In order to predict the scale dependency of fire property in oil tank fires, we estimated the thermal balance of generated heat from the fire with a simple fire model, a so called one mesh model. In this estimation, both experimental values of burning rate and air entrainment in heptane pool fires were used. For 1m tank fires, it was found that about 70% of the generated heat was lost as upward convective heat and about 30% was lost as radiative heat, and only about 1% was used for fuel vaporization. The ratio of upward convective heat became larger with tank size. Experimental results show that the radiative fraction went down in fires larger than 1m diameter because of the huge smoke emission.