Please help me determine the circuit's power consumption from the battery and help me outline the steps to calculate?

  • Battery power consumption is a tricky engineering problem. Many estimate then find them selves to be way off. Especially when using ultra low current processors (processors which support sleeping features). Added complexities may result when assumed features are unavailable in various levels of sleep mode. Like, for instance, an accurate clock. Further, special considerations are necessary when designing for low power consumption. Simple things like not using pull up or pull down resistors can spell the difference between an extra hour to extra days of operation. – st2000 Aug 4 '18 at 0:45
  • Use a multimeter to measure it in its different states (relay on/off; MCU in sleep mode, etc) – Gerben Aug 4 '18 at 9:09
  • Why the frog are you using a TIP122 to switch the gate of a MOSFET?!?! – Majenko Aug 4 '18 at 12:20
  • You can use higher values for r5 and r6, for example 100k and 10k. Don't connect aref to vcc. The crystal requires two 22pf capacitors. pd5 to q1 is missing a resistor. The buzzer might be inductive. Are those soil moisture sensors resistive or capacitive? If they are the resistive type, they will become useless very quick. What kind of battery is it? Are you relying on your programmings skills to prevent that the battery will be charged too much? The omih-sh-124l is a 24V relay and requires a minimal voltage of 18v. – Jot Aug 5 '18 at 10:16

It's not a simple matter of "calculating the power". Instead you need to:

  • Calculate (or obtain) the current consumption of each individual sub-module or component
  • Calculate (or obtain) the current consumption of the main MCU for each power state it operates in
  • Calculate the current consumption of static items such as voltage dividers and power LED


For each unit examine the percentage of time it spends in different power states. From that create an hourly average current for the unit.

And finally:

Sum up all the average hourly usages to get a final AH value for the whole circuit.

Most of the current values can be obtained either by calculation (e.g., Ohm's Law for resistive dividers and LEDs, etc), or from the datasheets for the devices you have attached.

Note that this will still only be an approximation, because (amongst other things) you have multiple power sources which will cause different drains on the system depending on which source is being actively used (different monitoring circuitry and components switched it), and that (I assume) is influenced by the weather.

| improve this answer | |

Not the answer you're looking for? Browse other questions tagged or ask your own question.