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Comment by caf | Hacker News Reader

So you want to subsidise the purchase of expensive cars to the point where it is financially a good deal. This magic money no doubt comes from other taxpayers.

The proposal isn't a subsidy. It's a straight market payment for an economic benefit - the difference in cost between off-peak and peak electricity is real, and this means that storing off-peak electricity and releasing it at peak is an activity that's worth money.

I'm not sure I agree that the economics work out exactly as described, though - if it would be worth paying for everyone's lithium car battery packs to do this, then it would be even more profitable to build a giant lithium storage battery in a central location.

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Well, the benefit is that it combines the capital costs of owning a vehicle and storing energy. Many people will be purchasing a 160+ mile range car, but day to day use only 20-30 miles of that - an unused capacity large enough to power an average house for over 24 hours.

This is in contrast to dedicated lithium ion batteries, which just don't work out economically. Otherwise you're right - the utilities would do it themselves at scale.

I can see how that's true if the car owner's continue to pay the capital costs of the battery - the part I'm taking issue with is where you say that it would likely that the car owners would have the capital cost of their battery packs paid for by taking part.

My back of the envelope calculations elsewhere in the thread were $1571/yr savings per car (10% overall reduction in grid cost); the Model S battery replacement insurance is $12k per car, which is a ~7.5 year payback (by which time, they claim the pack will be at 70% effectiveness).

Elsewhere someone corrected me that the peak grid usage in a year is actually significantly higher - the figures I was looking at were average daily figures - which may make grid savings quite a bit higher than 10%.

I don't think it necessarily is a clear financial win now; I'm just saying, if you squint just right it kind of makes sense now, and with the march of battery technology and PV, not to mention the inevitable adoption of EVs whether or not this scheme exists, it's going to look better and better from here on in.

No matter how you slice the numbers though, the fact remains that if it's economical to pay for someone's car-optimised battery pack, then it must be even more economical to build a giant fixed battery.

(Since we know that the latter isn't economical, then that also indicates a problem with your numbers - I suspect that for one thing, insurance couldn't be economically provided at $12k/pack if all of those packs were being used for daily peaking storage).

I suspect you're right, and it may well mean it doesn't make outright financial sense today.

The key is probably that the $12k replacement program is an investment in Tesla; they're cash starved, and are willing to sell the batteries at a loss in return for the cash advance. Also, they're betting on the batteries being ~half price in 10 years.

From further reading, they'd be $30k - 2.5 times the price - to buy upfront today.

But that doesn't mean that the subsidy doesn't exist - if 1% of consumers would buy a Model S due to its other advantages, maybe 2-3% would if that purchase returned $1500 a year back to them.

So while you're right that it's not outright going to pay for itself in 2012, it probably halves the gap between a Model S and a BMW in the same class, and it's only going to get better as battery/PV technology improves.

Wouldn't the increased cycling of the batteries significantly decrease their lifespan?

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