Hi Brian
My mom is considering a similar install to what you have and had some questions.
Could you elaborate on why you say that the summer rate is $180/kw and the winter rate is $40/kw when MassSave says that it's $225/kw and $50/kw, respectively?
This answers your last question as well, but it's because Tesla is taking a 20% cut here. When you fill out the program application with Tesla you have to choose to have them send the incentive to Tesla. So your utility will send the $225/$50 to Tesla, but Telsa only then sends you $180/$40 (in theory, although they have not yet but are responsive when I ask for the status). They say this is to cover the costs of running the program. Its annoying, but in their defense:
- At this small scale, I really do thing they are at best breaking even on the costs of running this. I think there are 1-2 people at Tesla who work on this close to full time, but only in the low hundreds of households enrolled. So optimistically they would decrease this cut as the program scales, but that also seems unlikely.
- Powerwall is still much better break-even than any other battery solution on the market (most kWh/$), much more than 20% better.
- If you are participating in SMART also, the way the DC-attached battery solutions work (SolarEdge, Sunrun which is still SolarEdge, and Pika), you end up not earning SMART generation credits for those kWh (unless they have fixed this) because the gen credits are done using an AC meter provided by the utility which is after the entire DC plant.
- I believe Sunrun is doing it where the end user doesn't get any of the credits and it's all "baked in" on the Sunrun lease/PPA.
Regarding the difference between a 2-hr and 3-hr event incentive rate, I take it that a battery can't charge and discharge at the same time? I thought that I had asked Tesla about not being able to sustain the full 5kw discharge rate for a 3-hr event and asked if her solar system could make up for that by charging the battery while it was being discharged. I guess I was mistaken?
It can charge and discharge at the same time during outage events (well really it either charges or discharges and is offset by the solar - as in if your solar generation exceeds the demand of the house, it will charge, if it does not, it is discharging but at a lower rate - note there is actually a 3rd state where your solar generation exceeds the demand of the house, and the battery is either fully charged or your generation exceeds the charging capacity of the power-wall, in that case the Powerwall signals to the solar inverter via a frequency shift to shutdown the inverters.
Anyways, during one of these events, since its not an outage, you export both your PV and your Powerwall discharge, so any PV generation does not count towards the event discharge, and is not stored in the Powerwall. Originally when this program was drafted they were going to include PV generation during events, but the final program did not, I'm not sure if that was for technical or economic reasons.
Regarding the discharge rate - what determines this? Is it just based on what you can provide (i.e, they take whatever you can/willing to give them)? Or does the utility decide?
I think it gives everything it can give, but calculated to attempt to be even across the event. So if its a 3hr event, I would expect to see 8.6kW discharge on 2 Powerwall's with Storm Watch disabled (which seems to preserve 20%) and 6.9kW discharge with Storm Watch enabled. But we have not seen any 3 hour events yet, so I don't have any data. (I export the csv out of the Tesla app for every day there is an event for my own records)
One final note - I think this is only worth it if you really want the backup (and the backup's value is somewhat questionable in the winter where we often have days that you would not be able to recharge, although if you are using fossil fuel for heating, perhaps your consumption is low enough), OR if you are also participating in SMART and get the storage incentive.
