Here are some numbers to ponder about our U.S. electricity generation.

Total generating capacity of power plants (in megawatts):

Coal-fired plants: 336,000
Natural gas plants: 449,000
(Source)

Total electricity generated per month (in terawatt hours, 2008–2009):

By coal-fired power plants: ~150
By natural gas plants: ~70
(Source)

That’s right. We have far more generating capacity with natural gas than coal, but we generate much more electricity — by a factor of more than two — from coal than natural gas.

The result is a lot more carbon dioxide (CO2) emissions.

CO2 emissions (in million metric tons/year, 2007):

From U.S. coal-fired plants: 1,980
From U.S. natural gas plants: 376
Sum from both: 2,356
(Source)

CO2 emissions if electricity were generated at existing natural gas plants instead of coal-fired plants: ~1,600**
(Source [pdf])

The reason natural gas plants are not used as much as coal-fired plants is that once the plant is built most of the expense is due to fuel costs, and BTU to BTU, natural gas is more expensive than coal.

Average fuel costs (per million BTUs, August 2009):

The average price paid for coal:  $2.22
The average price paid for natural gas: $4.09
(Source)

But when the investment (and operating) costs of building new coal plants compared to gas plants are considered, the cost of the electricity from both is similar.

Costs of electricity (including fuel, operations, and investments) in $/kilowatt hour:

Coal: ~0.06
Gas: ~0.06
(Source)

Current coal power:

Number of existing coal plants: 614
(Source)

Future coal power:

Number of new coal plants proposed between 2000 and 2009: 214
(Source)
Number currently in development or planning stage: 83
(Source)
Additional generating capacity from these proposed plants: 47,000 megawatts
(Source)

Annual CO2 emissions (in million metric tons) ...

Anticipated from the 83 plants: ~350
(Source)
If this capacity were fueled by natural gas instead: ~240 *
If this capacity were fueled by wind instead: ~0

So, not only is it less expensive to build and operate a natural gas power plant than a coal one, its CO2 emissions are also substantially less. Why exactly is it that we are building all these new coal-fired power plants?

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Notes:

*Assumes current fleet emissions for natural gas savings relative to coal–fired emissions, where electricity from natural gas emits 68 percent as much carbon as from coal-fired electricity. (Source [pdf]) EPA puts CO2 emissions from natural gas at about 50 percent of that from coal. (Source)