The formula to calculate the thermal efficiency of a biomass cookstove is:

\[ \xi_t = \frac{M_w \cdot C_p \cdot (T_f - T_i) + W_e \cdot H_v}{WFC \cdot HVF} \times 100 \]

Where:

• \(\xi_t\) is the thermal efficiency in percentage (%).
• \(M_w\) is the mass of water in kilograms (kg).
• \(C_p\) is the specific heat of water, 1 KCal/kg-C.
• \(T_f\) is the final temperature of water in degrees Celsius (C).
• \(T_i\) is the initial temperature of water in degrees Celsius (C).
• \(W_e\) is the weight of water evaporated in kilograms (kg).
• \(H_v\) is the heat of vaporization of water, 540 KCal/kg.
• \(WFC\) is the weight of fuel consumed in kilograms (kg).
• \(HVF\) is the heating value of fuel in KCal/kg.