It's not a simple calculation. It depends whose numbers you use for the calculation. First off, do you only care about CO2, or are you interested in other by pr

It's not a simple calculation. It depends whose numbers you use for the calculation. First off, do you only care about CO2, or are you interested in other by products of coal consumption like NOx? Which kind of coal plant, how efficient is it? What kind of coal is in use? Then, on the consumption side, it depends on your electric busses' efficiency, leakage of CNG into methane emissions, honestly there are too many specific parameters left unspecified in your question.