By the barrel, falling oil prices boost U.S. wealth XLE, the energy sector ETF, has been knocked back to its value in late 2010. [...] while the stock market has been sinking miserably when measured in dollars, it has been doing quite nicely in the stuff we all really need: barrels of oil. In 1970 - well before the first OPEC oil embargo - the price of oil was $3.18 a barrel and the S&P 500 index was 92.15. Not so good for America. [...] we've had a lot of ups and downs. Let's measure the value of America - of the collective net worth of all American households - in barrels of oil. Measured in another way, American households have a net worth that is about twice as large as the proven reserves of OPEC. Better still, in a remarkably short time our country has moved from serious energy dependence to an active debate about allowing exports of our surplus oil and gas.