volatile int i;
volatile int value[20];
volatile bool ready = false;

ISR(TIMER0_COMPA_vect) {
    //interrupt commands for TIMER 0 here
    if(!ready){
        value[i] = analogRead(A0);
        i++;
    }

    if(i==20){
        ready = true;
        i = 0;
    }
}

void setup(){
    // TIMER 0 for interrupt frequency 1000 Hz:
    cli();       // stop interrupts
    TCCR0A = 0;  // set entire TCCR0A register to 0
    TCCR0B = 0;  // same for TCCR0B
    TCNT0 = 0;   // initialize counter value to 0
    // set compare match register for 1000 Hz increments
    OCR0A = 249;  // = 16000000 / (64 * 1000) - 1 (must be <256)
    // turn on CTC mode
    TCCR0B |= (1 << WGM01);
    // Set CS02, CS01 and CS00 bits for 64 prescaler
    TCCR0B |= (0 << CS02) | (1 << CS01) | (1 << CS00);
    // enable timer compare interrupt
    TIMSK0 |= (1 << OCIE0A);
    sei();  // allow interrupts

    Serial.begin(9600);
}

void loop(){

    if(ready){
        noInterrupts();
        int total = 0;
        for (int i = 0; i < 20; i++){
            total += value[i];
        }
        total = total * 5 / 1023;
        total = total / 20;

        Serial.println("Vrms = " + String(total));
        ready = false;
        interrupts();
    }
}