> What's the manufacturing breakeven in carbon costs for a windmill or a solar panel?
This questions feels like you mostly try to justify your otherwise preexisting preference. A proper system would have external costs factored into the price, by the way.
They don't for example, wonder if no one has thought to calculate the carbon impact of renewables, because they assume the scientists and other authority figures they trust who recommend it would have taken that into account. And if they did wonder, it's a short Google away, and their usual info sources would provide facts, not scaremongering propaganda.
That's why you'll rarely see a defence of nuclear that doesn't quickly degenerate into attacking Californian hippies, or government interference, or the collapse of Germany into a solar powered Islamic no-go-zone, or flat out denial that climate change is even a problem anyway" that they've read about in their highly reliable news sources. Generally they're more anti-renewable than pro-nuclear.
"Using Megapack, Tesla can deploy an emissions-free 250 MW, 1 GWh power plant in less than three months on a three-acre footprint – four times faster than a traditional fossil fuel power plant of that size. Megapack can also be DC-connected directly to solar, creating seamless renewable energy plants."
We need more Gigafactories (regardless of manufacturer/owner), which needs demand to be proven (contracts with utilities), which allows for more capital to be acquired from capital markets to build more battery factories. Batteries are a known quantity though, versus nuclear where you can spend billions and a decade and still not produce a kWh of power.
Tesla built Gigafactory 3 in Shanghai in 190 days, from breaking ground to manufacturing validation of a Model 3. These are tractable problems.
[1] https://www.tesla.com/blog/introducing-megapack-utility-scal...
Also does your figure of 6000 Megapacks derive from total electricity consumption in the US, or just base load?
Historical cost declines would agree with that thesis. Renewables will approach a price so close to 0 that the battery storage will be the dominate cost in supplying dispatchable energy (but still a lower cost than fossil fuels). Utility lithium battery storage is already cheaper than the most expensive peaking plants, and the cost model improves as costs continue to decline.
https://data.bloomberglp.com/professional/sites/24/Capture2....
https://about.bnef.com/blog/behind-scenes-take-lithium-ion-b... (A Behind the Scenes Take on Lithium-ion Battery Prices)
https://www.nrel.gov/docs/fy19osti/73222.pdf (PDF: NREL Cost Projections for Utility-Scale Battery Storage)