You know what a carbon footprint is -- a measurement of greenhouse gases emitted as a result of human activities -- and you know that a smaller carbon footprint is better for the environment. You even know that you can reduce your carbon footprint by

flying less

, buying local products and driving a

fuel-efficient car


But your

carbon footprint

isn't based solely on your travel and shopping habits. If you're like most people, about 40% of your carbon footprint comes from your home. Shrinking your home's carbon footprint has the added benefit of reducing home energy use (and costs). Expect to knock at least $100 off your annual electricity expenses just by making a few small changes -- and if you go whole hog (see below), your next electric bill could be zero. So how do you reduce the carbon footprint of your house?

Step one

Start by figuring out your home's carbon footprint. For a two-person household in the U.S., the average carbon footprint is 41,500 pounds. To determine yours, consult a calculator at the

EPA's climate change Web site


Low Impact Living

or the

Nature Conservancy


For a more detailed assessment of your home's energy use, get an

energy audit

. An energy auditor won't be able to calculate your home's carbon footprint, but she will tell you how much energy your home consumes and how you can make it more efficient.

Go whole hog

If you're ready -- and able -- to make your home's carbon footprint a mere shadow of its former self, go off the grid. Stars like Julia Louis-Dreyfus, Will Ferrell, Daryl Hannah and Ed Begley Jr. run their homes on solar power.

You could too. Solar efficiency varies nationwide -- per installed kilowatt (about $9,000) you can expect to get about 1,300 kilowatt hours a year in Maine and 1,800 in California. To truly go off the grid, you'd have to heat your home with electricity (provided by your solar array) or with an electric-powered geothermal

heat pump

, which can cost up to $30,000 installed.

Of course, there's one much more affordable way to separate yourself from the coal-fired power grid: Sign up for

green power

through your utility company. For a small premium (about 12 cents a kilowatt hour), you can purchase your electricity from wind turbines, solar arrays, low-impact hydroelectric dams, or biogas or biomass sources.

A measured approach

If you'd like to take a more incremental approach, a few simple tweaks can shave pounds off your carbon footprint and dollars off your energy bill. Energy experts say phantom loads -- from any appliance that uses electricity even when it's turned off -- add about $200 to the average annual energy bill. Stopping these energy suckers is simple (and free): Unplug appliances that aren't in use. To avoid the unsightly appearance of unplugged cords -- or to make it easier to turn on and off a group of appliances like an entertainment center -- plug them into a surge protector or

smart strip


Hot water is another energy sink. Use it just when you need it: Take shorter showers, wash your clothes in cold water and only run the dishwasher when it's full. And don't overheat your water. Some water heaters push their contents to 140°F, but 120°F is plenty warm. Consult your owner's manual to safely adjust the water temperature.

Heating and cooling your home with oil, natural gas, propane or electricity certainly contributes to your home's carbon footprint. Keep the indoor temperature seasonable. Set your thermostat between 64° and 68°F in the winter.

To make your home heating and cooling as efficient as possible, beef up your insulation and fill any leaks with caulk or weather stripping. For a little extra money, invest in an Energy Star-certified programmable


. A programmable thermostat can help you cut your energy use by a third, compared with a manual thermostat, by allowing you to set different temperatures for different times of the day and days of the week.

If you haven't already done so, change your incandescent light bulbs to compact fluorescent lights (CFLs). CFLs are now available in different hues and styles including those designed for outdoors as well as dimmer and three-way switches. A single CFL uses about 75% less electricity than an incandescent bulb and lasts 10 times longer, adding up to about a $45 savings over the bulb's lifetime. Most retailers, including Home Depot, recycle CFLs for free. While CFLs do contain a small amount of mercury, there's no need to worry about bringing in a HazMat team if a bulb breaks. Just follow the instructions on the

EPA's Web site


Upgrade appliances

If you're able to invest a little more money, you can reduce your home's carbon footprint even further. Upgrade your appliances to

Energy Star

models. (To keep your old appliances out of a landfill, sell them on Craigslist or donate them to a nonprofit.) Energy Star appliances use 10% to 50% less energy than older models, saving you about $180 a year.

Kelsey Abbott is a freelance writer in Freeport, Maine, where she lives with her husband and their dog.