I totally agree on the overflow recoup not being feasible, unless of course your system was gigantic. Cool idea though, it's like regenerative breaking on a hybrid car.
One thing I've heard of people doing to make their tanks more 'green' is to reuse the waste heat in other areas of the home. The one tank I saw this done on was like 1200 gallons or something ridiculous, and it was powered by umpteen million 400 watt MHs. The guy who built it lived in Minnesota or somewhere with horrible winters like that, so he built his house's HVAC system around the tank. In the winter exhaust heat from the lights would be used to heat the air in his forced air system, offsetting the heating bill for his house. In the summer when it wasn't needed, he'd just switch it so the tank exhaust would vent outside.
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