Intantaneous vs Cumulative Data
There are two types of GRIDc Data.
When using GRIDc data, note that there are two types of data available: Instantaneous and Cumulative.
Be careful which you are using as it will impact your analyses.
Type 1: Instantaneous Data
On November 30, 2009, a student visited the GRIDc web site (www.gridc.net) to check the temperature outside of the North Carolina solar House in Raleigh, NC, at 15 minute intervals, between 12:00 am and 12:00 pm. Below is a snapshot of her screen
She reports the temperature to be 14°C at 12:00pm ... and she is correct.
Temperature is what we call instantaneous data. Instantaneous data does not require manipulation. In other words, at any given time, you can look up the data, and without manipulation, find the exact value you are looking for. Wind speed is another example that fits in the category.
Type 2: Cumulative Data
Example 1: Energy Consumption
Julie, the same student from the example above, decided to also check the level of energy consumption at the North Carolina Solar House. She used the data export link on the GRIDc web site (www.gridc.net) to find the amount of energy consumption at the house in 15 minute intervals between 12:00am and 12:00 pm on November 30, 2009. Below is a snapshot of her screen.
She reports the Solar House's energy consumption as 4,755.032 kWh at 12:00 pm. She has made a small, yet significant mistake.
Energy consumption is in the category of cumulative data. Cumulative data require some manipulation in order to properly represent the value we are looking for. Electricity consumption is measured with a meter which keeps on counting. In order to find the amount of energy consumed over a certain period of time, you have to subtract the initial value from the final value to find the change in the value reported by the meter. This change will give you the amount of energy consumed over a certain time period.
Energy production also falls in this category of data.
Example 2: Electricity Bill
The following table shows the kWh Usage History of a typical single family house in Raleigh, NC. You will find the "usage" table on customer statements.
- 38599 kWh
|kWh usage||850 kWh|
|Days in period: 31||Average kWh: 27.4|
The meter counts the total number of kilowatt hours used over time. The values given are from the dates of October 17 and September 17 which give the start and end values, respectively. The difference between these two values gives the total amount of energy consumption for a period (in this case a 31-day period).
If each kilowatt-hour is about $0.11, then the changes related to electric services for this period are:
850kwH * $0.11 = $93.50
The following options from the data export of www.gridc.net are either cumulative or instantaneous. Access the data and label whether the data is cumulative or instantaneous.
|Data Source:||Data Type:|
|Annex Array Current:|
|Annex PV Energy Produced:|
|Garage Solar Irradience:|
|House Energy Consumed:|
|Annex PV Power:|