Calculating the Calif Battery Rebate - SuperCaps vs Tesla PW
One of the most important competitive advantages is in Cycle Life. Forget about the fact that 13.2kWh nameplate capacity isn’t a reality if you intend to do daily TOU Arbitrage with your Powerwall. They won’t last two years at 100% DoD daily cycling. The real usable energy from a Powerwall is closer to 10.6kWh, or 80% DoD to maintain warranted cycle life. For the purposes of SGIP they can claim 13.2kWh of capacity but, it doesn’t work in the real world. So, back to the warranty. Supercap's is 10 years unlimited. Tesla has a 37MWh roundtrip (18.5MWh discharge) warranty. If you cycle daily (10.6kWh) that’s 3,869kWh/year. In 4.78 years you’d finish the warranty, not 10 years. They count on the fact that the majority of people don’t do TOU arbitrage and their batteries are UPS backup only. Now, say you make a warranty claim and Tesla actually honors it after looking at your usage data. They require the ability to collect data from your system. If you cannot connect the Powerwalls to the web, the warranty is 4 years. But even if they pay you the warranty it’s 1) prorated like on your tires, 2) you need to pay to ship the old Powerwall back, 3) pay to ship the new one in and 4) pay for the removal/install labor. No one is going to cycle their Powerwall daily. They’re good for UPS only.
If the residence installs a grid-tied smart inverter that is “battery ready”, then any of the 48VDC supercaps can be used in parallel for backup and provide the same capability as Tesla for a lower installed cost. So, if a developer uses Sol-Ark, Schneider Electric, Outback, Magnum or one of many grid-tied smart inverters, the Powerwall is not an economic addition. New construction is certainly an easier path to supercap adoption, but there are a lot of “upgrade” opportunities out there. Let’s discuss safety. The terminals can either have boots over the connections or a cleaner look, a sheet of plexiglass screwed to the front of the rack covering the entire stack of modules. These are insignificant cost and easy install solutions. Useable energy, regardless of what Tesla marketing says, is 80% of the Powerwall. Look to their cars for guidance, the batteries are the same as in the Powerwall. Jeff Dahn, renowned Li Ion battery expert recommends, don’t keep your Tesla at 100% charge. Why? Because it kills Lithium batteries to do so. He recommends 70% and a DoD no lower than 20%, but I’ll give them the benefit of the doubt at 80% useable. Tesla battery expert recommends daily charging limit to optimize durability - Electrek As mentioned before their warranty if these things are cycled daily ends in less than 5 years. They degrade 30% over the life of the warranty. They’re a known fire hazard. They have hazardous waste disposal issues. They have temperature, charging, and discharging limits that all reduce life cycles. Once again, look at real-world results from the Tesla cars. Car And Driver's Tesla Model 3 Has Lost 7% Of Its Battery Capacity In 24,000 Miles | Carscoops So three 3.55 supercaps provide the same useable capacity as one Powerwall. The installed cost of a Powerwall is $12k – 13.5k depending on the installer… if you can get one (wait time is nearly a year now). In a battery ready new home, three supercaps would cost $9052 ($850/kWh MSRP). Add a bit for installation so, let's say installed cost of $10k. What you get for $10k on a grid-tied smart inverter is 25-year life, 10-year unconditional warranty, and none of the bad stuff that goes with Lithium batteries from killing miners to fires and hazardous waste disposal. If homeowners want daily cycling against TOU rates and want to feel like they are contributing to the environment, Supercaps are the superior solution. Powerwalls do not provide an economic TOU Arbitrage value and they have no claim of being environmentally sound. In California, Hawaii, and elsewhere there are hundreds of thousands of “legacy” solar systems with no ability to provide UPS backup. California alone has over a million legacy systems, Hawaii 80,000, AZ, a couple hundred thousand, and so on… The Powerwall has an integrated, albeit small 5kW, inverter when coupled with an auto-disconnect switch, can add UPS to these systems. The Tesla killer needs to have an integrated inverter.
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