Solar, PowerWall, Car charging

[ammulder]

I hear people recommending not charging a car from a PowerWall.

If you have solar and a PowerWall, how do you manage the car charging? Is there a way to charge from solar directly if it’s being generated at the time but use power from the utility for the car when charging at night (even if the rest of the house is using daytime-generated power out of the PowerWall)? Or if not, how do you set up the car charging piece?

 

[Vines]

Charging a car from a Powerwall is like using a AAA battery to power a D size battery. It really doesn't make sense unless its required. a Single Powerwall can only output 20A continuous, so its also a slow charge if only 1 Powerwall is available.

In a grid outage situation, If you have a daily excess of PV power you can trigger to charge your car during the day after the Powerwall is fully charged with no penalty. The Powerwall would otherwise frequency shift and turn off or curtail the PV once it is too full to further charge. You could also charge at night straight from battery if it was an emergency.

In a grid up situation, your Powerwall will know the cost of power, and so I believe will not self consume Powerwall power to charge your Tesla during the cheap times if set up correctly.

Alternately you could charge from 2 different outlets, one smaller one on the backup side for grid outages, and a larger one or HPWC on the non backup side for fast charging during grid up.

 

[gpez]

Charging a car from a Powerwall is like using a AAA battery to power a D size battery. It really doesn't make sense unless its required. a Single Powerwall can only output 20A continuous, so its also a slow charge if only 1 Powerwall is available.

In a grid outage situation, If you have a daily excess of PV power you can trigger to charge your car during the day after the Powerwall is fully charged with no penalty. The Powerwall would otherwise frequency shift and turn off or curtail the PV once it is too full to further charge. You could also charge at night straight from battery if it was an emergency.

In a grid up situation, your Powerwall will know the cost of power, and so I believe will not self consume Powerwall power to charge your Tesla during the cheap times if set up correctly.

Alternately you could charge from 2 different outlets, one smaller one on the backup side for grid outages, and a larger one or HPWC on the non backup side for fast charging during grid up.

Adding to this - the inverter losses stack pretty dramatically going from PV (DC) -> Powerwall (AC) -> EV (DC), avoiding the "charging a battery from a battery" is wise. I personally do not have my level 2 (240v) EV charging plug backed up on the Powerwall. In an emergency I can use the level 1 (120v) and charge it off of the solar during the day.

To clarify I believe @Vines means 20a @ 240v as each Powerwall is capable of 5kW sustained.

 

[bob_p]

We have a separate 150A breaker panel for EV charging our S & X - which will not be connected to our solar/PW system. The installer will be adding a 14-50 outlet to the breaker panels that will be on solar/PW, which we'll use only for limited charging if the grid is down (hurricane) or if we find times of the year when we're generating excess solar (since local utilities aren't required to buy electricity from residential solar).

 

[Ampster]

We have a separate 150A breaker panel for EV charging our S & X - which will not be connected to our solar/PW system

But it should be noted that when solar is producing that consumption will be offset by the solar production at your meter before any excess solar is ent to the grid.

 

[ammulder]

Now that I look, our utility does support Net Metering... so do I understand right that if we set up the EV charging line completely outside the solar/PowerWall, we could have the solar send excess energy back to the grid for credit, while next to that consuming grid energy for the EV charging and thus using that excess solar credit? I have no idea how the wiring/meters would work, but is that possible?

 

[JayClark]

Now that I look, our utility does support Net Metering... so do I understand right that if we set up the EV charging line completely outside the solar/PowerWall, we could have the solar send excess energy back to the grid for credit, while next to that consuming grid energy for the EV charging and thus using that excess solar credit? I have no idea how the wiring/meters would work, but is that possible?

You won't get a utility credit from solar going to the grid at the same time you're charging your car, if the car charging is using more energy than the solar is producing. The utility will simply see the consumption of your car charging, minus the amount of solar produced (assuming no solar is going to the PW, if power is going to charge the PW you also have to subtract that amount).

If you're producing more solar than your car is using, then you'll get a credit that equals the amount of solar produced, minus the amount the car charging is using, and minus the amount going to charge your PW if applicable.

The totals all net out at the point the grid connects to your house. However, at least it's netting out using the grid, and not draining your PW if the car charger is on a separate circuit from the PW/Gateway.

In my case, my car charger is on the PW/Gateway loop, but I manage charging only from the grid by using the Balanced TOU Peak/Shoulder schedules. Then I set my car to charge only during non Peak/Shoulder periods (via my wall charger that lets me set a charging schedules, and my Volt lets me set schedules too, and I think the latest Tesla Car updates also allow schedules) . It is true that home power usage during that period is using grid power too, but it's minimal during the night when my rates are low.

 

[bob_p]

We're in Texas, which doesn't require utilities to support net metering, with only a few utilities supporting it right now.

So in planning for our solar/PW system, we assumed we would not have net metering - and believe what we've configured should reduce our grid usage by at least 50% and if we are at risk of having a few days when we'll generate excess solar power, we'll have a 14-50 outlet to charge our S or X off the solar/PW breaker panels.