I just signed the documents for my solar/powerwall install yesterday, so this is probably a bit premature, but I’ve been spending some time looking at powerwall documentation and configuration options to try to see what makes sense for me. This wound up being pretty long, so I put a tl;dr at the bottom.
First, here is a bit of background -
-My average power usage over the last year ranged from about 70-75kWh in the winter to 105-110kWh in the summer. Over the entire year of 2019 I used just over 33,000 kWh. Much of the high power demand is due to car charging. I have a model S and I typically average 2000+ miles a month.
-The system I’m putting in is Tesla’s XL size with 15.12kW of solar panels and 4 powerwalls. Tesla estimates that it will generate about 20,000 kWh annually, so it should meet about 2/3’s of my power demand. This system will pretty much fill my Southern and Eastern facing roof ares and nearly fill my Western facing areas, so even if I wanted to go bigger I really don’t have the roof space for it. Additionally, I don’t know that I will continue driving this much in the future, so if that happens I don’t want to wind up with a system that is way oversized for my needs.
-There is no time of use billing available in my area. My main goal for the powerwalls is to provide backup in the event of a power outage, and given that I’m in Florida we can have extended power outages after hurricanes. I lost power for more than three days after hurricane Matthew a number of years ago.
-In Florida net metering works on a 1:1 basis. For each kWh I export to the grid I can get full credit for a kWh I use later. Once every year they will true up and if I have excess credits they will pay me for those at their wholesale rates.
So with all that said, I’ve been looking at powerwall configuration options and at first blush it seems like backup only would be the right option for me, but the commentary here -
Tesla Powerwall says:
I’ve seen similar sentiments in other people’s comments which is making me reconsider, but I still think backup only makes sense for me. Here are my thoughts -
-I have no TOU billing option, so I don’t need to worry about time shifting
-I get 1:1 credit for power sent back to the grid. If I generate more than I use then I would get back less in credit at the end of the year, but given that my solar system will only meet 2/3’s of my power demand I don’t think there is any chance of that happening.
-The powerwall efficiency is only 90%. I think this is the big one for me. Say I was in self powered mode and I used 40kWh from my powerwalls every night, then I would need to put 44.5kWh into them each day, losing 4.5kWh a day. On the other hand, I could put that 44.5kWh into the grid each day and get credit for the full 44.5kWh. 4.5kWh a day is 1642kWh a year. At $0.13c/kWh that’s over $200 a year.
-I feel like there may be some benefit in terms of battery longevity to not drain and recharge them daily. I believe the batteries will last longer without all the use.
-Backup only mode will allow the powerwalls to stay at a high state of charge, so they will be ready for an unexpected power failure. As I said above, that is really my primary goal.
I love the idea of being self powered (at least as much as possible), but I’m just not sure that it really makes sense in my situation. What do you guys think? Is there anyone else who just uses backup only mode?
tl;dr:
Given that I don’t have TOU billing, that my state allows 1:1 net metering, and that the powerwall efficiency is only 90% I think it makes more sense for me to use backup only mode, rather than self powered mode.