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I greatly admire Elon Musk. He’s like one part Steve Jobs, one part Tony Stark, and he has a knack for making the impossible both possible and profitable. And usually just plain damned cool.

But his latest idea, batteries for houses to store solar energy, makes zero economic sense for the vast majority of US homes.

The sale pitch is as follows:

1. Make energy while the sun shines but you’re at work.
2. Use it while you’re at home at night.
3. Profit

The problem is, the numbers just don’t work out that way.

The average US home uses around 11 MWHr of electricity per year. However, that number has some serious biases that underestimate the usage among potential Tesla battery customers:

• The average in states where solar power makes the most sense have much higher electricity use, primarily due to a much heavier A/C load. The average in Louisiana, for example, is around 15 MWHr a year.
• The average is heavily skewed by apartment and condo dwellers, manufactured homes, vacation homes, and smaller, cheaper houses — all of which use far less electricity and are far less likely to buy a solar system, much less an expensive battery system to go with it. Larger homes (those whose owners can afford to drop $20-40k on a solar and battery system) generally have higher electrical usage, both for climate control and to feed more computers, TVs, refrigerators, pool pumps, and other power-suckers.

Because of the above, most rooftop solar installations barely provide enough electricity for daily use. Even if all residents are away (itself a real stretch), many of the most power-sucking devices still use a considerable amount of electricity. Air conditioning in the South, for example, doesn’t just run when residents are home, it takes hours to cool a house down from, say, 79 to 73.

Most parts of the country don’t have variable electrical rates, the rates are the same during the day and at night. So, trading electrical usage from one to the other has zero impact on the bill.

Even if rates were double the average at night ($0.20/KwH), which they are not, and you only used your solar power at night, it would take decades to make your money back on the battery.

Also, the battery system Tesla is selling doesn’t actually have the capability of running your entire house (again, see above, the “average” house of a would-be Tesla customer is not the “average” house), so you don’t even get the benefit of having uninterruptible power.

IMHO, Musk has solved a problem that simply doesn’t exist. He’ll sell some batteries, but would-be customers who actually sit down and do the math will find it doesn’t make financial sense, and unlike a Tesla car, there’s not much in terms of cool factor and bragging rights to having a glorified-but-undersized UPS hanging in the garage.