We're currently looking into upgrading our UPS and possibly having a small petrol generator on site in the eventuality that the UPS battery is used long enough to drain it completely.
Realistically, we can only afford a UPS that would give us around 45 minutes. It's not unheard of for our building to have outages of 4-6 hours. It is very rare though.
What products are available that are capable of sending notification (SMS, automated call, etc.) when a UPS switches to battery power? Some UPS's we've looked at have serial connections. How do you deal with this kind of thing?
The plan is that when the power goes down, the battery takes over and alerts the team. One of us (whomever is on call) drives the the building and starts the generator. Unless of course a notification comes through that the power is back on.
We do currently use Pingdom to monitor uptime of our services. It's just a ping test though so we can't rely on a lost host to explain why it's lost. For example; if the network connection went down and so did the backup link, we would get a notification. That doesn't necessarily mean the generator needs to be started.
The serial comms port is an indication of either an old or a small UPS (unlikely to provide you with the 45 mins of coverage for any notable number of server machines, network infrastructure and peripherals). It has been the state-of-the-art interface around 1995, but has been superseded by SNMP-capable Ethernet interfaces since.
Most of the available monitoring systems are capable of using a custom check or script to get the current UPS state over the serial port (obviously, it would need to be physically connected via a serial cable) or listening to SNMP traps and raising alerts and notifications in various ways - feel free to choose one to your liking.
If you already do have UPSes in place which cannot be upgraded with SNMP cards and have no other means of connecting to the UPS to monitor its status but are using servers with redundant power supplies, you might consider a setup where at least one PSU is connected directly to the mains, while the other(s) is(are) fed through through the UPS. Upon a power failure, you should get a PSU power loss event on the mains-connected PSUs, which you could forward and treat accordingly in your monitoring / notification system.
Note: if you are using your UPS as the single surge protection for your servers, you might want to add an external surge protector to reduce the risk of a potential PSU damage upon voltage spikes in the last scenario.
Most UPSs will include a rudimentary proprietary in house version of a notification package while others will co-brand a more commonly known solution, the way DVD burners include a stripped but functioning vesion of Roxio, your UPS will include something similar or at least a gnawed down version of APC Powerchute.
Regarding the serial connection, it is common and is typically connected to the RS232/serial port found in older computers and special-ordered as an offlist option in PCs today (harder than hell to order in a contemporary laptop) and the UPS sends almost primitive signals across the serial line to a listening/waiting software app that's captured the serial port for just that purpose.
Sometimes, the UPS has enough embedded code to be programmable with a phone number and the serial is jacked directly into an old external modem with serial cable between the PC and modem.
We prefer PCs as the monitoring agent and just deploy a serial/USB converter from any professional vendor (even Amazon sells them for less than $15 and it connects the serial to your PC's USB). We use GFI LanMon for this and other event tracking and its never failed us. I think we paid $7K for an unlimited GFI LanMon site license back in 2004 so I don't know much it costs today but they negotiate and their stuff suits us as a medium-sized company in six states.
Our site has a pair of relatively ancient 75KVA UPS units which keep on chugging, so we've had to get creative with monitoring. Luckily, in addition to the proprietary (multi-$k, and at this point hard to come by) serial interface from Mitsubishi, the units possess several dry contact relays outputs (street power good, on battery power, fault, etc.) which were fairly simple to interface to our Sensaphone IMS-4000 environmental monitoring system. That IMS also supports a dedicated AC power state sensor, which, in the absence of a "street power good" output from your UPS, might suffice for your needs. (NB: I haven't worked directly with the Sensaphone stuff, but it was simple enough for our relatively non-technical facility operations group to install and manage.)