Powercuts are quite common in Vietnam, in particular now when it comes to the end of the dry season where is no water left for the hydropowerplants. Companies know that, and there are different ways to deal with it. It also shows you a lot about your own company, how leadership is done and the state of your staff.
First rule: Be prepared. Do you know that many powercuts are announced in advance? Did you check already the website of your power company and local newspapers? If yes, your company seems to be prepared. If not, ask yourself why you didn't know it - have you been complaining instead of taking action?
Second rule: Deal with the situation. Wether the power cut is announced or not, it will come anyway. So what do you do then? You stop working? If yes, ask yourself why. You don't have a external power supply? And if so, does it supply only computers or also at least air-condition (what kind of productivity you can expect from staff that works in a 40 degrees environment?). Advise: Let the power supply run the mailserver (if you still maintain the server in your own place instead of a server farm where there is 24/7 availability), and let your staff work remotely.
That might not work for every company and every worker, but at least you get some things done. You can even see how takes leadership in your company: the one who tells you that they will work from home or from the coffee shop ist the one that needs to be on the promotion list next time, while the ones who are asking for advise are not.
I was once working a in a company in Cambodia where we went to coffee shops during power cuts, and when we came back in the afternoon the boss told us to stay longer because of the power cut. I was laughing at him and told him that we already did our work, and that work (especially office work) is not determined on the time you stay in the office anymore. So better pay the bill for the coffee shop instead of treating your stuff like cheap factory workers in the 19th century. (By the way, it seems to be more and more common that employees using their own laptops for work. That's quite nice, but do the get any reward for that?)
A powercut tells you, if you prepare in advance, if your staff is prepared as well, if your staff is looking for solutions, and if you are willing to accept different approaches.
The problem with productivity in office is that it is hard to put that in numbers. How much money you are loosing in an hour? What's the value of a workhour of your office stuff? But even if you can put that in account, motivation and engagement is still a soft value that is absolutely crucial. There is no excuse not finding a good solution or a problem.