(Or, as my friend Ryan referred to them, Renewable Energy Indulgences.)
The basic mechanism of RECs is subsidies of renewable energy
businesses (either profit or non-profit). In principle, this means
that more electricity from renewable energy gets put on the electric
grid, which displaces electricity from non-renewable energy (by
federal law). By paying a little extra, you in effect buy electricity
from renewable sources, rather than from non-renewable sources (like
coal and nuclear power, in the case of Washington, DC's, provider,
PEPCO).
There are at least two ways to purchase renewable energy: through
your local utility, or by purchasing RECs. These options are not in
principle different, since either way more green electricity goes on
to the grid somewhere (either in your area or elsewhere). In my state
(the District of Columbia), the first option (through Washington Gas and
Electric) is significantly more expensive than the second. (I
suspect the difference is administrative overhead.) So I buy RECs
from a wind energy company (Community Energy) that has
wind turbine farms in nearby states, ensuring that there is a possible
impact on my own air quality! In other states, renewable energy costs
may be subsidized by the state or local government (this is true in
Maryland), bringing down the cost of buying through your utility.
Community Energy sells RECs in blocks of 100 kWh (kilowatt-hour, or
one hour of continuously using 1 kilowatt of power) at $2.50 per block
(or 2.5 cents/kWh). My wife and I purchase 800 kWh to cover our home
electricity costs. If you want to use this also as a carbon offset
for, e.g., plane travel, you could buy more. One's total carbon
footprint is the sum of carbon dioxide produced through
electricity generation and personal travel, plus many other things.
In other words, a true carbon offset would require many more RECs than
purchased to simply cover home electricity costs.
To purchase RECs independently, you can find options on
the Green-E website. To find out about options through your local
utility, go to that company's website.
The details of what exactly RECs represent are a bit more
complicated (e.g., the Wikipedia entry below).
And don't forget that energy conservation is the best first step
towards cleaning up the atmosphere.