How to let multiple values contribute two one value constantly

You can use static variables and a get-only accessor to do all the math without having to re-write it each time.

 class Generator
 {
     // keeps a list of all generator instances
     static List<Generator> instances = new List<Generator>();

     // the amount of energy this instance has
     internal float electricity { get; set; }

     // returns a float[] containing the energy each instance has
     static float[] energyOfInstances
     {
         get
         {
             float[] value = new float[instances.Count];
             for (int i = 0; i < instances.Count; i++)
                 value[i] = instances[i].energy;
             return value;
         }
     }

     // returns the total energy across all instances
     static float totalEnergy 
     {
         get
         {
             float value = 0;
             foreach (Generator g in instances)
                 value += g.energy;
             return value;
         }
     }

     // constructor that adds the new instance to the list of instances. make sure you are also somehow removing the disposed instances from the list.
     Generator()
     {
         instances.Add(this);
     }
 }

Then, if you want to know the amounts of electricity being produced by each indivudal generator:

 float[] foo = Generator.energyOfInstances;

Then, whenever you want to know the total amount of electricity, you can just get the value of Generator.totalEnergy, and all the math is done within the get function.

 float f = Generator.totalElectricity;

From a performance standpoint, however, this would be essentially the same as writing it out each time you need the value. If you're going to be querying these multiple times each frame, and the values won't be changing between queries, I suggest caching it (like I did in the example above) instead of using the static variable each time you want to know.