I've accidently forgot to turn the power strip (heater, pumps, ato) back on after working on the 20L in our basement and didn't realize it till almost 24 hours later. The temp dropped to 68F in the tank before I found it. Everything survived, including a starry blenny. Not sure how common this is though.
My tanks are currently at 73F and 75F. I using one of our cars as a generator essentially (it has an inverter and outlet) to run heaters and pumps on each tank for 30 mins every two hours or so.