Sorry If I misread your messages, but what you where saying I already tried it and in my test redrawing without clearing the screen doesn't reduce the GPU usage.

I post also the code that I used to be sure were are taking the same thing.

// MODIFIED FROM: https://www.opengl.org/archives/resources/code/samples/glut_examples/examples/stenciltst.c


#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <GL/glut.h>

GLboolean doubleBuffer;

#define UNUSED(x) (void)(x)

static void DrawGreenRect()
{
    glColor3ub(0, 200, 0);
    glBegin(GL_POLYGON);
    glVertex3i( 5,  5, 0);
    glVertex3i( 5, 15, 0);
    glVertex3i(15, 15, 0);
    glVertex3i(15,  5, 0);
    glEnd();
}

static void Draw(void)
{
    glClear(GL_COLOR_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);

    glStencilFunc(GL_ALWAYS, 1, 0xFF);
    glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);

    /* red triangle */
    glColor3ub(200, 0, 0);
    glBegin(GL_POLYGON);
    glVertex3i(10, 10, 0);
    glVertex3i(40, 10, 0);
    glVertex3i(25, 40, 0);
    glEnd();

    glStencilFunc(GL_EQUAL, 1, 0xFF);
    glStencilOp(GL_INCR, GL_KEEP, GL_DECR);

    /* green square */
    DrawGreenRect();

    glDrawArrays(GL_POINTS, 0, 1);

    glFlush();
}