After the comments on the blog entry Increased Backup Reserve I was curious how well this would work:
I could see utilizing it during an extended outage to charge the car provided:
- Powerwall was full
- Sun was out without any cloud cover, meaning I could produce more than the house would use
As a reminder the circuits for the AC and the 14-50 outlet for my Tesla are in a new sub/side panel, which gets disabled if the grid goes down.
My uncle had been visiting Texas. First week was here in Houston, then he picked up a rental at Exotic Car Collection by Enterprise and drove to Bandera for a week.
He dropped the car off on Sunday so we picked him up, had a late Mothers' Day lunch, then dropped him off at the airport.
When we got home I lowered my charge point to the lowest option of 50% so I could do some charging experiments on Monday. I waited until the Powerwall was full, which occurred sometime after 10am, then plugged the car in via the dryer outlet from which the car charges at 24 amps. 240 * 24 = 5,760, or 5.76 kW. Per the Technical Specs at the bottom of the Powerwall page, the Powewall can output 5 kW continuous, with 7 kW peaks. So charging should be possible with a full battery + some solar. It was, though it turned out to be mostly solar + some Powerwall.
Actual voltage was 246 so charging was using 5.9 kW, meaning the rest of my home was using .9 kW at the time (all my work computers and lights in my office were powered up). So solar with fully charged Powerwall can handle this with ease.
If solar wasn't producing enough I could lower the amps drawn via the screen in the car. I tried 16 amps, which results in a 4.0 kW draw for charging:
That kept total usage below 5 kW, though it saw a significant reduction in charging rate from 23 to 7 miles per hour, so not the most efficient use of limited power in an emergency. I didn't try it at the time, but 5000 / 240 means about 20amp can be meet from the sustained draw of the Powerwall. That results in 16 miles per hour of charging, so quite a bit better.
Finally I plugged the car back into the 14-50 and was pleasantly surprised to see that Solar + Powerwall was able to handle it without tapping into the grid.
For some reason Tesla limits the draw when using the Gen 2 Mobile Connector with the 14-50 adapter to just 32 amps rather than the allowed 40 amps (a 14-50 circuit is rated for 50 amps, but for continuous loads like car charging you're only allowed to draw at 80%). If I had the older charging cable, or the Corded Mobile Connector (hardwired for 14-50), I could use the full 40 amps, which would increase usage by 1.9 kW. So even that could be could still be supplied by Solar + Powerwall under ideal conditions (sunny day without clouds).
I then headed out for lunch, plugged the car back in when I returned and allowed it to fully charge from 50% to 90% (max to charge to for daily use). I didn't monitor it, but by 5pm I could see in the usage graphs that grid hadn't been utilized, the car was full, and the Powerwall was full. I should have taken a screenshots of the graphs, but didn't think to at the time and the detailed info is no longer available (can only see today and yesterday).
Addendum - just got home from lunch. Car is charging, AC is running, everything's being powered by Solar + Battery.