Average Powerwall savings

[MorrisonHiker]

Let's put the tax credit aside for a moment. As I understand it, the Powerwall is designed to be unable to charge from the grid if there are solar panels connected. Does the same hold true for other battery systems?

It's just a software restriction, not a physical one. While I don't get the ITC, I can't choose to charge my Powerwalls from the grid because I have solar. Supposedly, they can be charged from the grid if Storm Watch is activated but I haven't seen that happen yet.

I have seen very minor amounts of energy being sent to/from the grid by my Powerwalls. I think this happens to keep the Powerwalls importing/exporting energy at a consistent level. For example, at this very moment, the app is showing I'm sending 12.9 kW to the grid. 12.6 kW are coming from my panels and 0.3 kW are coming from my Powerwalls while an additional 1.0 kW is being sent from the Powerwalls to power the household usage.

 

[Shygar]

Let's put the tax credit aside for a moment. As I understand it, the Powerwall is designed to be unable to charge from the grid if there are solar panels connected. Does the same hold true for other battery systems?

Mine charges fine from the grid during storm watch. There's no technical limitation preventing this. Other battery systems would have to decide whether they give you control over that or not. Tesla only lets you do it when storm watch is active.

Edit: Here's a screenshot when I'm charging from the grid and solar during storm watch

 

[miimura]

Let's put the tax credit aside for a moment. As I understand it, the Powerwall is designed to be unable to charge from the grid if there are solar panels connected. Does the same hold true for other battery systems?

The simple answer is yes. There are other inverter and battery systems that essentially give you complete control over what they do. Your utility interconnect agreement may also contain certain limitations on what you can do with battery-inverter systems.

 

[SDRick]

Especially in areas with TOU why would anyone purchase a Powerwall? Are there are advantages to the Powerwall system that overcome the inability to charge the batteries from the grid?

 

[miimura]

Especially in areas with TOU why would anyone purchase a Powerwall? Are there are advantages to the Powerwall system that overcome the inability to charge the batteries from the grid?

Everybody else's batteries are so much more expensive per kWh that they lose their appeal.

 

[Shygar]

Especially in areas with TOU why would anyone purchase a Powerwall? Are there are advantages to the Powerwall system that overcome the inability to charge the batteries from the grid?

TOU is precisely the reason I have a powerwall, a close second is allowing me to use solar when the grid is down and a distant third is backup options.

 

[SDRick]

TOU is precisely the reason I have a powerwall, a close second is allowing me to use solar when the grid is down and a distant third is backup options.

My line of thinking was assuming all things being somewhat equal and you can purchase a battery system that allows charging from the grid versus a battery system that does not, wouldn't most opt for the most flexible?

 

[Shygar]

My line of thinking was assuming all things being somewhat equal and you can purchase a battery system that allows charging from the grid versus a battery system that does not, wouldn't most opt for the most flexible?

I prefer to have the ITC and I'm hoping Tesla will give us the functionality in the future to charge from the grid if ITC requirement is gone (5 years I think?). Not to mention you just can't beat the cost and functionality of the powerwall.

 

[miimura]

My line of thinking was assuming all things being somewhat equal and you can purchase a battery system that allows charging from the grid versus a battery system that does not, wouldn't most opt for the most flexible?

Get some quotes for other battery systems and let us know if you change your mind.

Edit: My point is that all things are not equal. From my research, at best, most other battery systems have a battery-only cost that is similar to the battery+inverter cost of the Powerwall 2.

 

[TapHabit]

Especially in areas with TOU why would anyone purchase a Powerwall? Are there are advantages to the Powerwall system that overcome the inability to charge the batteries from the grid?

People like myself use massive power during off peak hours, and very little during peak hours. Having multiple Powerwalls allows one to not only fully charge them during the day, but use the excess power generated from my roof panels to power the home. Then, during off peak, I’m able to further supplement my power consumption along with the off peak rate to see a MASSIVE difference in my power bill. I have a huge solar system and use a ton of power tho. So subtle differences in savings for smaller systems become much larger saving with larger systems. The cost for a 6kw solar system and 20kw aren’t that different. My cost for the system should be recouped in 3-4 years.

There’s a ton of factors at play here though that will make each system different. This is just the scenario at our place, its hard to assume everyone’s case is the same

 

[bob_p]

How much capacity degradation is there if a Powerwall is taken from full charge and mostly discharged every day?

With our S and X vehicles, we expect to see slow loss over time - but we aren't doing a full charge/discharge every day.

Powerwalls are different - they are likely to see more charge/recharge cycles. Are they seeing more rapid capacity loss?

Or has Tesla done something to the design of the Powerwalls (such as providing extra capacity so that the batteries are always kept between 10-90%) to reduce the capacity loss over time?

 

[cwied]

Or has Tesla done something to the design of the Powerwalls (such as providing extra capacity so that the batteries are always kept between 10-90%) to reduce the capacity loss over time?

That's exactly what Tesla has done. I believe the actual capacity of the batteries is higher than what's used. In practice, I've never discharged more than about 60% of my batteries in a day either. Since part of the reason to have Powerwalls is backup, I usually have a backup reserve set at at least 30%.

 

[miimura]

People have also observed that Storm Watch charges more than you would expect after it reaches 100%. I estimated something like 1.5kWh above 100% on my two Powerwall system, IIRC.

 

[bob_p]

For anyone who has had a Powerwall 2 for an extended period, have you lost any capacity - and how long have you been using it?

 

[Shygar]

How much capacity degradation is there if a Powerwall is taken from full charge and mostly discharged every day?

With our S and X vehicles, we expect to see slow loss over time - but we aren't doing a full charge/discharge every day.

Powerwalls are different - they are likely to see more charge/recharge cycles. Are they seeing more rapid capacity loss?

Or has Tesla done something to the design of the Powerwalls (such as providing extra capacity so that the batteries are always kept between 10-90%) to reduce the capacity loss over time?

Remember each powerwall can only output man of 5kW, whereas a car will output a lot more than that. It's just easier on the battery. Plus you have a 10 year powerwall warranty at 70% capacity I think

 

[bob_p]

Each Powerwall can output up to 5KW sustained. However a 100 KWh battery pack is roughly 7 Powerwalls (assuming the Powerwall actually has more than 13.5 KWh of storage). With Powerwalls, that's 35KW of power, discharged in roughly 2.5 hours at full power. That's about how quickly an S/X will discharge a 100 KWh pack driving at high speeds.

The difference is that most Powerwall installations probably pull less than 5KW, to allow a Powerwall to last more than 2.5 hours.

So the slower draw rate coupled with keeping the Powerwall within 10-90% of the actual battery capacity, and usually slower recharging rates - the combination could reduce the rate of battery degradation in a Powerwall - or with the Powerpacks being deployed for use by utilities.

Is there any study or documentation on how the Powerwalls are designed or perform (other than the limited information on Tesla's website)?

 

[miimura]

In order to judge degradation, you need to come up with a procedure to measure the energy delivered from the unit from known starting state of charge to known ending state of charge. Then you need to do that same procedure repeatedly over time to see if it is changing. You also have to have a good idea about the measurement error in your procedure. While you're at it, you should check the energy in vs. energy out to see if that changes over time too.

I'm not too concerned about this issue, so I haven't spent much time thinking about the procedures.

 

[bob_p]

This issue has been studied for the S/X battery packs - with estimates of degradation over time/miles.

There probably isn't enough data yet to do the same with the Powerwalls, plus it may not be as easy for Powerwall customers to get that data to determine degradation. It's easier in an S/X, because you can see how many miles of range the vehicle has after a 90% charge has been completed - and when that number starts going down.

 

[cwied]

The Powerwall tracks the amount of energy that has been imported to and exported from the battery, so if someone has historical snapshots of those numbers, it'd be pretty easy to calculate the degradation numbers.