The cost for utilities in Panamá are the lowest in the region, which is just one of many reasons why so many foreigners consider moving to the country. This is in addition to several other benefits, especially if you are retired, in which case you can receive discounts almost everywhere with your "pensionado visa". You can enjoy this senior privilege when purchasing airline tickets, at restaurants, movies and many other places. The same low cost is available when purchasing vegetables and fresh fruits that you can get in public markets for ridiculously low prices, to make fresh juices or shakes on a daily basis.
Talking about electricity, you will find once in a while a short power outage, most of the time due a generator explosion outside in the street. A complete blackout is very rare and actually almost never happens, other than a for a very isolated area of couple of blocks when the generators have problems. Maybe in the "interior" you can experience power outages more frequently. If there is a long power outage, and especially for a tall building, there may be a problem, because electricity is needed to take water up to the buildings.
In the dry season (which Panamanians call "summer" on account of the the beautiful sunshine and breeze), if the lakes and rivers don't have sufficient water to generate enough electricity, the electric company takes special measures to prevent power shortages, and, if necessary, they will implement some basic rules. Marbella, as part of the city of Panamá, is under these regulations, but most of the buildings there have emergency back-up power if this problem arises.