How much energy is need to raise 50 kg of water from 45 c to 80c?

Based on your problem, what you are looking for is the quantity of heat. To solve for it, you will need this formula:

Q = mc(T2-T1)

Where: Q = Quantity of heat
             m =  mass of the substance
             c  = Specific heat
             T2 = Final temperature
             T1 = Initial temperature

Now the specific heat of water is 4.184 J/(g°C), meaning that is how much energy is required to raise the temperature of 1g of liquid water by 1 degree Celsius. 

Since your mass is in kilograms, let us convert that into grams, which will be equal to 50,000 grams. Now we can put our given into the equation:

Q = mc(T2-T1)
   = 50,000g x  4.184 J/(g°C) x (80°C - 45°C)
   = 50,000 g x 4.184 J/(g°C) x 35°C   
   = 7,322,000 J or 7,322 kJ or 7.322 MJ