Formula to calculate energy required to power an Arduino board Given an Arduino board with the following specs:

ATmega328 Arduino Nano AVR® ATmega MCU 8-Bit AVR Embedded Evaluation Board (link)

What is the mathematical formula to calculate how much energy/power would be required to keep the Arduino board on (meaning in "standby" mode, therefore not consuming any energy but the one required to keep it "alive")?

Please note I am new to all things electronics, Arduino but most importantly physics and maths. Thank you.
In the datasheet you can read how much every mode uses (normal mode, sleep mode etc). However, it also depends on which peripherals inside the CPU are active and all electronics around it. So make sure you read the datasheet of the entire development board, and not from the CPU itself.

The mathematical formula is reasonably simple: the total power is the sum of each component's Amperage * component's Voltage. But practicially this is hard to calculate.

The easiest way (but not mathematically) is probably just to measure the amperage in the development board, and the voltage and multipley them together.