When we moved to Managua in 2010, longer power outages (1 hour or more) were fairly common. Over time though, the situation improved considerably.
Nowadays (2015), there are hardly any longer outages. You can also always call the provider to either find out about the reason for the outage, and how long it will approximately take, or to report a problem. When you call the provider (the number is 125), have your invoice handy, as you will need your so-called "número NIS" (Número de Identificación de Suministro).
What still happens quite frequently, at least at the place where we live, are really short outages, sometimes just a few seconds... which can be annoying when you are just watching a movie, as your TV suddenly goes dark. ;-)
As I work from home, and need my Internet connection basically all the time, I bought myself a UPS battery. UPS stands for "uninterruptible power supply." It's a small rechargeable battery unit with several sockets. It protects your electrical devices from power fluctuations and supplies them with energy during an outage for about 30 to 40 minutes. I use it mainly to power my Internet router, so that I can continue working during a power outage.
(Pictured: uninterruptable power supply, or "UPS".)