An open fire place might run at about 10% efficiency which means that 90% of the potential energy in the log is either lost as heat through the chimney lost as smoke due to log not being burnt at high enough temperatures. There is a bit more to it than that but that describes it fairly well.
A “Pot Belly” might get up to 40% efficiency as it does a much better job than the open fire place in ensuring the log is burning at a good temperature and getting more the heat into the room rather than up the flue.
A modern slow combustion heater does an even better job and can get up to 60% efficiency with everything running really well.
That said you often find these heaters not being run at their optimum operating conditions with wood not being properly dry or sap free, manual feed of logs not being optimised and heaters being ’choked down’ at night cutting out oxygen and causing more smoke. Further more, because it takes a while to get the heater going and you can’t just switch them off when you go to bed or leave the house; the over all efficiency of the process can be considerably less depending on how its used.
With a pellet heater they automatically control the pellets and oxygen feed to keep the heater burning at the optimum efficiency. The pellets themselves are very high in energy density, have low moisture (less than 10%) and are sap free. Furthermore they have efficient heat exchangers that ensure that almost all of the heat generated in the burning process ends up in your room rather than outside.
The other big benefit is that you can turn it on and off when you need it and keep the room at the right temperature rather than guessing. So not only is the heater more efficient, but the way you will use it will be more efficient.