Tesla Kauai solar-energy generation and storage project [photo: Tesla]
Tesla’s summer announcement of the world’s biggest battery-storage system in South Australia characteristically kept key details confidential.
Fortunately, while mystery and mystique still shroud that project, more can be unearthed or inferred for Tesla's solar panel and battery energy-storage deployment in Kauai, Hawaii, which came online earlier this year.
In fact, there's now enough information that—to paraphrase Elon Musk—we can do the math. Inspiring math it is, too.
While the big story is that Tesla’s solution is cheaper than diesel, the bigger story is that it’s cheaper despite only using two-thirds of the solar panels’ actual production and two-thirds of the batteries’ actual capacity.
Half a Vatican
Tesla’s Kauai project comprises 55,000 solar panels capable of delivering 17 megawatts of peak direct-current power and 52 megawatt-hours of lithium ion battery storage in the form of 272 Powerpack 2s on a 44-acre site.
That's a bit bigger than Buckingham Palace (40 acres) and a bit less than half the size of the Vatican (110 acres).
Tesla CEO Elon Musk presents Powerwall 2.0 and SolarCity solar roof
Tesla Powerwall 2 AC battery storage specifications
Tesla Powerwall 2 DC battery storage specifications
Note that while the solar array is frequently referred to as being 13 MW (AC basis), the Kauai Island Utility Cooperative confirmed the 17 MW (DC basis) figure.
Tesla has contracted with the Kauai Island Utility Cooperative to provide up to 52 MWh of electricity to the grid every evening. The utility has agreed to pay a flat rate of 13.9 cents/kWh for this stored sunlight, about a 10-percent discount to the price they pay for power from diesel generators.
(The island will still need to burn diesel during peak electricity periods - it just won't need to burn as much. Plus, it’s occasionally cloudy and rainy, even in Hawaii.)
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As for why Tesla can’t sell electricity directly to the grid during the day, Kauai’s grid simply can’t absorb any more solar: at midday, photovoltaics can already produce upwards of 90 percent of the island’s needs.
Two out of three ain't bad: Battery Edition
From Tesla’s website, each Powerpack 2 is rated for 210 kwh and is made from 16 Powerwall 2s, which are themselves rated for 13.2 kwh. This makes sense, as 13.2 kwh x 16 = 211.2 kwh.
That said, the absolute energy capacity of each Powerwall 2 is certainly higher. The National Renewable Energy Laboratory estimated that the first generation Powerwall, with a nominal capacity of 7 kwh, was a 10-kwh battery designed to cycle to 70 percent of discharge.
2017 Chevrolet Volt
This would be similar to the two-thirds depth-of-discharge used in the Chevrolet Volt plug-in hybrid, which also uses a nickel-manganese-chromium cell chemistry.
At two-thirds depth-of-discharge, the 210-kwh output provided by the Powerpack 2 implies an absolute capacity of 320 kwh. Thus, the 272 Powerpack 2s on Kauai would have an absolute capacity of 87 Mwh.
The rated capacity of the Powerpacks is 57 Mwh, while the publicly-disclosed number is 52 Mwh.
These differences prove illuminating.