About this electric fan thing... have you done it? How big a difference did it make?

We have such underpowered alternators is it really a good idea? It sounds interesting, but I'm worried my new headlights and stereo combined with god-knows-what might be plugged into the cigarette lighter at any given moment (phones, laptops...) and various other electric items might to too much already. Does the fan draw a lot of power?

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'81 300TD Wagon 355k (Miss Diesel)
'83 300SD 180k
'84 500SEL 190k (Parting it out)