Need help with system size

[Alex_M]

Hello All,

I am in the system design stage with Tesla Energy. Due to a very limited space on my flat roof, and 4’ setback requirement here in the city of LA. I have a very limited space to work with. My usage over the past 12 months was a total of 14,600 kWh with an average monthly usage of 1,216 kWh. It is important to note that I have NET metering.

After a couple of re-designs, Tesla is recommending installation of 28, 340-Watt Panels, with System Size of 8.84 kW and “Estimated “annual production of 12,807 kWh.

I am very skeptical of the 12.8K number. I am looking to some help to understand if in fact my new 8.84 kW system will yield 12,807 kWh annually, here in sunny SoCal. I would appreciate any and all information.

 

[gpez]

PVWatts is a super helpful site - PVWatts Calculator.

I plugged in the following values for Sherman Oaks, CA (bolded are the ones I changed from the defaults):

DC System Size (kwW): 8.84
Module Type: Standard
Array Type: Fixed (open rack)
System Losses (%): 14.08
Tilt (deg): 20
Azimuth (deg): 135

Looks like your panels are facing south east so I used 135 degrees.

Results came back as 14,031kWh annually so it seems 12,807kWh may be a bit conservative. Double check my numbers.

 

[Alex_M]

Thanks gpez.

 

[jjrandorin]

my 8.65kW sized system in Temecula CA generates between 12 to 13k kWh a year, depending on how much rain there is in march and april.

I am no where near you, except for the fact that we are both in southern california, but its not out of the realm of possibility for a system that size to generate that much in southern california, is the point I am attempting to make.

 

[Nikola91423]

Hi,

I live in Sherman Oaks, CA, about a couple blocks away from you Alex_M (small world). I have a flat roof as well, and received the final design from Tesla today.

My usage over the past 12 months was a total of 18,458 kWh with an average monthly usage of 1,538 kWh.

The proposed system size is: 10.88 kW which Tesla estimates will produce 14,643 KWh

When I used the PVWatts Calculator, I get this value for the 10.88 kW system: 17,344 kW (I posted the data below)

Is the proposed system size of 10.88 good enough? (I can upgrade to a larger system of 12.4 kW if I redirect a gas pipe on my roof for 1K).

I would appreciate any and all information.

DC System Size (kwW): 10.88
Module Type: Standard
Array Type: Fixed (open rack)
System Losses (%): 14.08
Tilt (deg): 20
Azimuth (deg): 135

 

[gpez]

"Good enough" means different things to different people. What are your goals and non-goals for installing solar? Are you trying to get to exactly 100% usage or are you comfortable with 80%?

For me I was primarily interested in back up power for common outage scenarios (weather, tree branches, equipment failures) as well as rare but serious earthquake scenarios. Solar + Powerwall beat fossil fuel generator options. Secondary for me was ROI as I wanted to make sure I was getting a good return on my system. With my utility's full net metering and low tier / high tier usage rates I wanted to stay in "low tier" as best as I could to maximize ROI. That meant doing the math on usage and production and I settled on 75% of my electricity needs provided by solar. Lastly I wanted to hedge against what I expect will be substantial rate changes in the future. Where I live hydro is the primary source of power and they're not building any more of that any time soon. EVs are super popular already and will continue to be which will put a substantial new demand on the grid. Being majority energy independent will hedge against that.

Some people want to be 100% self sustaining and are willing to pay a premium. Some have maximum use tariffs or time of use charges that they want to avoid. Many utilities don't provide full net metering which changes the equation if you're looking at a Powerwall. These all change what makes the most sense for you.