In the region where I live, last year the electrical power was down on the average for 36 minutes, up from 29 minutes the year before. That constitutes an uptime of 99.996 %.
Research I have been involved in indicates that the best banking websites have a hard time getting 99.967 % uptime, equivalent to 3 hours of downtime per year. A more typical banking website has 99.8 % uptime, which is around 18 hours of downtime per year.
With numbers like these, power outages are not your main reason for website downtime.