Up-time requirements refers to the total amount of time that the system is available for end-use applications. The value is stated as a percent of total scheduled working hours.
- Less than 90% (downtime of 876 or more hours (36 days)/year)
- 90 to 95% (downtime of 438 to 876 hours/year)
- 95 to 99% (downtime of 88 to 438 hours/year)
- 99.1 to 99.9% (downtime of 8.8 to 88 hours/year)
- 99.99% (downtime of about 50 minutes/year)
- 99.999% (downtime of about 5 minutes/year)
Typically the cost per outage hour is used as a determining factor in up-time requirements. When talking about unplanned outages, the uptime requirements must be based only off of the scheduled working hours. This means the cost of an outage should be calculated based on the worst possible time.