@SolarUser i can confirm that the powerwalls use energy and there is an inefficiency to cycling them. I am a bit of a data nerd but my only data on this is from the Tesla app which I don’t think is precise. Here are a few thoughts from my 2 powerwalls, installed 3.5 years ago, which means they are not exactly what is being produced today.

If I leave them at 100% backup and don't cycle them, they use about 0.5 kwh per day.

If i cycle them down to 20% and back to 100%, the cost roughly doubles to 1 kwh per day. That is consistent with your calc of 383 kwh/year.

Do you really have your energy usage honed to feel the effects of 1 kwh/day?

I am curious your use-case for the powerwalls? I initially used mine to be self powered every day, but abandoned that because although it was fun, it doesnt work every day (because of low solar production some days), and in the end I judged it to be pointless. Now my reasoning for the powerwalls:

• So that solar production is maintained if there is a grid outage during the day.
• As a backup to a grid outage any time the grid is down.
• To stay off the grid during peak TOU rate period if I am out of banked kwh with my electric utility (sometimes at end of winter)

 

Thanks @Bigriver for your thoughts and experience. Like you I’m also a data nerd and have a lifelong passion of learning how things work. Sounds like the overhead you are seeing is in line with what others have reported, but am curious why the overhead goes up while you are cycling? Is the extra overhead really masking the Round trip costs? Are you able to see the Round trip costs?

@Bigriver asked: “Do you really have your energy usage honed to feel the effects of 1 kwh/day?” If I was off grid then every kilowatt could be critical to running the refrigerator for example, but for a grid tied system, it is mainly interesting to see how things work.

Usage wise I plan to use the powerwalls the same as you:

• So that solar production is maintained if there is a grid outage during the day.
• As a backup to a grid outage any time the grid is down.
• To stay off the grid during peak TOU rate period if I am out of banked kwh with my electric utility (sometimes at end of winter)

Lastly, I was intrigued by various sales pitches for batteries where people talked about how much money you would save with batteries, but the reality given PG&E rates and my usage is that last year I would have saved $65 for the year by time shifting with batteries. Offset by Round trip costs of -$57 to pay for the replacement of peak hours and offset by overhead costs of -$147. So going with batteries last year would have cost me $139. Not a bad price for a fulltime backup solution that didn’t involve propane or gasoline powered generators in case of emergency.

I’m going to increase my solar array by 20% and hopefully stay at net zero.

I would love to hear from others about their experience with overhead and round trip costs.

Thanks again.

 

As far as I can tell, if you have net metering that gives you retail price for the excess energy generated by your solar, then Powerwalls will cost you money. My present thought is that I should keep them at 100% backup, because it costs energy to discharge and charge the batteries to run the house and absorb solar. Even shunting the solar to the grid costs energy because this operation requires going through the Powerwalls' inverters to match the frequency to the grid. The Powerwalls don't seem to "pass-through" the solar to grid without using their inverters. You would think this was possible, but it does not seem to be the way they operate.