At 20 kilometers per second (high to typical asteroid impact velocity) a kilogram of mass has 200 million joules of energy.

(1 X 20,000^2)/2

At 500 kilometers per second (upper end of interstellar asteroid impact velocities, near worse case) a kilogram of mass has 125 billion joules of energy.

(1 X 500,000^2)/2

Twenty five times the velocity means it has 625 times more energy.

One megaton is equivalent to 4.18 x 10^15 joules

One megaton will raise the temperature of a 1 kilometer cube of water by about 1 degree Celsius.