I'm from a small village in Central Scotland and power outages are common in winter.
I had a small UPS, but it was only connected to the pumps. (Heater takes too much power and it would only have lasted an hour or so).
I designed and made a power alarm, which was plugged into an outlet in our bedroom, and when our power went out, it would let me know and I could react!
What I did discover from this, is that its not the drop in temperature which really harms the fish (within reason), but it's the toxins that the dying bacteria release (they are the first to die off at lower temps.)
It might take a few days for this to be seen, or if its not too drastic, the Nitrite cycle might survive and all will be well.
Just my 2 cents worth! lol
