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Saturday, December 24, 2022

Solstice, Storms, & Solar - Tesla Powerwall: StormWatch vs VPP

The winter solstice is a milestone day in solar energy. It's the shortest day of the year, so nothing but longer days from here for the next 6 months.

Usually for the solstice, I note our solar production, sunrise, sunset... I'll cover that, but something unexpected happened on the solstice this year. 

Winter Storm Event
Red Flag Warning

Unstoppable Meets Immovable

Like much of North America, our area is currently being hit by snow and ice storms during this yuletide. In response to the storm, on the solstice, two things happened. One, at 4PM our Tesla Powerwall went into Storm Watch mode. This charges it up to 100% and keeps it there so the battery pack has the energy needed to keep our home running if the grid power goes out. An outage is a real possibility during ice storms, so the precaution is smart. The second event in response to the storm is that our local utility scheduled a virtual power plant (VPP) event. The VPP event was scheduled to run from 5PM till 8PM. 

Portland General VPP event

So what happens when Storm Watch mode is trying to hold the pack at 100% charge and a VPP event is trying to discharge the battery to support the grid? The good news is we could to opt-out of either one or both of the events if we had a preference as to which one we wanted to win out. However, I was far more curious to see what happened if we did nothing and watched the result. 

Without further ado, the VPP won out. Our Powerwalls discharged for 3 hours at 3kW. This removed 9kWh from our ~40kWh pack.

Powerwall Discharging 
While in Storm Watch Mode

Solstice Energy Use and Production

Below is the graph of our energy use on the solstice. The colors tell you the source: grey is the grid, green is the Powerwall, and amber is solar direct from our roof.  

The sun didn't spend long in the sky on December 21st. Sunrise was at 7:47AM and sunset was at 4:29PM. That's just 8 hours and 42 minutes without the cold inky black winter sky overhead. Combine this with the sun low in the sky and storm clouds and the result is a yield of 12.7kWh of solar production for the day. 10.5kWh of that solar went directly into running our home with the remaining 2.2kWh going into the Powerwalls. 

For comparison, on the summer solstice, we generated 72.9kWh (almost 6 times more), along with feeding ~50kWh of that into the grid. Here's looking forward to sunnier days.

Other Solar Posts: 

Other Powerwall Posts:

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