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There's an episode in The Green Mile movie where the death row servicemen rehearse the upcoming execution that is done using an electric chair. One of them says (quoted from here):

Roll on one. [pause] "Roll on one" means I turn the generator up full. The lights go brighter in half the prison.

he says that and indeed ceiling lights in the building go brighter.

Which implies that the electric chair requires a generator that produces suitable voltage and also adds extra power required to sustain the peak power consumption of the chair. This part makes sense.

But the "lights go brighter" part implies that the generator is somehow connected to the mains in the prison. This makes no sense to me - connecting a high-voltage (more than a kilovolt) generator to 110 volts mains would cause electrical problems and would likely not give any advantage.

I couldn't find any evidence of such setups - electric chair powered by a generator connected in any way to the mains of the building. Do such setups exist?

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I don't see a contradiction here. I imagine the generator produces 110V and there is a transformer between the mains and the electric chair. – Jader Dias Jul 21 '11 at 11:47
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you already answered that. The generator is connected to the mains. The transformer draws energy from the mains. – Jader Dias Jul 21 '11 at 11:55
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@Jader Dias: Assuming the 110 volts generator is connected to 110 volts mains the voltage should remain 110 volts, shouldn't it? If it remains the same the lights should not go brighter. – sharptooth Jul 21 '11 at 12:00
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not if the voltages are slightly different, it never happened to you that a power hungry electrical appliance in your house diminished the lights when on? That's because consumption subtracted a few Volts from the mains. The opposite can happen when you have a generator. – Jader Dias Jul 21 '11 at 12:19
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It could make sense to connect the generator to the mains: because apart from being used in electrocutions, the generator would also be used for backup power (to the prison lights) if the mains electricity failed (in the same way that generators are also present in hospitals and in telephone exchanges). – ChrisW Jul 21 '11 at 12:21
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