Efficiency numbers for solid fuel stoves are usually just a lot of salesman BS.. the reason being that someone will start arguing about 5% yet the
fuel can vary 25%.. so the numbers are almost totally subjective..
IF someone took X fuel, tested it for BTU content and then tested the stove, then you may have a number that meant something..
But, have you ever looked at a chart for the BTU content of various wood species, and just how dry is DRY? Just too many variables.
Now coal, it is tested, and an actual spec sheet can be obtained, so that at least helps.. but then comes in the other Variables:
Operator style, experience,
Chimney type, draft,
Size of coal.
Desired heat output from stove : exterior of stove body temperature.
There are just too many variables and subjectiive opinions that are assigned to have meaningfull NUMBERS..
But, Coal you can load probably every 8-15 hours, depending on how hot you burn the stove.
With wood, you will add wood every 2-4 hours..
Just what type of chimney DO you have??
And make sure you buy and install at least TWO carbon monoxide dectors before you light a match to any coal !!