Tesla has received a lot buzz in the national press – even predictions about how they will fundamentally change the way we all use and store electricity. Buzz is one thing, facts are often quite different.
As the region’s leading installer of grid-tied, battery-backup solar systems, we field a lot of calls and emails regarding different battery-related components and capabilities. Lately we have had a lot of questions about Tesla batteries and our reasons for not using them at this point.
When talking specifically about the Powerwall lithium-ion batteries, there are two models targeted at residential use: A 7 kW hour daily cycling version and a 10 kW hour storage model targeted for true backup configurations.
First, the 7 kWh model: As indicated, this battery pack is a daily cycling technology. The batteries do not have a “float” state to allow for longer term storage . So the energy you store today must be used tonight. That’s why even Tesla’s own website claims only that its Powerwall battery “stores electricity generated by solar panels during the day and makies it available to your home in the evening.”
In our area, this is a mostly useless capability, because we still have net metering. So any extra power your system makes today can be sent back to the utility, via your bidirectional meter, so you get full credit for it. Some call this using the grid as your storage, which is not a bad analogy.
In areas with no net metering (like Hawaii), the idea is to send excess power to the Tesla for short-term storage and then use it tonight before it expires. I wonder about the ROI for this system, but that is the process.
Now the 10 kWh model: This was the system that was being tested for deployment by at least one of the big national solar companies for their own proprietary battery-backup system. But a not-so-funny thing happened on the way to a national rollout. According to multiple press reports, the 10 kWh batteries failed – a lot, and have since been withdrawn from availability.
As a result, the only Tesla for residential deployment is the daily cycling version, which is basically worthless in our area in my opinion. For now, the 10 kWh version is removed from the Tesla website, and one assumes an improved version will come out at some time. I strongly suggest that you do not want to be one of the early deployments of the next release.
The bottom line? If you are a customer with one of our systems, or considering one of our systems, be assured the battery system you want in the near term is Absorbed Glass Mat (AGM) technology. That is all we use and all we have ever used for grid-tied battery backup. I believe that an AGM battery bank, when coupled with Schneider Electric’s Conext/Xantrex inverters, charge controllers and balance of system components, represents the best residential grid-tied battery backup technology available today, and that is why we use it.
When looking at the systems we deploy, some of the words that come to mind are: mature, scalable, configurable, stable and predictable. As a person who has been engaged in designing and deploying high-end technologies for more than 30 years – in the military, federal government and Cisco Systems – I can assure you that is where you want to be.