Elements are powered either/or, not both. Thus 4500w & 4500w elements
draw 4500 between them.

IMHO resistive heating is lousy, inefficient use of energy input. Max
overall efficiency about 30%. One reason gas costs so much less.

Too bad you can't use generator's waste heat (exhaust) to heat the
water! About the same actual power (energy rate) available there as at
alternator output.

J