Hey Mr. Green, I would like to encourage my son-in-law to turn off the lights when he leaves a room. To do that, I would have to show the cost benefit. Can you help? — Ruth in Watertown, Massachusetts

In olden times, a household authority figure would say, "Turn the lights out," and that would be that. But today's contentious whippersnappers apparently need a detailed financial analysis before flipping the switch. Fortunately, the math is on your side. Electricity rates are based on the number of kilowatts used per hour, or kilowatt-hours (1 kilowatt equals 1,000 watts). All you have to do to find the daily cost of operating a lightbulb is multiply its wattage by the number of hours it burns, then multiply that by the kilowatt-hour (kWh) rate printed on your utility bill and divide the result by 1,000.

So if a 100-watt bulb burns for ten hours, and the power company charges ten cents a kilowatt-hour, it costs a dime a day to keep lit. That's about $3 per month, or $36 per year. Leaving a half dozen bulbs burning would waste more than $200 per year. If your son-in-law turns off the lights and puts the annual savings into an account that draws 5 percent interest, in ten years he will have about $2,650, a nice little sum he could invest in some booming alternative energy company.

Hey Mr. Green, If you had to recommend a particular brand of gasoline, which would be "best"? Because BP emphasizes "beyond petroleum" and was the first oil company to stop lobbying to drill the Arctic National Wildlife Refuge, I've been filling up only at BP-Arco stations. But with its recent pipeline leak in Alaska, I'm having second thoughts. — Helmuth in San Jose, California

Deciding where to buy gas is about as tricky as waltzing with Dick Cheney on his favorite oil slick. Different oil companies are responsible to varying degrees for different woes. (To name a few: polluting air and water, destroying wilderness, poisoning indigenous people's land, promoting denial of global warming, exploiting workers, and even collaborating with oppressive governments that don't give a dipstick about human rights or environmental protections.)

Further complicating the matter, most oil companies actually do some good things for the planet, such as supporting conservation groups or developing alternative energy sources. But all things considered--and despite its pipeline fiasco--BP-Arco is still your "best" choice, along with Philadelphia-based Sunoco. To see how Sierra ranks the eight biggest oil companies, visit sierraclub.org/sierra/pickyourpoison.

Hey Mr. Green, Why assume that you have to have a lawn on your grave site ("Hey Mr. Green," May/June 2006)? — Herb in Ithaca, New York

Because I've already bought a grave plot, and the cemetery association maintains conventional turf. But I'm sticking with the site, which features assorted ancestors and dearly beloveds, scoundrels and all. Call such a custom deeply spiritual, profoundly human, or just stupidly sentimental, but it remains the preference for many of us.

Environmentally conscious folks who are less attached to tradition can join a growing movement for green burial, which inters ashes or unembalmed corpses in simple caskets, shrouds, or urns in woods or meadows that retain their original character. To learn more, visit greenburialcouncil.org.

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Got a question on any environmental topic? Send it to Mr. Green via our Web site at sierraclub.org/mrgreen.

Illustration by Melinda Beck