"The board can operate on an external supply of 6 to 20 volts. If supplied with less than 7V, however, the 5V pin may supply less than five volts and the board may be unstable. If using more than 12V, the voltage regulator may overheat and damage the board. The recommended range is 7 to 12 volts"
If I give arduino only 5V my measures can be affected by an error. For example try to measure an analog temperature if you're under 7V and if you're in the 7-12V range. The value red will be very different. Anyway you're right, I can check out for a "9v boost-up", I only need to check if the value of ampere (max A)after this stuff would be ok for my system. Thanks
You don't want to power the board through the regulator (which is when you need more than 7V) as that would just produce inefficiencies. You want to power directly into the 5V and GND pins (avoiding the regulator) as your boosted voltage will already then be a regulated 5V (exactly what the arduino and your sensors require).
Mowcius