I ask is if I go with the small business owner I will save quite a bit and get the config I want. But if the business goes away,I would be in a dilemma how to get any issues taken care of at a later point. I wonder if the panel manufacturer addresses any issues with underproduction or issues requiring some diagnostics and/or adjustments. Kind of a open ended question but your thoughts on this are much appreciated.