I am here with you.
My plans, of course, center around hurricanes. Fortunately I was able to adapt to this problem. I have a 20 gallon and a 65 gallon. First off I just want to say that you should ALWAYS have backup up heaters. No please do as I say and not as I have done. Fortunately for me a friend offered a heater before the storm and I almost didn't take it. I am thankful I did.
I am running a Honda EUR1000i generator. It is keeping the tanks to temp but because I am running so much wattage in heaters I have to leave the lights off. During the day when it got down to 12F my indoor temps were at 45F. I am using two 300w heaters on my 65. My 20 had a 150w heater but it seems to have crapped out so I had to pull two 50w heaters to get my tank warm. I was able to keep the 20 at about 73 and the 65 at about 71.
The key to dealing with tank temps is to get your backup up immediately even in the summer every minute counts. I had a SNAFU with my generator and if you are bored you can read about it on my build thread. When you lose power you should not wait to see if the power comes back up. You will wait too long. I was watching my tank lose a degree every 15-30 minutes. Because it took me so long to get my back up in place my tank plummeted to 68F. You can't wait.
I have tested and worked out heat problems because hurricanes usually come when the weather is warm. The thing that people will forget about is that you will lose you house cooling and you have to test without a cooled house to get a true representation. Air is about your only choice for cooling and because it is usually really humid outside you won't have a nice dry household climate to drive your fans. Plan accordingly.
I have been in Houston for quite a while now and I have had family here for almost as long as I have been alive. I have to say this freeze is unprecedented. Not something I would have planned for, but fortunately I was planning for the power out and I was able to adapt my plan. Thankfully I had a plan.
Hope that helps.