Make a sum once per time, in the update function

I have this code, the commented part i want to do once per time, and because i need to put this code in the update function is constantly do the sum every frame, but i just need once per time when the player press the the arrowKeys. i Try to recreate this game: Tero beats PSX Bishi Bashi Special 2 Part 1 - YouTube especially the speedbar. Thanks and sorry if my english suck.

using UnityEngine;
using System.Collections;

public class Player : MonoBehaviour {

	
	
	private bool avanzar = false;
	private float velocidadAvanza = 15f;
	private float sumaPower = 0.1f;

	void Start () {
	
	}
	
 	void Update () {
		
		Avanzar();

	}
	
	void Avanzar()
	{
		if(Input.GetKeyDown(KeyCode.RightArrow))
		{
			if(avanzar==false)
			{
				transform.Translate(Vector3.right * velocidadAvanza * Time.deltaTime);
				avanzar = true;
				PowerBar.restaPower += sumaPower; // <== Do this only Once
				
			}
		}
		
		if(Input.GetKeyDown(KeyCode.LeftArrow))
		{
			if(avanzar==true)
			{
				transform.Translate(Vector3.right * velocidadAvanza * Time.deltaTime);
				avanzar = false;
				PowerBar.restaPower += sumaPower; // <== Do this only Once
				
			}
		}
	}
}

utiliza esto en tu powerbar para que con otro boton solo sea una vez como dices.

if (Input.GetKeyDown("space"))
{
     PowerBar.restaPower += sumaPower; // <== Do this only Once
}

aunque yo no veo que esten declarados esas variables, no vas a querer que te hagan todo el sistema que quieres verdad?

bueno, espero te sirva.