$${P_{rad.} = 9\cdot \frac{{15}\,\mathrm{^{\circ}C} - ({-20}\,\mathrm{^{\circ}C} )}{{90}\,\mathrm{^{\circ}C} - {0}\,\mathrm{^{\circ}C} }\cdot {4}\,\mathrm{kW} = {14}\,\mathrm{kW}\quad}$$