How to calculate the units consumed by electricity

hayee · Joined: 05 Sep 2007 Posts: 252

Hi,
I want to know how I calculate the numbers of unit consumed by electricity.
I am reading voltage and current in microcontroller. What should be the next step (what calculations/formula I should use to calculate the numbers of units).

Ttelmah · Joined: 11 Mar 2010 Posts: 19504

Power, is current*voltage*_time_.
This is the difference between a KW, and a KWHr.
A heater (say) draws 1KW. But the total power used, depends how long it is turned on for. If it is only on for 30mins, then 0.5KWHr, but leave it on for a day, and you have 24KWHr.
A 'unit', is normally 1KWHr.

Best Wishes

gpsmikey · Joined: 16 Nov 2010 Posts: 588 Location: Kirkland, WA

If you are working with AC, then you also need to consider "power factor" - if the load is NOT pure resistive (inductive etc), then the voltage and current are phase shifted and your measurements will not be correct unless you consider the phase shift as well (this can be a significant number for a big inductive type load for example). For a DC load, then the simple Voltage*Current=watts and if you multiply that by time, you get "watt hours" as was indicated in the previous post.

mikey
_________________
mikey
-- you can't have too many gadgets or too much disk space !
old engineering saying: 1+1 = 3 for sufficiently large values of 1 or small values of 3

hayee · Joined: 05 Sep 2007 Posts: 252

Thanks guys for your replies.
Yes I am working on AC voltages. My load is (2 enery savers each of 25W and 1 fan of 40W). How to handle the phase shift in software and what about power factor?

What i do for calculating kwh?
Can i do something like that, calculate the total power first, then whenever load increases start timer and when load decreases stop the timer. In between according to that value of timer calculate kwh.
Is this scenario right?

temtronic · Posted: Fri Jul 22, 2011 6:54 am

Ok.. first , forget about the 'power factor' correction as you can add that later after you get your program 'up and running'.

As 'T' says energy used = volts * amps * time device is on.
Depending on the PIC used and it's speed, you probably should only use integer math NOT floating point. FP math takes a LOT of processor time to compute answers.

Some 'energy meters' set a flag for 'device on' and 'device off' in an ISR,allowing the main program to compute the time the device was powered in a 1 hr period.Simple math of V x A will get you KWhr.

On the hardware side be very careful with wiring and testing !! 120 Volts can be lethal !!!