run a 1200w water heater with 900w Generator

[Flod]

I have tried to use my 1kw Honda to power my 1kw water heater. It works for like 2 seconds then the overload switch trip on the Honda. It’s simple laws of physics. A 1200w heater will draw 1200w. It won’t “feel” that the generator only can deliver 750w or whatever it’s rated for. I think you need at least a 2kw gen to power a 1.2kw water heater

[Sailmonkey]

Originally Posted by Flod

I have tried to use my 1kw Honda to power my 1kw water heater. It works for like 2 seconds then the overload switch trip on the Honda. It’s simple laws of physics. A 1200w heater will draw 1200w. It won’t “feel” that the generator only can deliver 750w or whatever it’s rated for. I think you need at least a 2kw gen to power a 1.2kw water heater

You just need a generator rated to deliver 1200 watts continuously.

In your case, a 1kw Honda is really like 900 watts continuous.

[geekoman]

I, and quite a few other cruisers I know, plug their suitcase generator, e.g. Honda 2200, into their shore power inlet and provide power to their vessel. There is no magic here, you only get what the generator can put out.

In your case, when you turn on your water heater, you'll most pop the fuse on the generator. Your generator is 900W, it will probably have some peak rating, but I bet eventually the breaker will pop to protect the generator.

The 1200W element will draw 10 amps at 120V. A 900W generator will be capable of outputting 7.5 Amps (900 W / 120 V). The heater will try to pull 10 Amps and the fuse will pop, hopefully!

A simple solution to your problem would be to buy a new element. Something less than 900W at 120V would work for you. Also, if you can find a 220V element at 1650 Watts or less, that should work, i.e. 1,1650 W at 220V == 900W at 120V.

[Reefmagnet]

Run the heater element via an inverter of adequate output. Run the genset for longer to replenish charge to the batteries. This way you can also heat the water from other charge sources too.

[geekoman]

Yes - this works, if you have the inverter capable of outputting 1200W.

[philcun50]

If you have shore power available why would you want to run your portable generator?
Next scenario, you are away from shore power and want to heat the water from cold. You need a generator of significantly more power than the element just heat the element up to operating temperature from cold. A cold element draws more power than one at operating temperature. Operating temperature of the element is way higher than the water temperature. Your generator will try to do it but, hopefully the generator’s overload protection will trip in a matter of seconds or the motor will stall or the generator windings will burnout.
I had a cheap 3.5kw generator to power a 1.2kw element. To get the element up to operating temperature I would start the generator with the heater turned off then turn it on for say 5 seconds then off let the generator recover then back on then off and over a few minutes of on and off with the on periods getting longer. Eventually the element would get hot enough that its power draw was low enough for the generator to handle and it would then be left to heat the water.]

Ultimately you seem to have no electrical knowledge so to be polite it would be best not to PLAY with it. Electricity is NOT a toy it IS deadly.

[CaptTom]

Wow, this forum can jump around!

This correctly and politely answers the OP's question, and doesn't make false assumptions about things the OP never said:

Originally Posted by geekoman

I, and quite a few other cruisers I know, plug their suitcase generator, e.g. Honda 2200, into their shore power inlet and provide power to their vessel. There is no magic here, you only get what the generator can put out.

In your case, when you turn on your water heater, you'll most pop the fuse on the generator. Your generator is 900W, it will probably have some peak rating, but I bet eventually the breaker will pop to protect the generator.

The 1200W element will draw 10 amps at 120V. A 900W generator will be capable of outputting 7.5 Amps (900 W / 120 V). The heater will try to pull 10 Amps and the fuse will pop, hopefully!

A simple solution to your problem would be to buy a new element. Something less than 900W at 120V would work for you. Also, if you can find a 220V element at 1650 Watts or less, that should work, i.e. 1,1650 W at 220V == 900W at 120V.

Well said, thank you! The idea of running the water heater off a (suitably sized) inverter, while running a battery charger off the generator, also has merit, depending on the size of the battery bank and charger, of course. There are even inverter/chargers which will parallel shore power (in this case, from the generator) and batteries, but all these ideas start to get pricy.

[Shrew]

Originally Posted by geekoman

The 1200W element will draw 10 amps at 120V. A 900W generator will be capable of outputting 7.5 Amps (900 W / 120 V). The heater will try to pull 10 Amps and the fuse will pop, hopefully!

This is the correct answer. Wild range of replies for a guy that is simply trying to plug his boat into a generator and, in this case, run a water heater.

I used a Honda 2K for over 12 years. It'll do everything, just not at the same time. Heat water, twice a day (shorter run times to maintain it on a regular basis, plus you have hot to warm water almost all the time). Then run battery charger. I could usually run fridge and water heater, or fridge and battery charger and phones/tablets. I couldn't run water heater and battery charger at the same time.

I spent a month unplugged from shorepower on a 40 foot boat with a fridge cold and a freezer full of meat and hot water for dishes and showers nightly with no issues.

[Cadence]

Originally Posted by Cadence

I doubt the gen. will sync with the shore power. I hope the breaker trips on the gen. prior to it frying.

Sorry! I may have misread the OP?

[mitiempo]

Originally Posted by LoudMusic

I agree you need to add more of your steps in here.

If you are running a 900W generator through a charger to charge your house battery bank, then an inverter from the house battery bank to the hot water heater (assuming this is an AC water heater?), you'll be drawing 300W from the battery (approximately - losses not calculated) and likely be fine.

Better be a large charger. A 1200 watt AC element in the water heater will draw 120 amps from the batteries if powered by an inverter.

[mitiempo]

Originally Posted by Cadence

I doubt the gen. will sync with the shore power. I hope the breaker trips on the gen. prior to it frying.

Will not have to. Shore power not connected, just generator.

[mitiempo]

Originally Posted by NorthCoastJoe

I would be worried about back feeding into the grid. This idea is flat out dangerous.

Your 30 amp shore power should easily handle a 1200w water heater. If not you have bigger problems and a potential fire from an overheated electrical connection.

Really hard to back feed into the grid when you are not connected to it.

[mitiempo]

Johnson XPXflow water heaters are available with 500 watt elements and Isotemp water heaters have 750 watt elements. Both at 120 volts.

If the thread is correct it is an easy fix.

[David B]

Originally Posted by sgtPluck

I meant connecting generator AC output to Shore Power AC input on the boat, then flipping on the heater 1200w element on AC circuit in the boat.

In my ignorance I thought it may be possible that the generator would happily put out its continuous rated load (750w) and that because the heating element is 1200w it would just not get as hot as designed. But it appears from responses that the generator itself will get overloaded/trip from the lack of resistance on the circuit.

Correct. I think I can see where you are coming from, thinking perhaps in terms of a 12V lamp running on 6V - running, but just being dim.

Not the same thing at all. Same voltage, and therefore the current draw is dependent on the load. In this case the load is too great. If the generator is half decent, the overload will just trip out. If not, it will overheat and kill the alternator on the generator, if it does not stall the motor first.

[RaymondR]

The correct solution here is to leave the electricity alone and get your local welder to make you a calorifier sleeve to fit in the exhaust of your genset. You'll also need a pump to circulate the hot water to the water heater tank. That way you can use the whole 800 watts to charge your batteries whilst the waste heat heats up your shower water. You'll double your return on the gas you are using.