Questions for a personal LED/ATmega328P-PU project
I will try to help you understand Ohms Law.  There are 3 forms of this law.  R=V/I; I= V/R; &amp; V=IR. For you to have a good understanding, I will give you some examples.

Lets start with an LED that has the following parameters: V=2 @ I=20ma.  

If you connect it to  a 5v source, then you have to use a series resistor of R=V/I = ((5-2)/.02 =) 150 ohms.  The power consumed by the resistor &amp; LED, is P=VI =5x.02 = 0.10W.  For 14 LEDs = 1.4W
If you connect it to a 3v source, then the resistor needed is ((3-2)/.02 =) 50 ohms, and the power used is P=3x.02 = .06W.  For 14 LEDs = 0.84W.
These examples show that less power is wasted, if the source voltage is closer to the LED voltage.