Fun with Powerwall 2 stats...

[Vostok]

I just received a notice from my “Big 4” energy retailer that my grid electricity costs are increasing from 1 July by about 18%. Quite a whack.

So I put the new tariffs into my massive PW2 spreadsheet and calculated that this price increase has taken 5 years off the battery payback time (as a standalone item), 1.5 years off the solar payback time (standalone), and 2.3 years off the total system payback time.

Maybe I should be happy about that

 

[billinghamn]

@Vostok I'm a few years behind you - my 10kW solar and single PW2 install was in 2021 and I'm just pulling in all of my data for the past 12 months into a massive spreadsheet. Sounds like I am taking the same approach as you. I have been very interested in your commentary here and I've found it useful to apply your lessons to my setup, so thanks for taking the time to post!

One thing I have noticed: in applying an efficiency factor to power going in and out of the PW2, I'm finding it difficult in identifying a factor that provides a calculated state of charge (SOC) that consistently fits between 0kWh and 13.5kWh. If anything it looks like the efficiency of my PW2 is changing and indeed getting worse over time. What have you found since you started monitoring your PW2?

For awareness, I'm based in the UK, so my solar generation is very much biased to the summer. I'm considering the value of installing a second PW2 to offset the big increase in electricity costs in the UK. I'm moving from an off-peak rate of 5p and peak rate of 17.5p (with a 25p standing charge) to a new tariff of off-peak of 7.5p and peak rate of 39.5p (with a 43p standing charge) so quite a hike.

 

[Vostok]

@Vostok I'm a few years behind you - my 10kW solar and single PW2 install was in 2021 and I'm just pulling in all of my data for the past 12 months into a massive spreadsheet. Sounds like I am taking the same approach as you. I have been very interested in your commentary here and I've found it useful to apply your lessons to my setup, so thanks for taking the time to post!

Glad to know that you have found my posts interesting and that I’m not the only slightly obsessive stats-head out there

One thing I have noticed: in applying an efficiency factor to power going in and out of the PW2, I'm finding it difficult in identifying a factor that provides a calculated state of charge (SOC) that consistently fits between 0kWh and 13.5kWh. If anything it looks like the efficiency of my PW2 is changing and indeed getting worse over time. What have you found since you started monitoring your PW2?

I quickly discovered that trying to track the SOC of the PW2 by cumulatively tracking power in and out of it was, at best, approximate and you will quickly build up a cumulative error, even when factoring in round-trip efficiency (which for me has calculated to be about 93.5%). The efficiency does appear to vary over time, probably temperature related. Also my PW2 has lost a fair bit of capacity (more than 20% over 3 years) and is probably on track for warranty replacement, and that also confuses the picture.

I used spreadsheet formulas to cap the SOC at 10% (my minimum) and 100% and then start tracking anew from that point, i.e. when it was clear no more power was coming out of the battery then its SOC must be 10%. Equally when there is solar export the battery must be full. This only works if you are in self-consumption mode. If you are in TBC then forget it - the PW2 can do all sorts of weird stuff and you have no idea.

People have worked out how to extract all the historical PW2 data including SOC (which is only recorded every 15 minutes though, and only to the nearest percent) by writing code to access the Tesla API. It’s not straightforward and code regularly breaks when Tesla changes the API.

For awareness, I'm based in the UK, so my solar generation is very much biased to the summer. I'm considering the value of installing a second PW2 to offset the big increase in electricity costs in the UK. I'm moving from an off-peak rate of 5p and peak rate of 17.5p (with a 25p standing charge) to a new tariff of off-peak of 7.5p and peak rate of 39.5p (with a 43p standing charge) so quite a hike.

What my analysis has shown is that the payback time of the PW2 in my case is not great - more than 10 years - and additional PW2s would have exponentially longer payback times (> 20 years). But I did not buy it for economic reasons - I had a personal desire to maximise self-consumption and minimise grid usage, just because.

But as my post above shows, the more grid prices go up, the payback time of the PW2 reduces

 

[EcoCloudIT]

Glad to know that you have found my posts interesting and that I’m not the only slightly obsessive stats-head out there



I quickly discovered that trying to track the SOC of the PW2 by cumulatively tracking power in and out of it was, at best, approximate and you will quickly build up a cumulative error, even when factoring in round-trip efficiency (which for me has calculated to be about 93.5%). The efficiency does appear to vary over time, probably temperature related. Also my PW2 has lost a fair bit of capacity (more than 20% over 3 years) and is probably on track for warranty replacement, and that also confuses the picture.

I used spreadsheet formulas to cap the SOC at 10% (my minimum) and 100% and then start tracking anew from that point, i.e. when it was clear no more power was coming out of the battery then its SOC must be 10%. Equally when there is solar export the battery must be full. This only works if you are in self-consumption mode. If you are in TBC then forget it - the PW2 can do all sorts of weird stuff and you have no idea.

People have worked out how to extract all the historical PW2 data including SOC (which is only recorded every 15 minutes though, and only to the nearest percent) by writing code to access the Tesla API. It’s not straightforward and code regularly breaks when Tesla changes the API.



What my analysis has shown is that the payback time of the PW2 in my case is not great - more than 10 years - and additional PW2s would have exponentially longer payback times (> 20 years). But I did not buy it for economic reasons - I had a personal desire to maximise self-consumption and minimise grid usage, just because.

But as my post above shows, the more grid prices go up, the payback time of the PW2 reduces

@Vostok - just wondering, how do you work out you have lost capacity (20% over 3 years)? If one is not being a slightly obesssive stats-head how do we "know" what our actual capacity "is"?

 

[Vostok]

@Vostok - just wondering, how do you work out you have lost capacity (20% over 3 years)? If one is not being a slightly obesssive stats-head how do we "know" what our actual capacity "is"?

Ah, you need to interrogate the Powerwall via the Tesla API. There are software tools to do this on github such as TeslaPy, but they are not plug-and-play or GUI-based. They are typically python packages requiring a bunch of stuff to be installed, so coding skills are useful.

Various PW2 status commands return a parameter called "total_pack_energy" which is the capacity in Wh. My current capacity is reported as 10234 Wh, or 77.5% of the initial installed capacity of 13.2 kWh. If the capacity drops below 70% within 10 years, Telsa is liable for a warranty replacement.

 

[EcoCloudIT]

Ah, you need to interrogate the Powerwall via the Tesla API. There are software tools to do this on github such as TeslaPy, but they are not plug-and-play or GUI-based. They are typically python packages requiring a bunch of stuff to be installed, so coding skills are useful.

Various PW2 status commands return a parameter called "total_pack_energy" which is the capacity in Wh. My current capacity is reported as 10234 Wh, or 77.5% of the initial installed capacity of 13.2 kWh. If the capacity drops below 70% within 10 years, Telsa is liable for a warranty replacement.

So I take that no way a pleb like me can know for sure so we need to trust Tesla will bring it to our attention that a warranty claim is claimable?

In a car it’s quite obvious (getting way less km’s) but on a battery it’s confusing and troublesome due to time of the year etc.

Surely, we as consumers must have a way of checking?

It worries me you have noticed a 23% odd drop off in such a short time….

 

[Vostok]

So I take that no way a pleb like me can know for sure so we need to trust Tesla will bring it to our attention that a warranty claim is claimable?

You can roughly work out what the current capacity is by using the Tesla App to see what it reports as “Powerwall Discharge” for a day, with the battery running down from 100% to whatever minimum percentage you have. Click on “Energy” to see the graphs and tap on the green battery icon, and PW discharge is reported at the top.

For this to be accurate, your battery needs to charge up to 100% and then be run down from 100% to 10% (or whatever) without being recharged at any point. Because any recharging during the day will result in double-counting of that amount of discharge.

Scroll back through the days and find one where there was only one continuous discharging event in the day (look at the top chart). For me, that reports 8.5 kWh discharge. Divide by the charging range (90% for me) and roundtrip efficiency (93.5%) and that translates to 10.1 kWh capacity which is close to what the API reports.

It worries me you have noticed a 23% odd drop off in such a short time….

Well it’s not great, but that’s why warranties exist. I will get a brand new battery - and possibly after only 5 years. We’ll see.

 

[Vostok]

Having now imported all 2023 power consumption and solar generation data into my spreadsheet, I have recalculated the payback time of my solar and battery system. As grid rates go up, the payback time decreases. For me, they are now at:

• Solar array by itself - 5.6 years (meaning my array will have paid itself off by December this year)
• Battery by itself - 12.3 years (yeah, not really economic, but I knew that before I purchased)
• Blended payback time - 7.7 years (so roughly by the end of 2026)

My annual electricity bill would be nearly 3 times higher without my solar+battery.