# Daily Power Consumption Calculations

Discussion in 'General Electronics Discussion' started by Ali Akber, May 5, 2015.

1. ### Ali Akber

2
0
Jan 30, 2015
Hello,
I am designing a project that records real time power measurements in WATT and store those values in a database each 10 seconds. I am confused that how can I calculate daily power consumption by using these values from the database...
Right now the calculation that I have applied is "taking average of all power values of the day and multiplying them by the number of hours of usage"
P(total) in kWh = P(avg of all values of day) * T (no. of hours of usage) / 1000
Is this calculation perfect? Or rather than taking the avg of all the values do I have to add all the values and then multiply them by the total no. of hours of usage?
Help would be appreciated thank you.

2. ### Harald KappModeratorModerator

11,160
2,548
Nov 17, 2011
Why the average?

Eenergy consumption is measured in WH (Watt hours) and for a given period T is calculated by: W = integral from 0 to T of (P(t)dt. where P(t) = instantaneous power in Watt.
The integral is approximated by a sum: W = sum from i=0 to N (P(i) * delta_t. Where N is the number of intervals and P(i) is the average power during one interval.

P(i) is what you already measure and store in the database, delta_t is the duration of an interval, 10 seconds in your case.
What is left to calculate the energy consumption for any interval is to sum up all database entries during this interval (e.g. during 24 hours). This gives a value in Watt (as the stored values are in Watt). Multiply this value by delta_t, namely 10 s, to get a result n Watt seconds (Ws). divide by 3600 to get a result in Watt hours (Wh).

Exampel:
assume you have 24 entries, 1 entry per hour. Each entry is 100 W (for simplicity). This makes P(i)=100W, sum(P(i)=24*100 W=2400 W, Energy consumption is 2400 W * 1 h = 2400 Wh
assume the same power consumption, but measured with a resolution of 1/10 hour, so you have 240 entries, each P(i)=100 W. sum(P(i)=240*100 W=24000 W. Energy consumption is 24000 W * 0.1 h = 2400 Wh, which is to be expected since the mean power was constant at 100 W.
With a fluctuating power consumption, the smaller the interval, the better the accuracy of the calculated energy consumption.