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How power is read by the utility company - Understanding power of a time interval

Jon Lee

Instantaneous power (demand)

The instantaneous power (or instantaneous demand, or instantaneous load) is the quantity of power that something is using (or generating) at any one moment in time. Instantaneous power is measured in kW (kilowatts).

ex: Put your laptop on standby and its instantaneous power will drop immediately. Bring it back to life and its instantaneous power will rise immediately.

If, at any particular moment, everything in your home is switched on, your home might be using 20 kW of power. That's 20 kW of instantaneous power. If, at any particular moment, everything in your home is switched off, your home should be using 0 kW of power. That's 0 kW of instantaneous power.

The instantaneous power of most homes varies constantly. People are constantly switching things on and off, and many items of equipment within the home have instantaneous power that is constantly changing too.

 

Average power

The average power represents the amount of instantaneous power used over a specific period of time (e.g. 30 and 60 minutes). It is measured as kWh(kilowatt hours)

Average power enables you think of complicated things, like entire homes, as if they were simple things, like light bulbs...

The instantaneous power of a typical home varies all the time. If you try to monitor instantaneous power you get lost in the noise. And figures of energy consumption are meaningless unless you know the length of the periods that they were measured over. But average-power figures smooth out the constant fluctuations of instantaneous power, and make it possible to calculate to the rolling average that is read by the utility.

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Using the speedometer analogy; instantaneous demand is how fast you are going at this second. Average demand is the average speed you traveled over time, such as 30 minutes or 60-minutes.

What the utilities do is capture the HIGHEST (of peak) demand for the period during on-peak hours. If you drove an average of 45mph the first hour, 50mph the second hour, 55mph the third hour, your highest average speed for the 3 hours is 55mph.

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Utility companies use a very similar method in order to determine demand rates

APS:

APS on-peak hours are 3-8pm M-F. The meter looks at the end of each on-peak hour to see if the demand over the hour was higher and if so, that becomes the new peak demand number (until reset).

Example:

4pm 4kWh used

5pm 5kWh used

6pm 5.5 kWh used

7pm 6.0 kWh used

8pm 6.5 kWh used

Since APS uses a 60-minute demand window they look at the accumulation over the hour. In this case the peak demand would be 6.5kW.

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SRP uses a 30-minute interval. So their meter looks at the :30 and :00.

They look at how much electricity was used during the 30-minutes, multiplies by 2 (to convert to kWh).

1:30pm 2kWh used (times 2) 4kWh of 4 kW average

2:00pm 2.5kWh used (times 2) 5kWh or 5kW average

2:30

3:00

..

..

6:00pm

6:30pm

7:00pm 3.5kWh used (times 2) 7kWh or 7 kW average

7:30pm 3.0kWh used (times 2) 6kWh or 6 kW average

8:00pm 3.6kWh used (times 2) 7.2kWh of 7.2 kW average

The highest 30-minute demand is 7.2kW

Details
Last Modified: 8 Months Ago
Last Modified By: jlee@inergysystems.com
Type: INFO
Level: Novice
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