Roger Long wrote:
The AC element on my hot water heater is 1250 watts.

Can you safely draw from an inverter while the engine is running? If
I put in a 1500 watt inverter and used it carefully to only run the
water heater (which would also be getting waste heat from the cooling
water coil) and was sure that my other 12 V loads were minimal at the
time, could I get my hot water tank heated up in the 5 - 10 minutes
that I do with shore power? (It takes motoring half a day to do it
with the waste heat alone from my 20 hp diesel).

Or, will this burn up my little 35 amp alternator? I have a 55 I can
put in but it needs a different voltage regulator.

I would never try to make hot water from the batteries through the
inverter for obvious reasons.

Your 35 amp alternator puts out 35 amps x 13 volts (approx) = 455 watts.
Your 80 amp alternator will deliver 1040 watts... both running flat
out and probably overheating. Neither would supply the 1500 watts your
heater needs, or the approximately 1700 watts your inverter would need
to make the 1500 watts your heater needs. Not even both together would
be enough.

Further, this electrical energy comes from the mechanical energy that
the engine develops. 1700 watts represents about 2.25 HP, or in your
case, about 10 % of your engine's maximum output. If you are not
running at full throttle, you could be delivering as much as 25% of your
engine's output to the alternator(s). Much better to use the waste heat
in the cooling water loop - that is free from an energy standpoint.

bob