Some things you want to keep in mind...
1- you do not have to run every bell & whistle on the tank when the power is out
2- concentrate on what power you need to run the heater/s & powerheads first
3- my furnace is hooked up to the power from my whole house generator - so that I will have heat myself, and keeps the tanks warmer
4- The circuit the A/C is on for the tanks is also hooked to the generator box -I have a room A/C to keep my tanks cool as they are all in the same room. Since the power doesn't only go off in the winter time
5- my fridge is also hooked up to that box (my system has a special box..wired to the breakers that need to be operated by the generator), so I will have food and not lose all in my freezer, including my frozen fish foods

6- I also have my koi pond (2,000g) wired to it along with the big TV in the living room (that caused an argument..lol) and my FW tanks upstairs.
While I can't tell you the size of my generator I recall it cost about $550, and the box that runs all the specific circuits cost about $150,so I only have to turn on the generator and flick a switch and I have power... but the peace of mind is priceless
My point is you may want to look further than just the tanks.