Fun with Powerwall 2 stats...

[Vostok]

The Tesla App update in August finally made it possible for PW2 owners to download all their data at 5-minute resolution (solar generation, household consumption, grid consumption and export, and charging/discharging of the PW2). Unfortunately you can only download one day at a time, so it's tedious to get 3 months' worth but I am a data guy

Anyway, having gathered together all the data for July-September (the first 3 full months of having solar) I can begin my stats-gasm. Two of the things I'm most keen to find out are (a) test different TOU grid rateplans to find out which one would be the most cost-effective and (b) calculate the real benefit of having the PW2 and calculate the payback time - this can be done by comparing what would have happened without a battery to what did happen with it.

I'm still setting up the spreadsheet to answer those 2 questions so I'll save that for a later post, but what was interesting was the battery data. Unfortunately the Tesla data is only recorded to the nearest 0.1 kW (2 decimals should be provided!) but if you run a cumulative total of all the charging and discharging of the battery over time, you get this weird result:


The battery accumulates more and more charge over time! Well clearly it doesn't, but I think I worked out why (and it's not cumulative rounding errors - they should go both ways). The input & output kW are measured at the external interface of the battery - what goes in and what comes out. But that of course is not what ends up in the battery cells due to system losses. 1 kWh in will not charge the cells by 1 kWh, and it takes more than 1 kWh of cell discharge to put out 1 kWh. So I did a goal-seek - if I applied an efficiency factor to power going in and power going out, what does that number need to be in order for the battery to have a long term state of neither charging nor discharging. I then got this result:



And what was the round-trip efficiency factor that produced this more level chart? 93.50% Pretty good! It's not the full story though, because there are occasional "offsets" that push the charge up... I think that is related to occasions where the PW2 actually draws from the grid to maintain a minimum charge. I don't yet know if there is a way to correct the data for that.

Much much more to come as I data-crunch...

 

[Grenadine]

I can begin my stats-gasm. .

This made me laugh! Looking forward to hearing more.

 

[Vostok]

Okey dokey... I waited until October was over then pulled those stats in so that I have 4 full months' worth (July-October).

This post looks at what our solar PV system has actually done. A reminder that it is a relatively small system (compared to others reported here ) of 5.84kW. Our house was continually occupied for this entire period, to put these results in context.

In those 4 months, our house in total consumed 2479 kWh, but we generated 3007 kWh Yay! We are in front already by 21%.

935 kWh of solar was used directly, 1071 kWh of solar was taken from the battery (i.e. when solar was not enough to meet demand). So having the battery has more than doubled our solar self-consumption. The battery is charged only from solar - there are a few blips where the grid puts some charge into the battery, but they are miniscule compared to the overall picture.

473 kWh was consumed from the grid - only 19% of all consumption. Critically, thanks to the battery, only 9% of that occurred at peak ToU rate (2pm-8pm weekdays). 26% of grid use was in the shoulder period, and 65% offpeak - which is mostly charging our LEAF and/or running the dishwasher (if we don't or can't do it during the day). We exported 847 kWh to the grid. We buy 100% Green Power, but even if we didn't, we have injected 80% more green power back into the grid than what we've taken.

Looking at the battery, it spent 18.2% of its time empty (well, at the minimum charge state I have set of 10%) and 12.5% of its time full - and the rest in-between. The charge state distribution is shown below.



I calculate the battery round-trip efficiency to be 93.5% and the solar PV system efficiency to be 94.9%.

The most solar we have generated in a day was 36.0 kWh. The least was 1.56 kWh. Here's the distribution of the daily power flows:



If I add up the off-grid times (i.e. grid draw zero), we have spent exactly 100 days (of the 123 days) off-grid. Meaning we drew no grid power for a total of 2400 hours. The longest continuous time off-grid was 68 hours (2.8 days). On 9 occasions we were off-grid for 48 hours or more, and on 25 occasions off-grid for 24 hours or more. The longest continuous time on-grid was exactly 1 day.

My next post next week will compare electricity rate plans - which one would be the best for this usage pattern? And the final one will be the what-ifs... what would this have looked like with no battery? With a larger batter? Larger solar array? That will answer the question of "payback time" and various optimisations.

So much fun to come...

 

[Blue heaven]

Okey dokey... I waited until October was over then pulled those stats in so that I have 4 full months' worth (July-October).

This post looks at what our solar PV system has actually done. A reminder that it is a relatively small system (compared to others reported here ) of 5.84kW. Our house was continually occupied for this entire period, to put these results in context.

In those 4 months, our house in total consumed 2479 kWh, but we generated 3007 kWh Yay! We are in front already by 21%.

935 kWh of solar was used directly, 1071 kWh of solar was taken from the battery (i.e. when solar was not enough to meet demand). So having the battery has more than doubled our solar self-consumption. The battery is charged only from solar - there are a few blips where the grid puts some charge into the battery, but they are miniscule compared to the overall picture.

473 kWh was consumed from the grid - only 19% of all consumption. Critically, thanks to the battery, only 9% of that occurred at peak ToU rate (2pm-8pm weekdays). 26% of grid use was in the shoulder period, and 65% offpeak - which is mostly charging our LEAF and/or running the dishwasher (if we don't or can't do it during the day). We exported 847 kWh to the grid. We buy 100% Green Power, but even if we didn't, we have injected 80% more green power back into the grid than what we've taken.

Looking at the battery, it spent 18.2% of its time empty (well, at the minimum charge state I have set of 10%) and 12.5% of its time full - and the rest in-between. The charge state distribution is shown below.

View attachment 473646

I calculate the battery round-trip efficiency to be 93.5% and the solar PV system efficiency to be 94.9%.

The most solar we have generated in a day was 36.0 kWh. The least was 1.56 kWh. Here's the distribution of the daily power flows:

View attachment 473655

If I add up the off-grid times (i.e. grid draw zero), we have spent exactly 100 days (of the 123 days) off-grid. Meaning we drew no grid power for a total of 2400 hours. The longest continuous time off-grid was 68 hours (2.8 days). On 9 occasions we were off-grid for 48 hours or more, and on 25 occasions off-grid for 24 hours or more. The longest continuous time on-grid was exactly 1 day.

My next post next week will compare electricity rate plans - which one would be the best for this usage pattern? And the final one will be the what-ifs... what would this have looked like with no battery? With a larger batter? Larger solar array? That will answer the question of "payback time" and various optimisations.

So much fun to come...

You haven't provided the stats on how many times per day you check the PW2 app, have you managed to limit it to twice per day? Just as solar production starts to check battery percentage and once again just before sunset.

 

[Techno-phile]

I am off grid again since September and based on last year will remain so until April (except for export to grid).
It is definitely not worth increasing my solar PV or my PW2 storage to cover the winter months, since PW2 simply tops itself up from offpeak then, which is so cheap.
Thank you Tesla.

 

[Vostok]

You haven't provided the stats on how many times per day you check the PW2 app, have you managed to limit it to twice per day? Just as solar production starts to check battery percentage and once again just before sunset.

Oh, we check it lots of time per day the novelty has not worn off!

 

[Vostok]

It is definitely not worth increasing my solar PV or my PW2 storage to cover the winter months, since PW2 simply tops itself up from offpeak then, which is so cheap.

Have you set your system to use Advanced Time-based control? What mode? Mine is currently on self-powered only, so the battery is (almost) never charged from the grid.

Time-based control looks interesting but obviously it starts making the decisions for you, and it might not always get it right?

 

[Techno-phile]

Have you set your system to use Advanced Time-based control? What mode? Mine is currently on self-powered only, so the battery is (almost) never charged from the grid.

Time-based control looks interesting but obviously it starts making the decisions for you, and it might not always get it right?

In winter months I use Advanced Time based Control, so that PW2 knows to charge sufficiently from offpeak so it can run the house at least until off peak starts again the following evening.
Most of the year this is not necessary, since PW2 charges sufficiently from solar to run the house until next sunrise.

 

[Hairyman]

I’m using Advanced Time Based control presently. With my current arrangements I export enough energy to the grid and import at times of very low cost (just to power the house, not needing to charge the battery). As a result I’m covering a lot of even the daily supply charge of electricity and so my bills are even lower. Powershop is my provider so imports are already carbon zero.

 

[Vostok]

Most of the year this is not necessary, since PW2 charges sufficiently from solar to run the house until next sunrise.

That implies you have more than one PW2. If we charge our LEAF overnight and run the dishwasher, the battery will be fully drained before sunrise, even if 100% at sunset prior.

 

[Techno-phile]

That implies you have more than one PW2. If we charge our LEAF overnight and run the dishwasher, the battery will be fully drained before sunrise, even if 100% at sunset prior.

No just the one, but we do have a very efficient passive solar house.
And I never charge the Tesla off the Powerwall - mostly whilst I am at my gym class each morning, or from Solar at midday.

 

[Vostok]

No just the one, but we do have a very efficient passive solar house. And I never charge the Tesla off the Powerwall - mostly whilst I am at my gym class each morning, or from Solar at midday.

Ah, well we can’t do that. My wife drives the LEAF to work on 4 days, so it has to be charged overnight. The other 3 days of the week we charge from 07:00, so directly from the sun if it’s shining. We aim to run the dishwasher on timer during the day but that’s not always practicable. Even still, I reckon an aggregate of 100 days out of 123 being off-grid is pretty bloody good for a 5.84 kW system!

 

[Vostok]

Time for the grid rateplan comparison. I went to the Green Energy Guide and pulled out some of the top rated suppliers, and some not-so, then went to their websites to pull out their rateplans. I looked only at rateplans with a solar FIT, and preferred ToU charging, but did look at two with a fixed general tariff. Sometimes it required digging to find the ToU plans. I ended up choosing 10 different rateplans from 5 different suppliers.

Because the PW2 provides 5-minute energy flow data, it's possible to calculate exactly what different rateplans would have cost. No more guessing! In all cases, any grid draw was priced at the relevant ToU rate (including whether the day was a weekday or weekend), daily connection charge, 100% Green Energy grid premium, and FIT income. Here's the results as to what my power bill would have been for the 4 months July-October, ordered from lowest to highest:

So the highest is more than 3 times the lowest. The takeaways for me:

• High FIT is king. The two cheapest plans both had FIT of $0.21/kWh - the highest available in NSW
• Higher FIT tariffs also seem to have the highest daily connection rates. The power bill predominantly becomes a battle between the daily connection charge and the FIT - but the high FIT wins.
• General (fixed) tariffs don't appear to be a good idea if you have a battery
• Eye watering peak grid rates don't matter much if you can demand shift most peak usage using a battery
• There was very little difference in 100% Green Power premium between retailers. Origin had the lowest (4.5c), most are 5.5c, the highest was RedEnergy with 5.83c. But if you've reduced your grid usage a lot, the cost difference becomes noise.

For the cheapest plan, the breakdown was as follows:

I am not currently on the lowest rateplan by a reasonable distance. Now I know what to do. There is an argument I should wait for a full 12 months of data to see how consumption patterns change across a full year and all seasons, but I've probably seen enough.

Next week will be the start of the "what-ifs".

 

[Hairyman]

Does the Powershop analysis include the discounts available when you buy their power packs in advance?

 

[Vostok]

Does the Powershop analysis include the discounts available when you buy their power packs in advance?

No, but a 3x gap is a helluva gap to close. I haven’t included pay-on-time discounts and similar that are available on some plans either, although those types of plans seem to be disappearing as the Govt tries to kill off “standing offers” that don’t have that kind of stuff - as a result, it seems those capers are being progressively removed from all plans.

 

[Hairyman]

No, but a 3x gap is a helluva gap to close. I haven’t included pay-on-time discounts and similar that are available on some plans either, although those types of plans seem to be disappearing as the Govt tries to kill off “standing offers” that don’t have that kind of stuff - as a result, it seems those capers are being progressively removed from all plans.

I have pretty much the same solar capacity and PW set up as you. With Powershop’s electric vehicle TOU offer, pack discounts and FIT I’m paying about $18/month for power with a higher consumption (three teenage kids - I called my PW Sisyphus for a reason).

Nevertheless your point is well made and I’ll run the numbers with Origin to have a look.

I’m very grateful for your insights and I’ll be following the adventure closely.

 

[Vostok]

I have pretty much the same solar capacity and PW set up as you. With Powershop’s electric vehicle TOU offer, pack discounts and FIT I’m paying about $18/month for power with a higher consumption (three teenage kids - I called my PW Sisyphus for a reason).

The Powershop breakdown is below. I'm curious as to where this goes off the rails compared to you. The daily connection charge is the highest of the ten plans I looked at, and the FIT is half that of the best Origin & AGL plans. Those two things really hurt it.

 

[Hairyman]

I have a different tariff regime. It is available to anyone with an electric car. It gives me a 9.02c/kWh rate from 0000-0400. At that time my battery supplies much less than the demand if the the car/house - the import cost is below what they pay me to export (10.2c/kWh). I run the HPWC at 11kW off 3 phase and this exceeds the output of the PW2 and the battery husbands its charge for later in the morning when power is more expensive. My solar system faces 307 degrees (a bit north of west).

Powershop also fully carbon offsets its base electricity. When you buy 100% green power from them you are paying them a bit extra for the carbon credits. I haven’t looked hard into how that actually works and this discussion will spur me to look closer. I do end up buying some of this power with accumulated credit. I also buy some if their community power - they use the extra funds to invest in various larger scale power projects by local communities.

I have attached the Powershop EV tariff sheet, details from my recent bill and some screen shots of my energy flows for the day.

I promise that I am not running a backyard aluminium smelter despite appearances!

 

[Vostok]

I have a different tariff regime. It is available to anyone with an electric car. It gives me a 9.02c/kWh rate from 0000-0400. At that time my battery supplies much less than the demand if the the car/house - the import cost is below what they pay me to export (10.2c/kWh). I run the HPWC at 11kW off 3 phase and this exceeds the output of the PW2 and the battery husbands its charge for later in the morning when power is more expensive. My solar system faces 307 degrees (a bit north of west).

I’ll need to mod my spreadsheet to add a fourth ToU rate but I can do that and then plug in the effect of the EV rate, since our Leaf is only charged after midnight and is usually done by around 3am. It also looks like you have set your PW2 to advanced time-based control, which is something I need to try. My solar array is almost perfectly oriented - roof pitch is 40° and azimuth is 23° and it regularly maxes out.

Powershop also fully carbon offsets its base electricity. When you buy 100% green power from them you are paying them a bit extra for the carbon credits. I haven’t looked hard into how that actually works and this discussion will spur me to look closer.

Carbon offsets and GreenPower are quite different things. To me, carbon offsets/credits are the option of last resort when there is no practical alternative to not generating pollution in the first place. The most obvious example is a flight. If you need to fly somewhere, you simply can’t avoid using fossil fuels and creating pollution. So carbon offsets (which all major airlines offer) are a way of trying to negate that pollution (the CO2 bit at least) with a carbon sink somewhere else (typically reforestation programmes). It’s trying to fix the problem after the horse has bolted, but certainly better than doing nothing.

But I think it’s a cop out when it comes to electricity, because of course electricity can be generated directly without pollution. And that’s where GreenPower comes in. Every kWh of 100% GreenPower must be purchased by your energy retailer from an accredited renewable generator, typically solar and wind. And that kWh is therefore not purchased from a coal-fired generator. It’s completely substitutional. If enough people do it, the coal-fired generator sells less and less product and scales down operation intensity accordingly. The benefits of that are obvious.

I’ve had robust email exchanges both with GetUp! (who spruik Powershop) and Powershop themselves over this. I think the fact that Powershop have been given gold stars by Greenpeace on their carbon offsetting policy is rewarding the wrong behaviour in this space, and arguably misleads customers if they don’t realise they are still buying a substantial part of their electricity from coal-fired generators. I asked Powershop what percentage of the electricity they sell comes from coal-fired generators and they refused to tell me.

Now I’m not saying AGL and Origin are better. Certainly not if/when they actively lobby against any form of carbon pricing or emissions target setting (AGL is way improved on that front though, compared to Origin). But in a way, it is more transparent. Buy coal fired power? Do nothing. Buy renewable power? Pick a GreenPower option. There is no fudging inbetween. Bizarrely though, Origin are the only retailer that I know of that offer a carbon-offset natural gas product. So we buy our gas from them, and our electricity from AGL.

 

[Hairyman]

Thanks very much for the clarification. I can easily choose to buy their Green energy power packs - they sell their green energy future packs at a discount and see how the costs go.

You are right to say I have my battery set for advanced. I am on the balanced setting of that presently. It is interesting to see how the behaviour of the battery evolves with changing conditions. Of note with this setting the battery charges itself to full only later in the day so less time is spent at 100% charge and this may help the longevity of the battery.

I hadn’t realised that Origin sold offset gas. I shall change to them shortly. If they have a referral scheme you should PM me your referral link.