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Powerwall/Car/Solar Charging Strategy

RubinRA

Member
I've just had a Powerwall and solar installed, and am trying to come up with the optimal way to balance home backup needs with charging my Tesla Model 3. I could use some feedback. Here are the details:

-- Solar 5.7kWp system
-- Single Powerwall unit, presently set to provide 50% backup power, 50% self-power
-- Utility uses Net Metering, rates do not depend on time of use ( typical rate for electricity in the Duke Energy region is presently 0.71 cents per kWh)
-- Household usage (other than charging the car) avg/day 20.29 kWh
-- Typically charge the car from 60% to 85%, twice a week (I work from home and don't put a lot of miles on it)

Right now, I've got my car set to start charging its battery every day at 11 am, about the time the solar unit begins to near peak output. The car draws from solar and from the battery until the battery goes down to 50%, then from solar and from the grid. Car charging ends about 2:00pm, and after that the solar recharges the Powerwall, sending excess to the grid to be banked. After the sun goes down, the Powerwall again begins discharging, but at a much slower rate than when the car is charging.

Does this make sense? Would it be wiser just to let the car charge from the grid at night, and keep the Powerwall on 100% charge and 100% backup? Some other arrangement? Ideas welcome.
 
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Reactions: Watts_Up

BrettS

Active Member
Mar 28, 2017
2,109
2,512
Orlando, FL
I went thought this exericise for myself a little while ago as my system is set to be installed in a bit less than two weeks. I’m in a similar situation with net metering and no time of use. Duke Energy is also my power company.

I posted a thread here where there was a bit of discussion about options:

Considering going with backup only vs self powered

In the end I think it makes more sense for me (and you) to go 100% backup and not be self powered. The big factor for me is that the powerwall is only rated at 90% effeciency, so for every 100kWh you put into the powerwall you only get 90kWh back. But with net metering, for every 100kWh you put into the grid you get the full 100kWh back.
 
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jrweiss98020

Tessa's Tesla
Jan 9, 2020
427
299
Edmonds, WA
With a small solar system like yours, I would second the Backup Only option. Also set the option in the Tesla App to only charge the car if the battery is over x% (75%?) when the grid is out.

If you want to occasionally test the system, just temporarily reset it to Self Powered.
 

jrweiss98020

Tessa's Tesla
Jan 9, 2020
427
299
Edmonds, WA
With simple net metering and no TOU differential, it makes no difference. Once the PW is charged, you either consume or sell the electricity, and there are negligible losses either way.
 

jjrandorin

Moderator, Model 3, Tesla Energy Forums
Nov 28, 2018
8,812
9,757
Riverside Co. CA
Oh my GOD... get more solar...

i pay .085 cents per kWh and am still doing a x-large system with 8 batteries.

I am suspecting that he has the zero in the wrong place (0.071, or 7 cents per, vs 0.71 or 71 cents per).. because if not it would be one of the most expensive rates I have seen, completely blowing our high CA rates out of the water.
 

RubinRA

Member
10.433¢ per kWh
I am suspecting that he has the zero in the wrong place (0.071, or 7 cents per, vs 0.71 or 71 cents per).. because if not it would be one of the most expensive rates I have seen, completely blowing our high CA rates out of the water.
Hmm. <grin> Okay, here are actual figures from a recent month’s bill: kWh=545, bill for power only = $54.28. Wouldn’t that make the rate 0.099? Not a math guy.
 

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