SDG&E EV-TOU-5, Battery Churning, Inadequate App Programmability

SDG&E EV-TOU-5, Battery Churning, Inadequate App Programmability

j.bowden | 27 March 2020

Some discussion in a thread on
has convinced me that I cycled my Powerwall battery through the month of February for no cost advantage at all.
The reason is the EV-TOU-5 peculiar 0.01 difference between PEAK and OFF PEAK rates. This difference is not worth time shifting using the battery due to the RTE (round trip efficiency).
However, the app does not know the difference is only 1 cent! If you simply set the TIME RANGES for the PEAK and OFF PEAK rates (Time-based control), the app will use the battery, and dutifully charge from your solar harvest excess until PEAK time then feed your house from the battery. Every Day!
I believe I would get the same bill if I turned the battery to Backup Only and sold all the excess solar harvest to the grid, then after sunset bought from the grid instead of draining the battery.
Anybody else frustrated by this?
(It will be different for SUMMER RATES when there is a BIG difference between Peak and Off Peak)

Thanks! | 27 March 2020

Here in the PG&E area there is always a huge difference between peak and off-peak - about $.30 /kWh with EV2-A TOU plan. Seems every utility has multiple rate plans and you'll have to figure out if using time shift makes sense on your plan, or if a different plan is better. The utilities don't always make it easy.

Up here time-shifting power makes a huge amount of sense, especially since they credit something like $0.02 when you send your solar to the grid.

Add to the fact the utilities are changing how each plan works, it's hard to keep on top of it. Our older EV-A plan was forced to change to EV2-A a few months back that has more off-peak, but all the rates went up. Hard to see if that is better or not.

Passion2Fly | 28 March 2020

Yes, I usually turn off my Time Based control from Nov 1st to Feb 28th each year. However March and April are great months because super off peak is 10am-2pm Mon-Fri when maximum solar production occurs. Make sure that you don’t miss these months before the June 1st summer schedule.

gregbrew | 29 March 2020

I'm convinced that utilities purposefully provide a moving target when it comes to their tariffs. It keeps customers too confused to be able to select the rate plan that best maximizes their solar investment.

Utilities hate competition. | 30 March 2020

@gregbrew - I agree. It's sort of amazes me that PG&E for as long as I can remember still refuse to adjust the tariffs for Daylight Savings Time. The entire utility is only in Califonia, so we have DST. This is is likely a 2-hour programming job in the billing software. By not shifting all the TOU rates an hour twice a year, it only adds confusion as to when various time-of-day tariffs start and end.

Jones | 30 March 2020

What about the issue of *NOT* cycling your battery ? Leaving it at 100% charge for months at a time. Do we have any battery experts to advise on this dilemma of constant cycling versus fully charge for extended periods ?
I decided to let mine cycle but keep the reserve at about 35%. It's not so much about the arbitrage as it is about battery health. Which is the better choice ? | 31 March 2020

Tesla uses a different battery formulation and strategy on the PW than the cars. They are designed to charge to 100% and discharge to some lower value. I set mine to 30%, but I think lower would be fine too. I think the default is 20%.

These are warranted for 10 years and expected to go through a full cycle every day, or about 3650 cycles over 10 years. My guess is the batteries are rated for 5,000 to 10,000 cycles before they degrade to 70% of the original capacity.

jamielnftaylor | 1 April 2020

I would like Tesla to upgrade the app for those of us who use solar systems, powerwalls, and Tesla vehicles, to allow us to customize our power flows to better match the Time-Of-Use demand periods of our local utilities.
For example, here in Ontario, there are currently two on-peak ($0.208/kWh) time periods per weekday, with a mid-peak ($0.144/kWh) period interposed during mid-day, and a 12-hour off-peak ($0.101/kWh) period overnight.
The price schedule editing tool in the app currently only allows me to set one on-peak period during the day. It is also allowing energy to be sent to the grid during a mid-peak period, without the powerwalls first being fully charged up. It should allow us to direct solar energy into the grid during mid-, or off-peak hours only once the batteries are fully charged.
Similarly, it should also allow us options to set the car not to charge from the grid, except during off-peak hours, or off-peak plus mid-peak hours.
Does that make sense?

gregbrew | 2 April 2020

Yes, but good luck with that... | 2 April 2020

@jamielnftaylor - Better TOU customization makes sense. On car charging, why not set the car to start charging at midnight? If the PW has power available - it can charge the car, but otherwise would use the grid at the lowest of-peak cost.

mikenorthrup | 2 April 2020

@jamielnftaylor I have the same setup as you here in California PG&E country (SolarCity/Tesla for 4 years, Tesla Model 3 for 3 months and Powerwall for 3 weeks.) I'd like to keep my solar from going back to the grid before my Powerwall is completely recharged. | 4 April 2020

@mikenorthrup - It's clear the Powerwalls have a limit to how much energy W/h they can take during charging - about 1.9 kW/h per Powerwall. Once your solar exceeds that amount, the rest is diverted to the grid. So nothing you can do to make the PW charge faster. In the app, ENERGY USAGE you can overlay the Powerwall and Grid to see the excess power going to the grid on a prior sunny day. To get to that screen, tap the grid icon on the POWER FLOW screen. Then tap the Powerwall icon at the top.

Atom12 | 9 April 2020

"about 1.9 kW/h per Powerwall"

No: "Real Power, max continuous 5 kW (charge and discharge)"

I've seen 9.9 kW on my dual PW system. | 11 April 2020

@Atom12 - Thanks for the reference. Not sure what may be going on with my system. I brought up the app and looked back over the last 7 days, and the maximum was 5 kW charging for 2 Powerwalls (2.5 kW per Powerwall) when solar was generating 7.1 kW, with the excess going to the grid. So more than the 1.9 kW from earlier days, but not a lot more. You're seeing 9.9 kW charging, then I'm sure that's the real limit.

Not sure why I haven't' got to the limit of the solar roof output, but as we get closer to the summer peak, maybe I'll see higher charging rates. I've had peak output from the Powerwalls at 11.2 kW so both are working. I'm also on software version 1.45.2, which I think is the latest.

Atom12 | 11 April 2020

Were your PWs in the mid 90's SOC? Charge taper kicks in there.

erneststephens21 | 14 April 2020

I'm convinced that utilities purposefully provide a moving target when it comes to their tariffs. It keeps customers too confused to be able to select the rate plan that best maximizes their solar investment.

gregbrew | 14 April 2020

"erneststephens21" is a bot/script. The post above is a direct copy of my post of March 29, with a link shoved in the middle of it. I've complained about bots and spam postings in this forum to Tesla customer support. I hope others do too.