Time shifting on SRP (Scottsdale, AZ)

[h2ofun]

oh interesting. so maybe this isn't a utility rule issue but a federal tax credit issue. interesting.
Ok, so who is checking if I get a federal tax credit to let me charge the powerwalls from grid or not? Is this an honor system thing?

I have been told that when the system is installed, the installers setup the units based on configuration. Meaning, if you have no solar, they set it can be charged from the grid. If you have solar, they somehow disable this "feature". So, seems not something us users can play with.

 

[miimura]

It seems like regardless of the reason, the idea of charging off-peak for use during peak is simple math in terms of payback" (peak - off-peak)*amount*0.9. This would be true whether or not you have solar. "amount" would be the lower of the battery capacity or actual use during peak. And the "0.9" reflects the loss in converting the energy to DC and back.

In my case, with my small solar system, my payback is also limited by my daily winter solar generation. Most days from Mid-November to Mid-February, my Peak usage is more than my solar generation. My batteries are only cycling between 65% or 75% and 95%. Even this week, I have hit my reserve of 65% before Peak ended on several days. The PWs never hit 100% either, so I set the Reserve high to preserve as much energy as possible for backup purposes. There's no point to cycling the batteries between 25% and 50% when I can cycle them between 75% and 100% with more safety cushion.

 

[h2ofun]

It seems like regardless of the reason, the idea of charging off-peak for use during peak is simple math in terms of payback" (peak - off-peak)*amount*0.9. This would be true whether or not you have solar. "amount" would be the lower of the battery capacity or actual use during peak. And the "0.9" reflects the loss in converting the energy to DC and back.

I do not know what the peak times are in AZ, but the typical use for PWs with TOU rates is to charge them up in the morning from solar, before peak, and then discharge them in the afternoon/evening during the peak. This would likely be more consistent with utility NEM rules. However, I don't see it as an abuse to use PWs as proposed, as long as the installation follows the utility rules and doesn't try to claim rebates/credits that would be disallowed by the use. In some areas utilities want, or are at least indifferent to, the idea of customers grabbing energy at night and using it during the day. It makes a lot of sense for utilities in some areas to support this.


Thanks - wasn't certain how that worked.

I was reading an article from an installer about this use shift for cost. They commented directly that they do not recommend a whole bunch of this since doing so would degrade the batteries faster. That is the piece I do not see many talk about, is these batteries do lose they abilities if exercised too much. For the same reason I hear Tesla does not want to allow the batteries in their cars to be used for home backup/storage/cost shifting.

 

[wjgjr]

In my case, with my small solar system, my payback is also limited by my daily winter solar generation. Most days from Mid-November to Mid-February, my Peak usage is more than my solar generation. My batteries are only cycling between 65% or 75% and 95%. Even this week, I have hit my reserve of 65% before Peak ended on several days. The PWs never hit 100% either, so I set the Reserve high to preserve as much energy as possible for backup purposes. There's no point to cycling the batteries between 25% and 50% when I can cycle them between 75% and 100% with more safety cushion.

Yeah - I should have been clear I was talking about charging from the grid off-peak, as was proposed here. In that case, presumably obtaining enough charge will not be an issue.

 

[wjgjr]

I was reading an article from an installer about this use shift for cost. They commented directly that they do not recommend a whole bunch of this since doing so would degrade the batteries faster. That is the piece I do not see many talk about, is these batteries do lose they abilities if exercised too much. For the same reason I hear Tesla does not want to allow the batteries in their cars to be used for home backup/storage/cost shifting.

That is a good point, and I just double-checked the Tesla warranty. If you are using it for other than "Solar self-consumption/backup only", the warranty is limited to 10 years or "37.8MWh of aggregate throughput." This comes out to around 8 years of daily charging/discharging at 13 kWh, but it might actually be closer to 10 years anyway if the battery degrades towards the warranted 70% over that time. So it is something to consider - particularly if you are banking on getting more than 10 years out of the PW - but it seems like the warranty suggests Tesla expects the PW to still make about 10 years at 70%, even with a daily charge/discharge cycle.

 

[h2ofun]

That is a good point, and I just double-checked the Tesla warranty. If you are using it for other than "Solar self-consumption/backup only", the warranty is limited to 10 years or "37.8MWh of aggregate throughput." This comes out to around 8 years of daily charging/discharging at 13 kWh, but it might actually be closer to 10 years anyway if the battery degrades towards the warranted 70% over that time. So it is something to consider - particularly if you are banking on getting more than 10 years out of the PW - but it seems like the warranty suggests Tesla expects the PW to still make about 10 years at 70%, even with a daily charge/discharge cycle.

Folks just need to remember the 70% in their numbers. Just like solar panels degrade. Just like we degrade