I propose the plantball receive a direct fuel value.
Since it can be made from 8 reeds (reed fuel value 50 heat / 120 EU / 0.25 items each), among other possibilities, I believe it should have a fuel value 8 times that of a single reed (plantball fuel value 400 heat / 960 EU / 2 items / .25 charcoal).
This value is numerically consistent with the fuel values for cactus and saplings, the other burnable items that can be made into plantballs (cactus is the same as reeds, and saplings have twice the energy as reeds but give twice as many plantballs) and logically consistent with the many other things that can be turned into plantballs that cannot be burned on their own (leaves, wheat, etc.).
In addition, the main use for plantballs, biofuel, still produces a product with much more energy than the plantballs themselves (a biofuel can provides ~10,000 heat / ~26,000 EU, whereas the six plantballs required to make the biofuel only provide 2,400 heat / ~6,000 EU on their own). Therefore, this process remains profitable, if inconvenient.
Lastly, some may argue plantballs would be unable to burn well. Yet some of the ingredients used to make plantballs, reeds and cactus, themselves should be subject to such logic, yet they still are useable as fuels, albeit poor ones; and if the plantball's ingredients can burn, why can't it as well?
Summary: A plantball should be as good a fuel as the sum of its ingredients.
Sources: Suggestion: "Brown" Charcoal, or a less bulky way to burn plants like reeds A previous thread of mine on a related topic. I did not post there because necroing is bad.
Wheat should be able to be burned for fuel points. Another person's thread on a similar topic that showed up on my search. Although they had some good ideas, I did not want to necro a year old thread.