How much heat is needed to heat water of mass $m = 2$ kg from a temperature of $t_1 = 20°$C to $t_2 = 100°$C? The specific heat capacity of water is $c = 4200$ J/(kg·K).
Heat needed for heating:
$$Q = cm(t_2 - t_1) = 4200 \cdot 2 \cdot (100 - 20) = 4200 \cdot 2 \cdot 80$$
$$Q = 672\,000 \text{ J} = 672 \text{ kJ}$$
$Q = 672$ kJ (672 000 J)