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Too big of a system?

I'm hoping to get solar on the house before NEM 3.0 and the tax credit reduces to 22%

I have less than a year of energy usage in the new house but in the last few months, with this heat, I've been running the AC at "free will" more often than not. I have 1 Tesla vehicle which I charge maybe once or twice a week (whenever it hits 50% charge) and I am planning to add another EV in another 3-5 years. In the life of solar, I'm sure we will convert our gas water heater to electric. Everything else is already electric (except stove, we want to keep gas).

With that said, for the first 4 months (Jan - April) of this year, we averaged 630kWh/mo. We saw drastic increases in May (815kWh) and June (1000kWh)

Tesla's system with my $350/mo bill input generated a 9.6kW system. Is that sufficient or should I up for the next system which is 12kW. I'm thinking with both systems I'm going to go with 2 Powerwalls unless advised against. The cost difference between the two systems is about $5K for an additional 3200kWh/year
 
I don’t think I can answer your question directly. But it is good to be cautious about installing more solar power generation than you need if you live in CA. If NEM 3.0 is approved in its current form, there will be a monthly “grid participation” charge of about $8 per kW of installed solar capacity, so a $96 tax per month for a 12 kW system. This clearly is a huge disincentive to installing more solar power than is needed. The NEM 3.0 decision has been delayed while the CPUC accepts comments about alternative approaches.
CALSSA plans protests for first CPUC meeting after reopening NEM 3.0 record
 

miimura

Well-Known Member
Aug 21, 2013
7,383
7,382
Los Altos, CA
I don’t think I can answer your question directly. But it is good to be cautious about installing more solar power generation than you need if you live in CA. If NEM 3.0 is approved in its current form, there will be a monthly “grid participation” charge of about $8 per kW of installed solar capacity, so a $96 tax per month for a 12 kW system. This clearly is a huge disincentive to installing more solar power than is needed. The NEM 3.0 decision has been delayed while the CPUC accepts comments about alternative approaches.
CALSSA plans protests for first CPUC meeting after reopening NEM 3.0 record
If you are getting solar today, you won't face those rules for 15 years. You will have saved the entire cost of the system by then. If the rules become so draconian after 15 years, you can just rip it out.
 
If you are getting solar today, you won't face those rules for 15 years. You will have saved the entire cost of the system by then. If the rules become so draconian after 15 years, you can just rip it out.
The way I read the proposed “glide path” approach which is now being considered as part of NEM 3.0, the additional cost of $8 per kW of installed solar power would phase in over a 4-year period for customers on existing NEM 2.0, not 15 years.

(A) Glide Path Approach
The “glide path” refers to the process for transitioning customers from the existing NEM tariff to a successor tariff. The December 13 Proposed Decision included a residential customer glide path approach in the form of a Market Transition Credit (MTC), which is a fixed dollar per kilowatt ($/kW) of solar system size, provided as a monthly electricity bill credit. The MTC would apply in addition to the electricity bill credits provided to a NEM customer for exported energy, which the PD proposes to calculate on hourly Avoided Cost Calculator (ACC) values. The PD proposes to apply the MTC to residential customers for the first four years on the NEM 3.0 tariff, but the MTC credit would step down by 25% for customers over a four-year period.

source: Commission Ruling Reopens the NEM 3.0 Record to Invite Comment on and Consider Limited Issues
 
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BGbreeder

Active Member
Jun 19, 2020
1,053
720
Bay Area
Get the bigger system. The marginal cost isn't much, and you are likely to use the additional production one way or another. e.g. more electrical appliances or more cooling/heating, or more miles driven.

If your system lasts for 15 years, that additional $5k will be generating power at a locked in rate of $0.12/kWh. Think of all the miles that you could drive for that 3,200x15=48,000kWh.

All the best,

BG
 
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The way I read the proposed “glide path” approach which is now being considered as part of NEM 3.0, the additional cost of $8 per kW of installed solar power would phase in over a 4-year period for customers on existing NEM 2.0, not 15 years.
The report you cite is a news article. The source for the article is
ADMINISTRATIVE LAW JUDGE’S RULING
I don't see any suggestion that existing NEM 2.0 customers would be subject to the glide path approach.
 
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With you excessive usage in the summer get the bigger system and 3 Powerwalls.

Get the bigger system. The marginal cost isn't much, and you are likely to use the additional production one way or another. e.g. more electrical appliances or more cooling/heating, or more miles driven.

If your system lasts for 15 years, that additional $5k will be generating power at a locked in rate of $0.12/kWh. Think of all the miles that you could drive for that 3,200x15=48,000kWh.

All the best,

BG

I don't live in CA and don't know what NEM 3.0 is, but with your air conditioning, you may want to consider 3 Powerwalls, as well as the bigger system.

Perhaps a dumb questions, but why the bigger system and 3 powerwalls?

My monthly usage was

- January 605
- February 644
- March 603
- April 660
- May 815
- June 976 (as of 6/28)

If I were to assume 8 months to be 650kWh and 4 months to be 1000kWh, it'd be 9,200kWh/year. How much would another EV add and then changing a gas water heater to electric?

Tesla 9.6kW system is projecting 14,394kWh/year, which would be an excess of 5,194kWh/year from the 9,200kWh/year assumption we calculated above
Tesla 12kW system is projecting 17,562kWh/year, which would be an excess of 8,362kWh/year from the 9,200kWh/year assumption we calculated above

What is the benefit for all of this excess and adding 3 powerwalls instead of 2?
 
With the AC on and EV charging, even I would've liked more panels now (not really any good roof space left) . EV charging uses a massive amount of energy and more panels = not having to drain batteries any/much when you run your AC or charge the car during peak sun.

As I mentioned before, I think "adding" stuff later is not worth the hassle and installers don't add to existing systems. The added cost of a few more panels is more incremental (sunk cost of big project), mostly panel costs.

I really hope they don't change the 20 year to 15 years or force folks to transition out of NEM1.0/2.0. Similar to ToU, maybe the IOUs will try to "spin" it as being on NEM3.0 is better for you and get people to switch.
 
First of all, I assume you have not submitted your order and the 9.6 kW projecting 13,394 kWh annually is simply a very generic estimate without taking your location, roof and environment into consideration, so the projection may come down a lot once they start the design phase. Second of all, it does not have to be 9.6 kW or 12 kW, you can add or subtract panels as needed in the design to achieve your energy goal. With AC and anything around 10K kW system you want at least 2 PWs (that is just my personal opinion), 3 would be better. I would do a rough calculation on how much power you use from 3pm-12am, and would 80% of your chosen no of PWs give you enough kWh through that. If you think you would use 9200 kWh annually then I would add at least 10% to that rounding it to 11000 kWh and ask Tesla what size they would design for. Also, to make a rough estimate you can use PVWatts to model your roof, keep in mind it would only be a close estimate if your roof does not have shading issue. Also, moving from gas water heater to hybrid is good and keep in mind that if you add recirculation pump you would need more kWh to run that.
 
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With the AC on and EV charging, even I would've liked more panels now (not really any good roof space left) . EV charging uses a massive amount of energy and more panels = not having to drain batteries any/much when you run your AC or charge the car during peak sun.

As I mentioned before, I think "adding" stuff later is not worth the hassle and installers don't add to existing systems. The added cost of a few more panels is more incremental (sunk cost of big project), mostly panel costs.

I really hope they don't change the 20 year to 15 years or force folks to transition out of NEM1.0/2.0. Similar to ToU, maybe the IOUs will try to "spin" it as being on NEM3.0 is better for you and get people to switch.
In the 3 years I've owned my Tesla, I've only charged it at Midnight onward so AC is never running in parallel to the car charging. I thought most charge their Tesla using the grid during off-peak hours instead of using solar/batteries?

I definitely don't want to add more panels later on. I'm trying to just find a balance of not oversizing it too much to the point where I have a ton going back to the grid even after I recharge 2 powerwalls.
 
In the 3 years I've owned my Tesla, I've only charged it at Midnight onward so AC is never running in parallel to the car charging. I thought most charge their Tesla using the grid during off-peak hours instead of using solar/batteries?

I definitely don't want to add more panels later on. I'm trying to just find a balance of not oversizing it too much to the point where I have a ton going back to the grid even after I recharge 2 powerwalls.
Once you have solar, you will probably want to change your car charging to the daytime on days that you aren't out, as there is no point in taking the 10% energy loss of taking solar energy, putting the energy into the Powerwalls and then taking it back out.

A second EV will cost you 25-35kWh/100 miles, depending on the vehicle. How far do you plan to drive the second vehicle per year? Fifty miles a day is an hour of production for the larger system.

A heat pump hot water heater will set you back 12-20kWh/day, depending of course on how much hot water you use, and how hot it is. (Heat Pump Water Heaters) That is one to two hours of production from your larger system.

Yes, excess solar power won't pay you much (+$0.03/kWh net), but under sizing your system will cost you at least $0.12/kWh going forward, assuming no rate increases, which is almost certainly false. $5,000 for the extra 3.2MWh of production seems like a great deal to me.

Having an AC gets into complexities when off grid and starting up (read up on soft/hard start and inverter ACs and powerwalls) and a third Powerwall might help and simplify that for you. My advice is dig in to the details of your particular AC, and what the LRA (locked rotor amps) for your unit is and how many powerwalls would be required to start it in a off grid scenario.

Finally, as @SaveOurPlanet pointed out above, the details do matter and until you sort out where the panels go, and where they face, and what your roof angles are, you won't have a good idea of how much a "12kW" system actually produces, but it will mostly likely be less. PVWatts is your best buddy for that.

I guess that I look at your existing usage, and your planned additional consumption and it looks to me as if you are likely to increase your loads considerably over time. I wouldn't be going by the existing use or bills, I would be going by what the future loads look like.

If it were me, I would get the 12kW system, and think seriously about a third Powerwall. You live in an area with earthquakes, and and a less than outstanding electricity provider...

All the best,

BG
 
Seeing an average below 3 kWh/day, family of 4, actually <2.5 this last month as the incoming water is warmer.
Resistive heating will 4x that though.
My results are very similar for my Rheem with an ER=4.0 has averaged 2.33 kWh/day since it was installed 10/2021. Worst was 3.02 kWh/day in December and I'm at 1.71 kWh/day for May and June.
 
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In the 3 years I've owned my Tesla, I've only charged it at Midnight onward so AC is never running in parallel to the car charging. I thought most charge their Tesla using the grid during off-peak hours instead of using solar/batteries?

I definitely don't want to add more panels later on. I'm trying to just find a balance of not oversizing it too much to the point where I have a ton going back to the grid even after I recharge 2 powerwalls.

Heh, we run our AC at night since it's hard to sleep (no whole house fan) when it's hot. But yeah, I charge usually at peak sun from like 11am - 2pm or so. As someone who uses like 15 kWh per day normally, summer hot days and EV charging and we're well over 50+ kWh easily (and that's not going crazy with the AC neither).

We don't charge daily due to WFH and I micro-manage my energy since solar/batteries.

Even though rates are cheaper overnight, I figure sun out, not too hot, charge away instead of overnight since sun energy is free and I am less grid reliant if I use the sun energy vs. selling it back. I try to sell everything at on-peak though.
 
Based upon the data points below, how much oversizing of the system should I do to account for another EV in 3-5 years (10K miles/year) and slight increase in use of heater/AC? Should I add/get Powerwalls? If yes, why and how many would you recommend?

Total kWh/mo
- January 605
- February 644
- March 603
- April 660
- May 815
- June 997 (as of 6/29)

Average kWh/day
- January 19.53
- February 23.03
- March 19.48
- April 22.03
- May 26.30
- June 34.37 (as of 6/29)

Systems of Consideration
9.6kW - Estimates 14,394kWh/year, 1,199.50kWh/mo, 39.43kWh/day - $25,824 before ITC
12kW - Estimates 17,562kWh/year, 1,463.50kWh/mo, 48.12kWh/day - $32,280 before ITC
Other?
 
Once you have Powerwalls, it becomes a game of "how much money can I avoid paying to the energy company, and how much money can I get them to pay me for my exports"
To that end, the first and most important number is not the overall export, but the peak time (e.g. 4pm-9pm) exports. My 1 battery barely covers that usage (e.g. 8-10kWh) while leaving a small amount as reserve.
If I used more during peak that my batteries can hold, and the next day they're not going to get back to 100%, I'm eating away at the prize.

My way of playing the game is to also run the A/C harder up to 4pm, then let it coast into the evening on fan alone, but that's only easy because of this local climate.

My other thought on system size is , what fits comfortably on your roof. If it's laid out so there's an obvious way to put a few more panels on, just do it.
Don't leave easy to use roof space unused, you'll never get another chance to expand to it.

With a system size above 10KW, you're going to have enough to charge an EV during the day, so you also need to ask yourself if that vehicle is actually at home during that time. Otherwise you're forever relying on NEM to timeshift the value of your solar, and that isn't going to last forever.
 
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