Outages and Power Quality

Reasonably good statistics can be obtained from SAIDI (System Average Interruption Duration Index), a reliability index that utilities publish that measures the average amount of time a customer experiences an outage. The following links provide both data and background.
https://www.eia.gov/electricity/annual/html/epa_11_01.htmlhttps://en.wikipedia.org/wiki/SAIDI

SAIDI information is comprehensive, but misleading. The numbers are reported on a per-customer basis, which dramatically understates the duration of an outage for those experiencing an outage. They take the total number of outage hours divided by the total number of customers served by that utility.

Since only a small percentage of customers are actually without power, you need to divide the SAIDI number by the percentage of customers who actually do not have power. For example, if the SAIDI number is 1 hour but only 5% of customers are actually without power, the effective outage duration for those without power is 1 hour / 5%, or 20 hours.

Averages are also misleading. There is a huge difference between being down once a year for an hour, and being down 60 times in a year for 1 minute each time. A one-minute power outage could require several hours to recover. It’s important to remember that how long your operation is down is more important than the length of the outage.

Outage duration follows a probability curve. The following graph displays the probability of an outage as a function of the outage duration.

The statistics can be confusing and very granular. Exergy helps you get the right information for your decision. Book time with one of our consultants here, it’s free.