Welcome to Tesla Motors Club
Discuss Tesla's Model S, Model 3, Model X, Model Y, Cybertruck, Roadster and More.
Register

Moving to new house.. solar system sizing

tfan2018

Member
Apr 14, 2018
71
20
USA
PG&E (San Jose Clean Energy) customer. I will be moving into a new home in the next few months and want to install solar as soon as possible to start saving money on my energy costs. I've settled on Tesla solar panels and I currently already drive a Model 3 with 2 years of free supercharging.

In the next two years, I think 4.08kW system should cover most of my electricity needs, but should I install 8.16kW to future proof? I estimate that my energy bill will be approx $100-$130/month during the next two years. Based on Tesla savings estimation, 4.08kw will cover $92 of the estimated ($100-$130/month) energy bill each month. I plan to replace the gas wat/er heater with a hybrid heater and also replace the gas cooktop with induction. I will likely need to start charging at home once my free supercharging ends and my partner will likely get an EV 5-7 years from now. What would you do in my situation? 4.08kW will cover the majority of my energy needs while 8.16kW system will definitely be oversized but in 3-5 years my energy needs to increase due to two EVs. Not sure whether it makes sense financially to install 4.08kW and install add-on panels later on, or just go big at the beginning with 8.16kW. I'd love to get a powerwall but from ROI perspective it doesn't seem to save any money and more for backup power.

As I understand it, there is a minimum $10/month bill with PG&E even if I cover all my energy needs from solar. But San Jose Clean Energy also provide 4.19cents/kW for any excess (Rooftop Solar - San Jose Clean Energy)

Will the $ generated from excess energy generated be sent to me via a check or does it stay as a credit on the power bill and rolls over into future years until it can be consumed? If I receive a check, then technically the excess power I generate can pay towards the $120 annual minimum bill?

Should I submit an order for solar as soon as I move in, or wait a few months to see my energy usage trends?
 

ohmman

Plaid-ish Moderator
Feb 13, 2014
9,965
18,008
North Bay, CA
Given that the CPUC will be hearing a joint proposal on NEM 3.0, which would entirely erase much of the value of solar (specifically by introducing monthly true-ups which don't allow carrying summer surplus into winter), I would get my order in as soon as possible for the largest system allowed for grid interconnect, and hope to get grandfathered in on NEM 2.0. Ideally with storage, because adding storage will probably move existing NEM 2.0 customers to NEM 3.0.

So in answer to your specifics, I'd shoot for the 8.16kW, assuming you can get in before NEM 3.0 is adopted. Note that parts of the joint proposal will absolutely face opposition from solar rights groups and other solar advocates, but at least some of it will be adopted.

I have a 9.54kW system and wish I had more. Given the NEM 3.0 potential, I've been looking into expanding my system ASAP as well.

Good luck.
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
7,679
8,458
Riverside Co. CA
The size solar you are talking about, OP (4.0k) is EXTREMELY small, for a person who also has an EV. You may be living off supercharging now because its "free" but that doesnt mean you should size your solar (which should last 20+ years) to exclude charging your car.

Its highly unlikely that a person with an EV can cover their usage + car in any home with a PV system that small, and it would also be very rare for someone to have a bill that averages $100 to 130 a month in any california utility area, if they have an EV, unless the home is like <800 square feet with all gas appliances, and basically nothing but a fridge hooked up.

TL ; DR, I highly doubt that 8.1kW sized system is oversized.
 

wjgjr

Active Member
May 11, 2020
1,051
792
Silver Spring, MD
I agree with the advice given, but I will also add that, unless Tesla has changed their policies, it is likely that you do not have to choose between a ~4 kW and ~8 kW system. If you find that 6 kW or 10 kW, or some other number in the middle is what works the best for your requirements and/or the available roof space, you can customize the number of panels with Tesla after placing the initial order by speaking with your assigned advisor and requesting an updated layout.
 

h2ofun

Active Member
Aug 11, 2020
1,561
250
auburn, ca
The size solar you are talking about, OP (4.0k) is EXTREMELY small, for a person who also has an EV. You may be living off supercharging now because its "free" but that doesnt mean you should size your solar (which should last 20+ years) to exclude charging your car.

Its highly unlikely that a person with an EV can cover their usage + car in any home with a PV system that small, and it would also be very rare for someone to have a bill that averages $100 to 130 a month in any california utility area, if they have an EV, unless the home is like <800 square feet with all gas appliances, and basically nothing but a fridge hooked up.

TL ; DR, I highly doubt that 8.1kW sized system is oversized.
From what I am sizing, it is hard to believe anyone can make an oversized system. All it takes is adding an EV or 2 and one would be way undersized. Get the largest you can afford, and get it under NEM2
 

bob_p

Active Member
Apr 5, 2012
3,630
2,764
We're in Texas, so our experience will be different from California...

We sized our system to provide around 50% of our total electric usage (including EV charging).

After installing the system, we switched to a "Free Nights" plan with free electricity from 9PM to 9AM, and paying about 2X for electricity used between 9AM to 9PM.

With Tesla Gateway set in Advanced - Cost Saving mode, we maximize use of solar/PowerWall power during the expensive daytime hours and use grid power during the free nighttime hours. We also shifted as much electricity usage (EV charging, pool pumps) to the free hours.

Doing this, we reduced our electric bill for last year by 85%.

And even though our system is sized to only provide 50% of our house power, during extended power outages (primarily hurricanes, though there is a small chance of having outages during the winter), we have enough solar and PowerWalls to operate the house during those outages, by only powering "essential" systems [and not EV charging].

If there are plans that provide periods of free or reduced electricity charges (and don't exclude homes with solar/energy storage), you may be able to benefit from a system smaller than providing close to 100% power...
 

cobra

Member
Feb 8, 2021
101
19
Montgomery, PA, USA
Wow, free power hours... I live in an usual town that still has flat rate (0.16, so not outrageous by other area standards) electric, no net metering either, it seems. It would be fun to have overnight free power!
 

nwdiver

Well-Known Member
Feb 17, 2013
7,578
9,669
United States
Wow, free power hours... I live in an usual town that still has flat rate (0.16, so not outrageous by other area standards) electric, no net metering either, it seems. It would be fun to have overnight free power!

Well... free*

*TDU charges still apply. IIRC ~$0.03/kWh?
 
  • Funny
Reactions: cobra

jrweiss98020

Tessa's Tesla
Jan 9, 2020
412
290
Edmonds, WA
I think 4.08kW system should cover most of my electricity needs, but should I install 8.16kW to future proof? I estimate that my energy bill will be approx $100-$130/month during the next two years. Based on Tesla savings estimation, 4.08kw will cover $92 of the estimated ($100-$130/month) energy bill each month. I plan to replace the gas wat/er heater with a hybrid heater and also replace the gas cooktop with induction. I will likely need to start charging at home once my free supercharging ends and my partner will likely get an EV 5-7 years from now.
The water heater and cooktop will instantly increase your electricity use significantly. A single burner can take over 3kW, and you may use 7.5kW if all 4 are running. A hybrid water heater will draw 4.5kW during peak demand. Do you have the required 40A and 30A circuits already in the house for the conversion from gas?
 

Redhill_qik

Member
Aug 16, 2020
144
69
South SF Bay, California
@tfa
PG&E (San Jose Clean Energy) customer. I will be moving into a new home in the next few months and want to install solar as soon as possible to start saving money on my energy costs. I've settled on Tesla solar panels and I currently already drive a Model 3 with 2 years of free supercharging.

In the next two years, I think 4.08kW system should cover most of my electricity needs, but should I install 8.16kW to future proof? I estimate that my energy bill will be approx $100-$130/month during the next two years. Based on Tesla savings estimation, 4.08kw will cover $92 of the estimated ($100-$130/month) energy bill each month. I plan to replace the gas wat/er heater with a hybrid heater and also replace the gas cooktop with induction. I will likely need to start charging at home once my free supercharging ends and my partner will likely get an EV 5-7 years from now. What would you do in my situation? 4.08kW will cover the majority of my energy needs while 8.16kW system will definitely be oversized but in 3-5 years my energy needs to increase due to two EVs. Not sure whether it makes sense financially to install 4.08kW and install add-on panels later on, or just go big at the beginning with 8.16kW. I'd love to get a powerwall but from ROI perspective it doesn't seem to save any money and more for backup power.

As I understand it, there is a minimum $10/month bill with PG&E even if I cover all my energy needs from solar. But San Jose Clean Energy also provide 4.19cents/kW for any excess (Rooftop Solar - San Jose Clean Energy)

Will the $ generated from excess energy generated be sent to me via a check or does it stay as a credit on the power bill and rolls over into future years until it can be consumed? If I receive a check, then technically the excess power I generate can pay towards the $120 annual minimum bill?

Should I submit an order for solar as soon as I move in, or wait a few months to see my energy usage trends?
Instead of estimating based on $100-$150/month you should be estimating on your annual usage. You could try asking the sellers to get their data from the PG&E portal for the last 1-3 years.

The PG&E PTO application allows estimating usage of 12 months isn't available as square footage * 3.32, so a 3,000 sqft home would be 9,960 kWh. That wouldn't include EV charging.
 
Mar 1, 2021
117
31
San Diego, CA
If you have or are planning on EVs, get more. My neighbors 4kW system makes 7,000kWh/yr, and the model x uses 3,500kWh/yr to charge based on my conversation with my tesla advisor. So... an 8kW system would give you enough for 2 EVs and your house.
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
7,679
8,458
Riverside Co. CA
If you have or are planning on EVs, get more. My neighbors 4kW system makes 7,000kWh/yr, and the model x uses 3,500kWh/yr to charge based on my conversation with my tesla advisor. So... an 8kW system would give you enough for 2 EVs and your house.

How much power an EV will use is directly related to how much someone drives, so I dont know how anyone can say it uses XXXX a year. What if you drive 3000 miles a year, or 30,000 miles a year?

We also dont know where this OP is, their location says "USA". In many places in CA, systems perform like you mention (my 8.7 k system produces 12-13k a year). In other places,. it produces nameplate rating or less (some places back east).
 
  • Like
Reactions: jhn_ and ohmman

Redhill_qik

Member
Aug 16, 2020
144
69
South SF Bay, California
How much power an EV will use is directly related to how much someone drives, so I dont know how anyone can say it uses XXXX a year. What if you drive 3000 miles a year, or 30,000 miles a year?

We also dont know where this OP is, their location says "USA". In many places in CA, systems perform like you mention (my 8.7 k system produces 12-13k a year). In other places,. it produces nameplate rating or less (some places back east).
OP is buying a house in San Jose, CA.

Agreed that panel wattage cannot be easily translated to annual production. I think that this and the industry's obsession of starting with "how much is your monthly bill" are major stumbling blocks to system sizing.
 

Ampster

Active Member
Oct 5, 2012
1,675
432
Sonoma, California
The water heater and cooktop will instantly increase your electricity use significantly.
I used a heat pump water heater and the increase is not significant compared to a resistive heat water heater. Yes, it will reach higher levels if run in fast recovery hybrid mode. . Also induction cooktops are more efficient and can modulate more quickly so I do not think a single burner will consume 3kW unless bringing a large pot to full boil. Both those choices, when used efficiently could make sense.
 
  • Helpful
Reactions: ohmman

tfan2018

Member
Apr 14, 2018
71
20
USA
Thanks all!

I'd love powerwalls to complete the system but I can't justify the $ for it because the payback isn't there (just based off of the Tesla's website of savings over 20 years). But, it seems with NEM 3.0 the math would all change and power walls would increasingly be favored.

My energy bill will not reflect my actual usage since I am not charging my car at home, will I receive any pushback if I want to an oversized system relative to my existing energy needs? I heard that PG&E will push back hard on allowing customers to get an oversized system as they don't want to be in the business of writing surplus checks.

Also, given TOU I heard that it may be advantageous to have the panels be west facing rather than south facing. South facing will generate the most kW but west facing will generate more power during the peak hours. Will Tesla walk through all the scenarios and create the best layout to optimize against the existing TOU plans? Or, will they simply put as many panels as possible south facing because that direction generates the most kW (without regards to west facing generating more energy during peak times)?
 

jhn_

Member
Jan 21, 2021
85
63
Northeast United States
Or, will they simply put as many panels as possible south facing because that direction generates the most kW (without regards to west facing generating more energy during peak times)?
This. Tesla’s first draft of plans will be to generate the most power from the cookie cutter sizes they provide that you choose. Once you get those in hand, work with your project advisor to move the panels, add panels etc. I had good success just drawing on their PDFs and sending them back via email for layout changes. Turn around can be slow, like a week or two. But from what I hear trying to get your project advisor on the phone can sometimes involve a lot of phone tag and waiting that I wasn’t up for.
 

bob_p

Active Member
Apr 5, 2012
3,630
2,764
Well... free*

*TDU charges still apply. IIRC ~$0.03/kWh?
Actually - under our Direct Energy Free Nights plan, we aren't charged TDU charges during free nights...

When projecting energy costs for a typical home, they are assuming customers have 45% free electricity and 55% daytime paid electricity.

With solar panels and energy storage, you should be able to do much better.

Before solar panels - we were using 68% of our electricity between 9AM-9PM. After adding solar panels and PowerWalls and shifting EV charging to overnight, we reduced daytime usage to 22%!

When we estimated long-term break-even, we assumed a fixed price 24/7 plan - at least until the utility companies start excluding solar/energy storage companies from free nights plans - we should be able to break-even faster than we had planned...
 
  • Informative
  • Like
Reactions: nwdiver and ohmman

About Us

Formed in 2006, Tesla Motors Club (TMC) was the first independent online Tesla community. Today it remains the largest and most dynamic community of Tesla enthusiasts. Learn more.

Do you value your experience at TMC? Consider becoming a Supporting Member of Tesla Motors Club. As a thank you for your contribution, you'll get nearly no ads in the Community and Groups sections. Additional perks are available depending on the level of contribution. Please visit the Account Upgrades page for more details.


SUPPORT TMC
Top