For those concerned. I am not intending to use this as a perminante installation, but rather a semi-portable hotwater + generator combo. I have a 55 gallon rain barrel, with a RV type- pressure regulating pump (120V). I'm not concerend about recovery time and more concerened about power requirements (of generator vs 10guage extension cord).
I see that home depot has replacement elements in stock for reasonable prices. I have been unable to find a supplier for these rare 750-1000w, 120V elements, but have been able to find lots of 240V elements for near $10. When I look at the elements, I can make guesses why they don't advertise a 1000W, 120V element as a capable 4000W, 240 element. But why can't a 240V, 3200W element run at half voltage to achieve quarter power.
Resistance of a 3200W, 240V; P = V^2*R; R = P/V^2 = 3200/240^2 = 3200/57600 = 0.055555 [ohms]
Resistance of a 1600W,120V; P = V^2*R; R = P/V^2 = 1600/120^2 = 1600/14400 = 0.11111111 [ohms]
Power of a 3200W, 240V element running at 120V: Power = V^2 * R = 120^2*0.055555 = 800 watts (PERFECT).
Other thoughts I've had, include... (1) using a power diode to rectify the wave. (2) using an variac (variable transformer). (3) using a stove top resister dial. But these seems risky because I don't know if the thermostat circuit requires exactally 120V or can run at arbitary values.