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Too big of a system?

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

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?
First, will you be able to run your air conditioners and nominal house loads on 2 Powerwalls? You can get 5kW continuous (7kW or 7.2KVA peak) from each PW. Startup loads on air conditioners can be twice the running loads. Each EV can take up to 9.6kW when charging. How many miles will you drive? Estimate 0.3 kWh/mi for charging. If you lose the grid at night or on a cloudy day (no PV) when the A/C is running and 2 cars are plugged in, can you disconnect the cars fast enough to keep the PWs from shutting down? What will your new water heater draw? Even the new heat pump types use resistance heat during peak loads.

The larger system will help future-proof your house when you add all those new high-draw appliances/cars. Any load that the PV can't supply will be borne by the PWs or grid. With a grid outage, you will have to closely monitor your loads with 2 PWs.
 

BGbreeder

Active Member
Jun 19, 2020
1,113
791
Bay Area
You might also want to check with how big of a system your local utility approves. I have seen some utilities where they have mandated no more than 110% of your last 12 month bill and some utilities saying go as big as you want with payback in peanuts credits.
For PG&E, you can tell them that you plan a second EV and more electrical appliances. On that note, you may want to wire in the power for those appliances before Tesla arrives. Altering your electrical panels after solar/Powerwall installation becomes complicated.
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?
Based on your numbers above, you are already using 33.5kWh/day in June. That excess 6kWh/day isn't going to get your second vehicle very far, especially if your hot water heater uses 2-4kWh of it. Granted, you are using less in January, but you are likely to have much lower production in January. By much lower, I mean on the order of only 25-50% of what your system will produce in June. The exact number depends on your panel layout, orientation, etc. Again, PVWatts is your best advisor.

According to your above numbers, you will be getting an extra 8.7kWh/day for$6,400, again for less than you can buy it from PG&E under most TOU rates today, which means it will only be better with the passage of time.

Unless you have a phenomenal investment vehicle, that seems like a bargain.If it doesn't seem like a bargain, then I am curious, if you do have a much better investment vehicle, why bother with solar?

All the best,

BG
 
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?
I'm going to take a stab at what a full year might look for you based on my pre-solar usage, so definitely your mileage will vary.

MonthHardHitterRedhill
Pre-Solar 2017
RatioHybrid Water
Heater ER=4.0
Jan
605​
602​
1.00​
87​
Feb
644​
564​
1.14​
64​
Mar
603​
516​
1.17​
66​
Apr
660​
501​
1.32​
67​
May
805​
554​
1.45​
53​
Jun
1046​
754​
1.39​
51​
Jul-est
1292​
1037​
1.25​
51​
Aug-est
1189​
955​
1.25​
54​
Sep-est
995​
799​
1.25​
60​
Oct-est
590​
474​
1.25​
69​
Nov-est
555​
445​
1.25​
70​
Dec-est
614​
493​
1.25​
94​
Total
9597​
7693​
785​

So, I would estimate that your full year would be 9,597 kWh switching your gas water heater to a hybrid water heater with an ER of 4.0 would add 785 kWh and adding an EV that you drove 12,000 miles a year with 330Wh/mile would add another 3,960 for a total of 14,342 kWh/year, so I think that the 9.6 KW panel systems is right in the ball park for you.

PG&E doesn't want you to over install solar and their interconnect form 79-1151A says that the installed generation can't exceed 100% of your last 12 months of usage and projected future increase. Since you don't have 12 months of service the form estimates your usage based on 3 kWh * house square footage. You have posted the size of your house, but to get to 9,597 kWh your house would need to be at least 3,199 sq ft. Which gets us to the projected future increase grey area, in most cases Tesla will calculate the annual production of the system and use your actual 12 months or the 3kWh estimate and then simple add the extra as your projected future increase. PG&E often accepts that at face value as they don't have any reason to question your plans and adding an EV is an easy justification if it was questioned. However, a few forum members have gotten pushback/refusals, but that also might be related to other issues like the size of their service or local transformer/grid issues.

I would go with the 9.6KW panels and I would also add at least two Powerwalls. This likely won't raise any flags on your interconnect agreement and will ensure that you still have power in the case of an outage.
 
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Reactions: yblaser
I'm going to take a stab at what a full year might look for you based on my pre-solar usage, so definitely your mileage will vary.

MonthHardHitterRedhill
Pre-Solar 2017
RatioHybrid Water
Heater ER=4.0
Jan
605​
602​
1.00​
87​
Feb
644​
564​
1.14​
64​
Mar
603​
516​
1.17​
66​
Apr
660​
501​
1.32​
67​
May
805​
554​
1.45​
53​
Jun
1046​
754​
1.39​
51​
Jul-est
1292​
1037​
1.25​
51​
Aug-est
1189​
955​
1.25​
54​
Sep-est
995​
799​
1.25​
60​
Oct-est
590​
474​
1.25​
69​
Nov-est
555​
445​
1.25​
70​
Dec-est
614​
493​
1.25​
94​
Total
9597​
7693​
785​

So, I would estimate that your full year would be 9,597 kWh switching your gas water heater to a hybrid water heater with an ER of 4.0 would add 785 kWh and adding an EV that you drove 12,000 miles a year with 330Wh/mile would add another 3,960 for a total of 14,342 kWh/year, so I think that the 9.6 KW panel systems is right in the ball park for you.

PG&E doesn't want you to over install solar and their interconnect form 79-1151A says that the installed generation can't exceed 100% of your last 12 months of usage and projected future increase. Since you don't have 12 months of service the form estimates your usage based on 3 kWh * house square footage. You have posted the size of your house, but to get to 9,597 kWh your house would need to be at least 3,199 sq ft. Which gets us to the projected future increase grey area, in most cases Tesla will calculate the annual production of the system and use your actual 12 months or the 3kWh estimate and then simple add the extra as your projected future increase. PG&E often accepts that at face value as they don't have any reason to question your plans and adding an EV is an easy justification if it was questioned. However, a few forum members have gotten pushback/refusals, but that also might be related to other issues like the size of their service or local transformer/grid issues.

I would go with the 9.6KW panels and I would also add at least two Powerwalls. This likely won't raise any flags on your interconnect agreement and will ensure that you still have power in the case of an outage.

I met with a solar contact today and this is the table they came up with to say they think a 10.28kW system would be sufficient to cover current and future needs

Current Usage
Projected Future Use
MonthCurrent UsageSystem SizeAdditional UsageFuture EVTotalSystem Size
January6060.45612569230.68
February6450.48652569660.72
March6040.45602569200.68
April6610.49662569830.73
May8150.68225611530.85
June9970.7450025617531.3
July8750.6550025616311.21
August8750.6550025616311.21
September6060.4550025613621.01
October5920.44592569070.67
November4830.36482567870.58
December5510.41552568620.64
Grand Totals8,310.006.172,496.003,072.0013,878.0010.28
 
A good reason to oversize a system is for cloudy days. If it's choppy sun, solar does pretty bad and when you one can normally get 45kWh+ daily production, cloudy can drop that by a massive amount down to like 10kWh or less. We work from home and don't even drive much, but if $$ isn't a huge issue and you can "afford" more, I'd still stand by going with as many panels as you can get away with from the IOU. You drive tons more than us at 10k miles too. I guess I just don't feel people are going to miss that few $K for a few more panels assuming they have normal/half decent jobs (which I assume anyone getting large solar/batteries do).

There is also the tax credit so the increase is partially offset by that.

We got energy storage (2 batteries) and so far, I'm not even missing that massive $$ since I feel protected from the IOU a little bit and simply like having something which can protect against my non-existent power outages...(still waiting here for one).
 
What do the 500 kWh in June-Sept represent under Additional usage? Also, If you are 997 kWh in June expect to be higher in Jul-Sep as those are the hotter months.

So I told the guy that the usage that I have is me being "conservative" on running the heater/AC as I heard horror stories of $500-$600 PG&E bills. I was also still new in the house, so for the majority of the months, I was "feeling out" how much use would correlate to the bill.

So while those were my usage numbers, I told him that I'd have a feeling that when I go solar, I'm going to be more open in running the AC and so he entered 500kWh estimates for those months he thinks I'd run the AC more.

As you know, we had a pretty decent heat wave in June and because I knew I was going to start considering solar, I wanted to be more freely in running the AC to be comfortable and also see what my peak consumption for the month could be with that type of us. I ran the AC probably 9-15 years of the month each around 4 hours each day with one day being 9 hours. I think months like June would be rare occasions but if I have the "excess" power, then perhaps I run it more.
 

jboy210

Well-Known Member
Supporting Member
Dec 2, 2016
7,278
4,943
Northern California
So I told the guy that the usage that I have is me being "conservative" on running the heater/AC as I heard horror stories of $500-$600 PG&E bills. I was also still new in the house, so for the majority of the months, I was "feeling out" how much use would correlate to the bill.

So while those were my usage numbers, I told him that I'd have a feeling that when I go solar, I'm going to be more open in running the AC and so he entered 500kWh estimates for those months he thinks I'd run the AC more.

As you know, we had a pretty decent heat wave in June and because I knew I was going to start considering solar, I wanted to be more freely in running the AC to be comfortable and also see what my peak consumption for the month could be with that type of us. I ran the AC probably 9-15 years of the month each around 4 hours each day with one day being 9 hours. I think months like June would be rare occasions but if I have the "excess" power, then perhaps I run it more.
I am not sure if this June with as anomaly or the "new normal".

On the good side, in August 2019 we had an electricity $550 bill. We added solar in March 2020. In August 2020 our bill was around $150, and this was with me turning the solar off to prevent sending power back to grid because we did not have PTO. In August 2021 (hot will temps in the near 100 most days) our bill was a credit of 800 kWh + a $10 connection fee (grrr).
 
I am not sure if this June with as anomaly or the "new normal".

On the good side, in August 2019 we had an electricity $550 bill. We added solar in March 2020. In August 2020 our bill was around $150, and this was with me turning the solar off to prevent sending power back to grid because we did not have PTO. In August 2021 (hot will temps in the near 100 most days) our bill was a credit of 800 kWh + a $10 connection fee (grrr).

I think this will be an anomaly which will happen every now and then. I don't expect my usage to be like this month over month.
 

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