Well, here it is, the depressing truth about your new refrigerator, your storm windows and your fireplace glass screen, not to mention the extra layer of insulation you just put in and the neighbors' spiffy new solar water heater: Energy-wise, they were all probably a waste of money.

According to John Rothchild, you might have done better financially just by turning the furnace down and buying a couple of electric spot heaters and an electric blanket.

In this post-crisis era of energy worry, where the new wisdom brands anything electric wrong by definition, this sounds like heresy. But Rothchild says it's hard-wired energy economics.

"I am not moved by the joys of woodburning or the moral values of a solar greenhouse," Rothchild writes in "Stop Burning Your Money." "To me, the only worthy energy conservation decision is one that pays off."

In this new book, endorsed by Harvard energy wunderkind Daniel Yergin, Rothchild treats energy investments like stock market buys: If you save $50 a year on your heating bill after installing $600 worth of insulation, he sees that as an 8.3 percent tax-free return on your investment, or 16 percent if you are in a 50 percent tax bracket.

Anyone with enough money and/or smarts to have done some thinking already about tax shelters and investments can understand that. Rothchild is clearly aiming at those folks, for the book provides graphs, tables and equations to be worked out to fit individual situations. An investment of planning time, he makes clear, will pay off, and very handsomely, too.

Readers who just want to be told what to do and are not prepared to calculate alternatives rather closely will have to look elsewhere for blunt advice. Similarly, virtuous readers who want to save energy for the national good will probably be offended by the Rothchild approach, which is merely to save money for the personal good.

The author, a former Department of Energy analyst who built two energy-efficient houses himself, has provided a how-to, where-to book for energy conservation in the persnickety, half-caulked houses that most of us occupy. It doesn't offer furnace loads of blanket admonitions -- with a few exceptions.

It is an unqualified good, Rothchild says, to caulk up the air leaks around your doors and windows, behind the electric plugs and along the chimney seams in the attic floor: up to a 120 percent return on investment. It is beatific to have at least six inches of insulation above that leak-proof attic floor (40 percent return), and it is unreservedly wondrous to have a 10-cent water flow controller in your shower head. Returns approach 7,600 percent on that one.

But an energy-unconscious refrigerator can undo you. California could give every household in the state a free, new, energy-saving refrigerator or freezer and save 5 percent of the state's current electricity consumption, enough to avoid building two new nuclear power plants that together would cost $3 billion or more, Rothchild says.

Storm windows will double the R-value (insulation efficiency) of your windows, but going from R-1 to R-2 still leaves the windows miserably inefficient. Insulated shutters or shades can go to R-14. Six inches more insulation in your attic may not provide as high a return on investment as many other things, Rothchild cautions, and it is percentage of financial return he is after far more than actual savings in BTUs.

Probably the best part of the book is a detailed and complex section on evaluating your existing furnace and boiler and figuring out whether to update, modify or chuck them. Major investments, as always, pose major risks. Another helpful chapter tells how to evaluate a prospective new house: Take a ruler along to measure the insulation.

Rothchild finds woodburning stoves to be financially poor risks in the city, spicing the numbers with a few horror stories about chic urban energy warriors pouring their wallets into the black iron bellies.

Storm doors and attic ventilators are probably zero profitmakers, as are water heater timers. As for solar furnaces, "I can't imagine a sillier heating choice" financially, Rothchild says.

The most heartbreaking news is that the only thing good about fireplaces is the view. Those glass fireplace screens are a waste of money, Rothchild says, citing various studies to prove it. So are those newfangled grates that move air around the fire. In fact, Rothchild says brutally, "The way to get the most out of your fireplace is to plug it up and forget about it."

The verdict is harsh, but then tradeoffs are possible. The $15.50 invested in this book is a good way to find out what they are.