Powerwall Efficiency

[LesMoss]

Does anyone know the round trip (charge/discharge) efficiency of a Powerwall 2?

My Winter off peak rates on CA PGE E-6 offer only an 8% savings over part-peak. My guess is that the round trip losses are at least that 8% making time shifting uneconomical. Are there any measurements or published data.

Ideally, the Powerwall stats would report battery in/out data just like they do for the grid. That would make it transparent.

 

[wwhitney]

Per the current Powerwall 2 NA spec sheet available here:

Powerwall Owner Documents | Tesla

the quoted round trip efficiency is 90%, for 3.3 kW charge/discharge at beginning of life.

Cheers, Wayne

 

[LesMoss]

OK, sounds like my guess was correct. I can leave it strictly in backup mode until summer rates start. I missed the CA SGIP, so no minimum discharge motivation for me.

My typical charge/discharge rate would be more like 2kW. I wonder if that would change anything significantly.

 

[woferry]

My typical discharge is around 600-800W, when charging during the day it will charge as high as 5kW from solar. I see around 85% round-trip efficiency with my single PW (~10kWh in to get ~8.5kWh out). It's harder to determine long-term averages since the app stopped reporting total kWh in to the PW beyond the single-day view (at first that sounded like a bug, but they've done a bunch of app updates since then and the measurement has never returned), but any time I've calculated it the answer has been right around 85%.

 

[LesMoss]

woferry, What is your motivation for discharging the battery? Are you on SGIP?

 

[woferry]

No, no SGIP. With my typical household consumption and solar system size one PW2 basically keeps me off the grid all but the very cloudiest days (maybe 7 days since I flipped the switches in May, sometimes only for 10-30 minutes and less than 1kWh). So I'm hardly accumulating any NBCs as a result, which should minimize what I need to pay at true-up. Though I also don't personally see any point in having it and not using it.

 

[mikemillar]

You can work out your efficiency by accessing the total battery energy exported and imported. To do that you need to download the raw data from the powerwall. Type in x.x.x.x/api/meters/aggregates into your browser, where x.x.x.x is your gateway IP address. You'll get a long string of data in 4 sections, "site", "battery" "load" and "solar" In the battery section is the data you need. Here's my current state:
"energy_exported":1240020,"energy_imported":1414290,
That works out at 87.5%, so not quite the 90% advertised.

 

[Not Sure]

woferry, What is your motivation for discharging the battery? Are you on SGIP?

I'm not sure but..... isn't that the purpose of having a battery?

 

[jjrandorin]

I'm not sure but..... isn't that the purpose of having a battery?

People buy these batteries for different reasons. Some buy them to get mostly "off the grid" (even though grid connected) and use the solar they are generating (or about to be generating, if its a new install) to run their house both during the day when the sun is shining, and during the evening when it isnt. some do this for cost reasons, some for environmental reasons, and some a bit of both.

Some others buy the batteries as a replacement for a whole home generator, with the intention that they only really get used in a power loss / backup situation. For these people, a powerwall can represent an "easier, more seemless / faster" transition between not having power and having power during a power loss situation.

Still others either got hit by the "new normal" of forced power outages by their local power company, and a few days without power at home in a modern society will make someone contemplate almost anything to "not have to go through that again". For them, any potential monet savings likely takes a back seat to "Im never going days without power at my residence again".

So, not everyone cares about saving money by using a powerwall... although its my opinion that most purchases have some combinations of the reasons above.

 

[LesMoss]

Good summary by jjrandorin.

My situation is similar to woferry. I have had solar power for 10 years and have never paid more than the monthly minimum for my grid connection. So, I have no way to save money with a Powerwall. I got it recently as a backup system. If fact, I ordered it in September while I was in the middle of a "forced power shutdown.

There is no economic case for a Powerwall. Its just an expensive convenience. Its always cheaper to add more solar and or a backup generator. There is an environmental case to be made for it.

I was attempting to be an environmental good guy by using some of my backup capacity to to avoid peak time usage which might also avoid some "dirty power" usage. It was then that I realized that I could be using more power by time shifting as a result of the charge/discharge loss.

So, to answer woferry and NotSure: That would be the reason for "having it and not using it".

Its possible (probable?) that future tariff changes will give me a reason to time shift to save money. I will also probably be a good guy in the "Summer" when the peak/off peak differential larger than the Powerwall efficiency loss.

 

[JohnRatsey]

My PW reports energy imported 3752660, energy exported 3192980 (unit not stated - I presume Wh) which gives almost exactly 85% efficiency. I suspect that Tesla's 90% is measured by first fully charging the battery and then immediately discharging it. If the battery is left for a day before discharging then it will use up some energy to keep itself functional and comfortable (heating or cooling if needed). My total energy import is equivalent to about 6 kWh/day so the battery is having a relatively easy life in terms of energy in and out, but the overheads remain constant. The upside is that I would expect the battery to have useful capacity well beyond 10 years.

I decided to buy the battery when I saw the big mismatch between solar generation and household demand. The former going into the grid at a relatively low price to be bought back, sometimes only a few minutes later, at triple the price. In addition, the ability to do night time charging with cheap electricity when solar generation is poor (note: I'm in the UK) has provided another financial benefit. However, it's going to be a very long time before the investment is justified financially. In the mean time it keeps me entertained!

 

[Odiemac]

There is certainty an economic case for the Powerwall, at least at the grid operator level who fire up a bunch of expensive and dirty peaker plants as the sun sets on grid scale solar arrays.

Whether the grid operators chose to pass on these costs or savings to customers is another matter. So far they’re mostly trying to gut NEM credits by shifting peak hours (4PM to 9PM now for PG&E EV2-A), while Arizona SRP demand charges are closer to the true economic cost of peaker plants.

 

[mikemillar]

People buy these batteries for different reasons......

Well put. In my case it's partly economic, partly future proofing and partly environmental.

Economics - I did a lot of analysis before buying and I think it will break even or maybe slightly reduce net costs over 15 years.

Future proofing - if electricity prices go through the roof it will make more economic sense. Plus, grid outages may become more common in 5-10 years time if the government doesn't get it's finger out - in the UK, several nuclear power stations will reach the end of their life.

Environmental - it helps to shift the grid load from day to night so helps to reduce the number of power stations required. (but if lots of Ev's will be charging at night?)

My 87.5% efficiency is based on total energy import 10-12kWh/day. Interestingly, it's working better in the winter than the summer, in that since we get very low solar during winter it always charges to 100%, so cloudy days don't significantly affect things.My system is very well balanced, in winter the battery usually gets very low but rarely runs out. However, in summer it doesn't charge enough before a cloudy day if it follows sunny days.

 

[LesMoss]

You can work out your efficiency by accessing the total battery energy exported and imported. To do that you need to download the raw data from the powerwall. Type in x.x.x.x/api/meters/aggregates into your browser, where x.x.x.x is your gateway IP address. You'll get a long string of data in 4 sections, "site", "battery" "load" and "solar" In the battery section is the data you need. Here's my current state:
"energy_exported":1240020,"energy_imported":1414290,
That works out at 87.5%, so not quite the 90% advertised.

 

[LesMoss]

Thanks mikemillar, my initial reading based on 4 days and a total export of 7.3 kWh says my efficiency was only 75%.

 

[JohnRatsey]

Thanks mikemillar, my initial reading based on 4 days and a total export of 7.3 kWh says my efficiency was only 75%.

You need a much longer period of charging / discharging to get a representative number for the efficiency. Alternatively, a calculation over a short period needs to allow for the state of charge at the start and end of the measurement period.

 

[LesMoss]

You need a much longer period of charging / discharging to get a representative number for the efficiency. Alternatively, a calculation over a short period needs to allow for the state of charge at the start and end of the measurement period.

Yes, I made sure to measure at 100% charge level.

I am currently running on Backup Only. I will check for a loss with no power output.

 

[cwied]

Also don't forget the fixed use each day from battery conditioning and running the controller in the Powerwall. I think at least 500 Wh a day per Powerwall go to that. That would be significant when your use is only 7.3 kWh over four days.

 

[JayClark]

Also don't forget the fixed use each day from battery conditioning and running the controller in the Powerwall. I think at least 500 Wh a day per Powerwall go to that. That would be significant when your use is only 7.3 kWh over four days.

This matches my calculations. I have four PWs, and I see about 2kW losses per day if I'm not drawing from the PWs at all on the weekends, and I'm just letting extra solar go to grid... the PWs still seem to slurp up about 2k/day no matter what.

 

[Regnad]

Agree... My 2 PWs discharge to 85% each night and charge back up to 100% in the morning. Usually it’s a 3.6 KwH discharge and a 4.6 KwH charge, so about 1 KwH loss per day.