 # Calculating the energy used by the system and choosing a battery

Hi, I have a project and I need to calculate the energy usage of the whole system to choose a battery which I'd like to use lipo. I have already some information about the calculation but I'm confused at some points. I know the formula ; P = I2R

∆E = Pt

and I know that if you want to calculate the whole system's energy usage, you should sum the sensor's energy need. And there are also websites to do this, it just wants to sum up mA, then gives the battery life that you need. But how? It also gives a formula but I can't understand. I give the system to you.

• Arduino nano
• MPU-6050
• Lora e45-ttl-100
• GY-NEO6MV2-GPS
• Adafruit BMP280 I2C
• Time 30 Minutes or more.

This is my first subject sorry if I have made mistakes.

You calculate the power consumption first, then multiply by time to get the energy.

Typically you do this by, for each supply voltage used, summing the current consumption, multiplying by voltage to get the power.

However if you only have one voltage its simpler just to sum the currents, and calculate the battery capacity by multiplying by time.

Battery capacity is given in ampere-hours, Ah, so if you device uses 100mA and you want it to last 5 hours, 0.1A x 5h = 0.5Ah.

*** You then double this figure *** to get a realistic battery capacity requirement (capacity of batteries always drops over time, considerably, many people get caught out by this).

There are two ways to represent battery capacity, either in coulombs (amps x seconds = coulombs), or Ah (amps x hours = Ah). 3600 coulombs = 1Ah.

If you want the energy in a battery multiply amp-hours by volts to get watt-hours (Wh), or multiply coulombs by volts to get joules (J). 1 Wh = 3.6kJ

And of course 1W = 1J/s

Use a multi meter to measure the total current drawn by your system.

The measured current may not be exact that there will be a failure to measure surges but you'll get a good starting point.

Add in a fudge factor say (measuredcurrent/2) + measuredcurrent.