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Old 20-02-2019, 17:48   #1
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Does a 120 amp alternator put enough load on diesel?

I have a theoretical question about the load a 120 amp (or around there) would put on a 40 something HP diesel engine.



What we are wondering is if you have your motor running and a 2500 watt inverter on and you are running a 1500 Watt heating element in you water heater. Would the drag from the alternator load the engine enough so you could safely idle for 30 to 45 minutes?
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Old 20-02-2019, 17:54   #2
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Re: Does a 120 amp alternator put enough load on diesel?

That's not a very efficient way to heat water. you'd be better to use a water exchange coil to circulate hot coolant in the water heater. Leave the alternator for charging your batteries
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Old 20-02-2019, 18:02   #3
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Re: Does a 120 amp alternator put enough load on diesel?

It wouldn't be a problem for the engine, but it would take about 50 amp-hours out of your batteries. The heater/inverter will be pulling about 140-150 amps with losses, and the 120 amp rated alternator will only put out about 90 amps if you are lucky.
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Old 20-02-2019, 18:05   #4
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Re: Does a 120 amp alternator put enough load on diesel?

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That's not a very efficient way to heat water. you'd be better to use a water exchange coil to circulate hot coolant in the water heater. Leave the alternator for charging your batteries



Actually that is what I am talking about, using both. Sorry if I was unclear. The engine doesn't get hot quickly if you are just in idle but would the load of the alternator be enough to maybe get the engine hot faster and keep you from getting in trouble just idling with no load to heat water
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Old 20-02-2019, 18:05   #5
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Re: Does a 120 amp alternator put enough load on diesel?

Let's see, it's a 120 amp alternator at 12 volts; that's 1440 watts or about 2hp; not much of a load... that said it's not a huge problem so long as you get a heavy load before and afterwards. That said, you are probably spending $10/kw hour this way. I'd find another way...
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Old 20-02-2019, 18:13   #6
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Re: Does a 120 amp alternator put enough load on diesel?

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Actually that is what I am talking about, using both. Sorry if I was unclear. The engine doesn't get hot quickly if you are just in idle but would the load of the alternator be enough to maybe get the engine hot faster and keep you from getting in trouble just idling with no load to heat water
If you could load the engine with an engine driven water maker and/or fridge compressor that would help but running an engine just for the alternator and water heating is going to be inefficient. take a while to get the engine hot when its just idling , you're right.
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Old 20-02-2019, 18:29   #7
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Re: Does a 120 amp alternator put enough load on diesel?

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Let's see, it's a 120 amp alternator at 12 volts; that's 1440 watts or about 2hp; not much of a load... that said it's not a huge problem so long as you get a heavy load before and afterwards. That said, you are probably spending $10/kw hour this way. I'd find another way...

Thanks Scott, that 2 hp is the number I was trying to come up with. I don't know that formula. I am just trying to figure out a way to get 3 showers and some dishes out of a 6 gallon heater in the cold Channel Islands. I have put the engine in reverse on occasion to put more of a load on it. (Only if the anchor is really set) I figure 45 minutes max on something like this. about 1/3 gallon of fuel.
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Old 20-02-2019, 19:48   #8
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Re: Does a 120 amp alternator put enough load on diesel?

The alternator at idle does not add any load and will. Barely be charging. Maybe 20a at idle. They work at higher revs
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Old 20-02-2019, 19:53   #9
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Re: Does a 120 amp alternator put enough load on diesel?

All the above is great info...Just to be sure you understand...your alternator makes 120 amps, at the very best, at WOT. At idle, as stated above, you will make 15 to 25 amps. 746 watts = 1 hp
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Old 20-02-2019, 20:19   #10
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Re: Does a 120 amp alternator put enough load on diesel?

Lots of assumptions. As noted at idle you likely would not get much current out of your alternator. That is one avenue to explore.

1 horsepower is around 745 watts. And watts are voltage times current.

So if you could get 90 amps out of your alternator and it was running at 14 volts you would be loading the engine with (14v * 90a / 745w = 1.7 HP). But, things are not 100% efficient so the actual load would be greater, say 60 % efficient so you would need 2.8 hp.

But and this is the big but, as noted your alternator will not put out 90 amps at idle.

Also, your engine will not put out 40 hp at idle. Something like the Yanmar 3JH40 makes around 10 hp at 1000 rpm. This engine comes with a 120 amp alternator but "typical" automotive service alternators quickly heat up under load and derate their output.

So to get 90 amps out of your diesel at idle you would need to install an alternator/regulator combination that would be capable of continuous 90 amp output, have a pulley ratio that would spin the alternator fast enough at engine idle to get the alternator to output 90 amps.

On the plus side the residual heat from the engine would help keep the boat warmer as it cools down.
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Old 20-02-2019, 20:54   #11
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Re: Does a 120 amp alternator put enough load on diesel?

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Actually that is what I am talking about, using both. Sorry if I was unclear. The engine doesn't get hot quickly if you are just in idle but would the load of the alternator be enough to maybe get the engine hot faster and keep you from getting in trouble just idling with no load to heat water

1. Just in idle will do squat for charging. Period. You need to run at at least 1500 rpm to get the alternator to actually produce anything worthwhile.


2. Depends on the SC of the house bank, too. Low it'll draw more amps from the alternator. Basic Charging 101, alternator or any other source.


I do this regularly: house bank down to 12.1V after a night on the hook; run engine at 1500-1800 rpm, get a charge and after an hour I get tepid water, without using the inverter.


One of these days I'll remember to use the inverter, but so far after 20 years, this works for me.
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Old 21-02-2019, 09:23   #12
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Re: Does a 120 amp alternator put enough load on diesel?

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Thanks Scott, that 2 hp is the number I was trying to come up with. I don't know that formula. I am just trying to figure out a way to get 3 showers and some dishes out of a 6 gallon heater in the cold Channel Islands. I have put the engine in reverse on occasion to put more of a load on it. (Only if the anchor is really set) I figure 45 minutes max on something like this. about 1/3 gallon of fuel.
Could you try having shorter showers?
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Old 21-02-2019, 09:31   #13
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Re: Does a 120 amp alternator put enough load on diesel?

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I have a theoretical question about the load a 120 amp (or around there) would put on a 40 something HP diesel engine.



What we are wondering is if you have your motor running and a 2500 watt inverter on and you are running a 1500 Watt heating element in you water heater. Would the drag from the alternator load the engine enough so you could safely idle for 30 to 45 minutes?
Seems as though you have fallen into the notion that it is a terrible thing to let a diesel idle. Pull into any truck stop in cold weather and you will probably hear at least 50 of them idle all night.
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Old 21-02-2019, 09:37   #14
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Re: Does a 120 amp alternator put enough load on diesel?

You can safely idle your engine for 45 minutes. You can safely idle your engine for 45 hours though you might develop some substantial carbon deposits. Have you ever seen a diesel-powered crane in operation? They’ll often idle for days on end with no ill effects. Your engine will be fine.

Fair winds and calm seas.
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Old 21-02-2019, 09:48   #15
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Re: Does a 120 amp alternator put enough load on diesel?

Just FYI, you can do any conversion you want on the internet.

In your case, to convert Watts to HP, just search for "Watts to hp".
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