Question: how much energy does wood produce?
Answer: Each kilogram of (EN Standard) dry wood contains 5.14 kWh of "potential heat" so 1 kWh of energy is contained in (1/5.14=) 0.195 kg of wood. If your wood heater's Gross Efficiency was (say) 70% then the actual amount of wood that you would need to produce 1 kWh of heat is (0.195/0.70=) 0.279 kg.