How to calculate the battery standby time according to the load?

Things we should know:

  P = V . I                   

(ie. power in watts = volts x amps)

Every battery has a amps/hr (Ah) rating which signifies for how many hours it can deliver that much amount of current?

for eg. 2000mAh battery can deliver 2000mili Amps per hour 

Eg.1 Inverter battery standby calculation

1.Calculating the total battery capacity in Watts/hr:

 if our inverter battery has a rating of 150Ah and voltage 12V

thus it can deliver 150Amps per hour.

thus power delivered per hour will be 1800Watts/hr 

(1800 = 12 x 150) .............................(  P = V . I  )

2.battery threshold level:(actual battery capacity)

now before proceeding take into consideration that the battery wont be fully discharged due to the internal protection circuit in the inverter so we have to first take 85% of our total battery capacity. 

thus 0.85 x 1800 =1530W/hr

 3.Load in watts:

your general ceiling fan eats up 75W 

lets assume 4 such fans running thus 4 x 75 = 300

now lets calculate how long can your inverter battery can standby

4.Battery standby time in hours:

as calculated before before 4 fans eats up 300W

so the battery will last for 1530W/300W = 5 hrs

Eg.2  Mobile battery standby approx calculation

1.Calculating the total battery capacity in Watts/hr:

 if our mobile Lithium ion battery has a rating of 2000mAh and voltage 3.7v

thus it can deliver 2000 mili amps per hour we can also say 2Ah

thus power delivered per hour will be 7.4Watts/hr 

(7.4 = 3.7 x 2) .............................(  P = V . I  )

2.battery threshold level:(actual battery capacity)

now before proceeding take into consideration that the battery wont be fully discharged due to the internal protection circuit in the battery so we have to first take 85% of our total battery capacity. 

thus 0.85 x 7.4 =6.29W/hr

3.Load in watts:

lets say your GSM edge module eats up 128mW (general consumption for 1200 bit rate)

now lets calculate how long can your mobile battery can standby

4.Battery standby time in hours:

as said before the GSM module eats up 128mW

so the battery will last for 6.29W/128mW = 49hrs = 2 days 

but this wont be the actual case of standby as we have not considered many loads which will eat power such as display, Graphic CPU etc. its just an approximate calculation. : )

program to calculate the battery standby:
Battery standby c program
Battery standby (onedrive)