Legally I don't think you can tap on after the meter and make it run backwards. Your input to the system has to be greater than what you're taking in for it to make any difference on paybacks.
Say your total house load is 48Kw. You'd need at least 48Kw to equal the house load and cancel out the power. 49Kw to feed power back into the system.
32 amps @ 12 volts = 384 watts, 32 amps @ 24 volts = 768 watts
32 amps @ 120 volts = 3840 watts
Check with your power company before undertaking this task.
