user13696: According to the Atmega328P-PU Datasheet (side: 589) the current consumption of the microcontroller should be around 9.5mA (at 16MHz and 5V).
But I measured 15mA. (Nick Gammon who did some pretty good power saving research(gammon.com.au/power) got the same)
There is nothing connected to the outputs. The code I ran was this:

void setup () {}
   void loop () {}

Now I'm wondering if this is just a normal deviation (datasheet to real world) or if Atmel measured their current consumption in a state that is different from mine?
EDIT:
I forgot to mention.. I didn't use it with the Arduino Uno board but as standalone device. There was only a 16Mhz oscillator with two capacitors connected. Power came directly form Power supply.
Now that I think about it..  Datasheet says this regarding the current measurement: "A sine wave generator with rail-to-rail output is used as clock source."
Is it possible that the oscillator needs that extra 5mA?
EDIT2:
Here is a screenshot of my schematic:

And a screenshot of the ATmega328P-PU Datasheet:


user10794: According to the datasheet:

All current consumption measurements are performed with all I/O pins configured as inputs and with internal pull-ups enabled.


That was the critical point. Trying this sketch:
int main ()
  {
  for (int i = 0; i < 20; i++)
    pinMode (i, INPUT_PULLUP);
  }  // end of setup

Note that timers are not running (your sketch would have timer 0 running):
With that sketch, even using the crystal and capacitors (I actually had a resonator) the consumption at 5 V at around 20 °C was 9.2 mA which is within spec.
Fuses:
Signature = 0x1E 0x95 0x0F 
Processor = ATmega328P
Flash memory size = 32768 bytes.
LFuse = 0xFF 
HFuse = 0xDF 
EFuse = 0xFF 
Lock byte = 0xFF 
Clock calibration = 0x96 


If we turn pull-ups off:
int main ()
  {
  }  // end of setup

Now it consumes 14.4 mA, closer to what you measured.

And with your actual test sketch:
void setup () {}
void loop () {}

It consumes 15.5 mA, which is more, because Timer 0 is running (started in the init function). It is also close to what I had on my page about power - 15.15 mA.

So, working as expected. Those pull-ups are important, to stop the input drivers consuming current.
I did mention further down in my page the importance of configuring the pins:

Testing in SLEEP_MODE_PWR_DOWN:

All pins as outputs, and LOW: 0.35 µA (same as before).
All pins as outputs, and HIGH: 1.86 µA.
All pins as inputs, and LOW (in other words, internal pull-ups disabled): 0.35 µA (same as before).
All pins as inputs, and HIGH (in other words, internal pull-ups enabled): 1.25 µA.


Notice the difference there of 0.9 µA when asleep. It would appear that there is an ever greater difference if the processor is awake.

13696: <p>According to the Atmega328P-PU Datasheet (side: 589) the current consumption of the microcontroller should be around 9.5mA (at 16MHz and 5V).<br>
But I measured 15mA. (Nick Gammon who did some pretty good power saving research(gammon.com.au/power) got the same)<br>
There is nothing connected to the outputs. The code I ran was this:</p>

<blockquote>
  <p>void setup () {}<br>
   void loop () {}</p>
</blockquote>

<p>Now I'm wondering if this is just a normal deviation (datasheet to real world) or if Atmel measured their current consumption in a state that is different from mine?</p>

<p><strong>EDIT:</strong></p>

<p>I forgot to mention.. I didn't use it with the Arduino Uno board but as standalone device. There was only a 16Mhz oscillator with two capacitors connected. Power came directly form Power supply.</p>

<p>Now that I think about it..  Datasheet says this regarding the current measurement: "A sine wave generator with rail-to-rail output is used as clock source."</p>

<p>Is it possible that the oscillator needs that extra 5mA?</p>

<p><strong>EDIT2:</strong></p>

<p>Here is a screenshot of my schematic:
<a href="https://i.stack.imgur.com/GZBCx.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/GZBCx.png" alt="enter image description here"></a></p>

<p>And a screenshot of the ATmega328P-PU Datasheet:
<a href="https://i.stack.imgur.com/Ae11H.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/Ae11H.png" alt="enter image description here"></a></p>
 10794: 

<p>According to the datasheet:</p>

<blockquote>
  <p>All current consumption measurements are performed with all I/O pins configured as inputs and with internal pull-ups enabled.</p>
</blockquote>

<hr>

<p>That was the critical point. Trying this sketch:</p>

<pre class="lang-C++ prettyprint-override"><code>int main ()
  {
  for (int i = 0; i &lt; 20; i++)
    pinMode (i, INPUT_PULLUP);
  }  // end of setup
</code></pre>

<p>Note that timers are not running (your sketch would have timer 0 running):</p>

<p>With that sketch, even using the crystal and capacitors (I actually had a resonator) the consumption at 5 V at around 20 °C was 9.2 mA which is within spec.</p>

<p>Fuses:</p>



<pre class="lang-none prettyprint-override"><code>Signature = 0x1E 0x95 0x0F 
Processor = ATmega328P
Flash memory size = 32768 bytes.
LFuse = 0xFF 
HFuse = 0xDF 
EFuse = 0xFF 
Lock byte = 0xFF 
Clock calibration = 0x96 
</code></pre>

<hr>



<p>If we turn pull-ups off:</p>

<pre class="lang-C++ prettyprint-override"><code>int main ()
  {
  }  // end of setup
</code></pre>

<p>Now it consumes 14.4 mA, closer to what you measured.</p>

<hr>

<p>And with your actual test sketch:</p>

<pre class="lang-C++ prettyprint-override"><code>void setup () {}
void loop () {}
</code></pre>

<p>It consumes 15.5 mA, which is more, because Timer 0 is running (started in the <code>init</code> function). It is also close to what I had on my <a href="http://www.gammon.com.au/power" rel="nofollow">page about power</a> - 15.15 mA.</p>

<hr>

<p>So, working as expected. Those pull-ups are important, to stop the input drivers consuming current.</p>

<p>I did mention further down in my page the importance of configuring the pins:</p>

<blockquote>
  <p>Testing in SLEEP_MODE_PWR_DOWN:</p>
  
  <ul>
  <li><p><strong>All pins as outputs</strong>, and LOW: 0.35 µA (same as before).</p></li>
  <li><p><strong>All pins as outputs</strong>, and HIGH: 1.86 µA.</p></li>
  <li><p><strong>All pins as inputs, and LOW</strong> (in other words, internal pull-ups disabled): 0.35 µA (same as before).</p></li>
  <li><p><strong>All pins as inputs, and HIGH</strong> (in other words, internal pull-ups enabled): 1.25 µA.</p></li>
  </ul>
</blockquote>

<p>Notice the difference there of 0.9 µA when asleep. It would appear that there is an ever greater difference if the processor is awake.</p>
