The report below shows Downtime (in hours) for a range of Devices/Systems (like all reports you can Schedule this and have it emailed to you).
The report can use Service Level Agreements (SLA). If you've not encountered SLAs before, here is how they would affect the Downtime calculations.
Suppose that the ERP system needs to be available for your business from 08:00 to 18:00 Mon-Fri, excluding public holidays. Outside of these hours it's OK to take the system down, because no-one is using it.
Now suppose that a failure occurs at 17:00 on Friday, and continues until Friday 23:00. IT Service View Pro will use the SLA to determine that just 1 hour of downtime has occured, rather than 6 hours.
Of course, for IT systems that are needed 24/7, you can use the default SLA that runs all day, 7 days per week.