Having trouble with a points-based fuel system for an entity I'm making

So im attempting to create an entity that takes in fuels like coal and kelp, and stores it as a value which can be expended later to smelt items. think of it as a furnace that only uses the amount of smelting that it requires (sort of like the brewing stand not using a full blaze powder). However, i can't seem to figure out how to store the fuel reserve as a variable that's entity specific without either making it universal between all identical entities or breaking it entirely.

any help would be appreciated as ive been slamming my head against a brick wall for a solid few hours now
im willing to clarify things if ive failed to explain them well in this initial post too