run a 1200w water heater with 900w Generator

[David Mathis]

If you're trying to supplement shore power with additional power from your generator, don't do it. You can't sync them (they're alternating current) which can quickly damage the generator. If you want to run the water heater on a separate circuit, it would work but you need a larger generator. The 900 generator won't do well with a 1200 load. Just get a generator large enough or address the problem of insufficient shore power. Your marina should have at least 30 amp but preferable 50 amp which would make a generator unnecessary unless you want ac while anchored.

[sgtPluck]

Originally Posted by pendragon

Never heard such stupidity..
If you connect the shore power and your gen set together. The genset will blow up, or the shore power fuse will blow, as they will not be in phase with each other.

That was never the plan. The question was poorly worded, but the was actually about doing this at anchor: 'shore power cable' ... not shore power.

Still a terrible idea, which has been very clearly established

[RaymondR]

Originally Posted by Pete7

Hmm, just be aware of this tragedy:

https://www.bbc.co.uk/news/uk-england-34810360

Anyone who takes a gas generator into the cabin of a boat to power a heater is no great loss to the gene pool.

[highside]

Originally Posted by sgtPluck

Pardon me if this is a silly question, but I've never owned a portable generator until now.

If I run a 900w generator, then connect shore power cable and turn on my water heater (1200w element) will it just heat up more slowly or could it cause a problem?

Oh my.

[Topspin]

Originally Posted by RaymondR

Anyone who takes a gas generator into the cabin of a boat to power a heater is no great loss to the gene pool.

Darwin Award !

[Gerrit Coetzee]

Originally Posted by Pete7

I think and hope he means feed the generator power into the boats shore power connection as you might do when at anchor.

Sgt P, a 900w generator is a bit of a toy. You have a couple of other choices, ditch the generator and buy one twice the size. The Honda 22i would be nice and still a one arm lift. Alternatively swop out the heating element for a smaller size. A 750w size is available in the UK but we have 230v mains. A bit of investigation ought to find something better suited.

Sensible and practical advice

[skenn_ie]

Start the generator then plug the already live electrics into shore power ? Big BANG will likely result, killing the generator, and tripping at least the nearest breaker on shore, quite possibly more. IF the generator produces 120V, and the heater is rated for 240, then, as others have stated, it will work, just take a long time to heat. Depending on thermal insulation, the hot tank could lose heat faster than the heater can produce it, and you will never reach a satisfactory temperature. Also, as others have stated, 900W is a toy. Sell it and buy a decent one, I suggest at least 5kVA.

[RaymondR]

Originally Posted by Gerrit Coetzee

Sensible and practical advice

The high AC voltage and CO hazards introduced into the marine environment by the use of gas generators on a small vessel is a recipe for disaster. It's a Rube Goldberg solution at best and neither practical nor sensible.