This mostly happens in case of DC appliances. These devices don’t state power with respect to 220V or 120V, they just provide power ratings to tell power dissipation of components.
Here is the formula
Bill Amount=
(V*I)/1000*T*D*$
To calculate the power consumed by users you need to
know the input current written on the power adapter of the device.
1. I= Rated Current of devices
If device label doesn’t state wattage then it definitely labels the current ratings written as “Amperes, mA or Amp” (there will input and output current ratings, you need to note the input current).
For Example 12V submersible water pump consumes 0.6 Amps.
2. V=
Standard Voltage in your country
Find the voltage in your region. In the United States and a few other
countries, the standard household voltage is 120V. In the EU and most of the rest of the world, the voltage is
between 220 and 240V. if you don’t know, don’t worry just click at this link ''"Voltage Standards'' this will direct you to a page containing different voltage levels in the
world. Let’s assume voltage to be 220V.
For above Example: P=220*0.6= 132W
divide the answer by 1000 to get power in kWh.
132/1000= 0.132 kWh
3. T=
Number of hour you use your appliance in one day
At
this stage, you must know that for how long, in a day, you use the appliance.
a pump is used for 3 hours: 0.132*3= 0.396 kWh
4. D=
Number of days you are measuring for
Now you know how
many watthours the device uses every day. To calculate your watt-hours per month
or per year, just multiply by then number of days in that period.
Lets Calculate it for 30 days: 0.396*30= 11.88 kWh
5. $=
Cost of one electricity unit in your country (One electrical unit =kWh)
Your electricity
bill lists the cost per kilowatt-hour. If you don’t find their go to you
electricity provider’s website and search for electricity charges. Like my
country charges five rupees for one unit.
11.88 kWh*5=59.4 rupees