"Being," if this is one sentence.
Less coal was burned in 1980, with its figure being just over 15 billion units.
Less coal was burned in 1980, its figure being just over 15 billion units.
The latter case seems a bit questionable to me. I'd be more likely to use a dash than a comma here. That's kind of my style, though.
If you omit "with," then you may need something stronger than a comma as the two parts of the sentence are no longer strongly linked. If you split it into two sentences (or strongly, using a semicolon), you should use "was."
Less coal was burned in 1980. Its figure was just over 15 billion units.
Less coal was burned in 1980; its figure was just over 15 billion units.