It will work as long as the lower tank is lower than the upper tank. The larger the change in height, deltaH the more flow there will be. Try it with a pitcher of water and a small hose. As long as the discharge is below the top of the water it will flow. The lower the discharge of the hose is the more volume (flow) you will get. I was trying this the other day, I was cleaning and emptying a 65 gallon tank. I had it outside on my deck, as long as the discharge of the hose was lower than the top of the tank I was emptying it drained. But as you have shown in your diagram you would be draining from the bottom of the tank, there will be nothing to stop it from draining when the power goes out, unless you put a valve that closes when the power goes out. I think it is risky like Shawn said. Now if you drained from the top of the tank with an overflow box the siphon would stop when power goes out and the water level drops to the bottom of the teeth on the overflow.