Commercial coal has a carbon content of at least 70%. Coal with a heating value of 6.67 kWh per kilogram as quoted above has a carbon content of roughly 80%, which is

\frac{0.8 \ \mathrm{kg}}{\mathrm{12} \cdot \mathrm{kg/kmol}} = \frac{2}{30} \ \mathrm{kmol} , where 1 mol equals to NA (Avogadro Number) atoms.

Carbon combines with oxygen in the atmosphere during combustion, producing carbon dioxide, with an atomic weight of (12 + 16 × 2 = 44 kg/kmol). The CO2 released to air for each kilogram of incinerated coal is therefore

\frac{2}{30} \ \mathrm{kmol} \cdot \frac{44 \ \mathrm{kg}}{\mathrm{kmol}} = \frac{88}{30} \ \mathrm{kg} \approx 2.93 \ \mathrm{kg}.

This can be used to calculate an emission factor for CO2 from the use of coal power. Since the useful energy output of coal is about 30% of the 6.67 kWh/kg(coal), the burning of 1 kg of coal produces about 2 kWh of electrical energy. Since 1 kg coal emits 2.93 kg CO2, the direct CO2 emissions from coal power are 1.47 kg/kWh, or about 0.407 kg/MJ.

The U.S. Energy Information Agency's 1999 report on CO2 emissions for energy generation, quotes a lower emission factor of 0.963 kg CO2/kWh for coal power. The same source gives a factor for oil power in the U.S. of 0.881 kg CO2/kWh, while natural gas has 0.569 kg CO2/kWh. Estimates for specific emission from nuclear power, hydro, and wind energy vary, but are about 100 times lower.