Generator capacity in real-world use

[tom2018]

Evening all,

The boat we're looking at has a 7kw Onan, which appears base spec for the model (2011 Princess V56/53), all the other ones we've seen have the up-specced 13.5kw Onan.

Given our intended use of mostly daytrips (Med), with the occasional overnight at anchor, do you think we'd notice the difference with our gen having practically half the KW of the others?

Obviously it'll depend on usage, and I imagine we'd want the aircon on downstairs, bbq, couple of ipads charging, a few lights etc. I expect the aircon will be the biggest user - I don't have the exact model number of the units but I think there's 4x of them, Cruisair before rebranded Dometic, and I think they're split gas systems from what I saw. I was a bit surprised to see 4 of them, 2 on the engine room firewall port and 2 to stbd. Perhaps several smaller units better than one big one? Boat has aircon in cockpit too.

What kw generators do you all run and in real-world use do you find yourselves running to full capacity and tripping CBs etc?



Thanks

 

[jrudge]

That will give you up to 30 amps.

I have a squadron 58.

30 amps is a fairly typical draw for me with ac running. In the morning after night on anchor the chargers and immersion will push that to 40 and then the kettle etc can take it higher.

60 amps is the max my gen can deliver and when cooking dinner we turn chargers off and reduce from two to one ac pack to keep within 60.

Are you sure you have 4 central chillers ?

30 amps will be fine if you manage it but I’d suspect you will have no ac whilst cooking dinner ( depending how many hob rings / microwave / bbq you use

 

[Andy Bav]

We have the same generator in a V42. Last week we had 2 x aircon units, bbq, battery charger and 2 rings on the electric hob running. It was only when the wife put the 3rd ring on that the hob turned itself off, note didn't trip any CB. We think it has a self protect, low current cut out as it restarted about 60 seconds after we turned one aircon unit off.

I wonder if your engine room mounted units service the bedrooms to give relatively quiet cooling at night? .. One of our units is under our bed!

It would probably be rare that you would have all the electrical items that you listed on at the same time, but some simple power management will sort that, your panel will give you the amps being drawn.

Personally, although I haven't done it yet, I don't think I would anchor overnight with the genny running

 

[Portofino]

It’s a taboo subject - Aircon load tripping out gen sets on boats .
You are correct highlighting it ,
Visit Frigomar s site .
They do a brushless DC inverter motor .
My 42000 btu draws 2 A only .
Another issue is start up spikes if conventional AC powered have to keep switching on / off - thermostat controls the on / off .
With D.C. it just a rheostat, slows down etc so never actually stops ie re starts .
Some AC motors have so called soft starts but even so it’s not that soft and can trip the breakers ,say if 2/3 out of the 4 re start at once with other stuff running like your hob etc .

Industry has stated to wake up to this .

You really need to know the start up spikes in the 4 AC motors as well as obviously the normal running amps .
Then work out how much headroom you have for hob , kettle , and charger (s) if @ anchor .

Now that’s an interesting one - how are you gonna use your boat .
Idyllic anchorage , stay overnight ?
Start the geny turn the charger ( s ) on .
How much juice will they sap restoring those domestic low ( fridges / ice maker ) bat banks .How bigs the charger 40 / 60/80 / 100 aH ?
Oh “ somebody turn the Aircon on to cool the boat “
“ can we have grilled prawns for evening meal “ ?
Teenage daughter “ can I use a hairdryer dad “?

 

[jrudge]

Porto

I simply don't believe that a 42,000 but AC system pulls 2 amps!

We can all agree the gen is a bit small but it is manageable. It is really no different to managing load on shore power when you are limited to 32amps etc which is a typical supply for a mooring the size of the OPs boat.

Something has to be turned off - but it is not hard to do, and you dont use all the rings of the hob for long.

Chargers off is pretty common for me as it takes a big draw if you have been out for a while - but again it is no hardship - just flick a breaker and flick it back on when you are done.

Big the seller!

 

[mcanderson]

Insist on seeing it tested when you do a viewing and ask to see it under load.

We have a mase 5.0 on our 40 ft boat and with aircon, stove and water heater on it is running close to max.

 

[MapisM]

'scuse me for the slight o/t folks, but you are making me curious. What is the normal power supply for residential usage, in the UK?
Down here, standard is 3kW, with the option of upgrading to either 4.5 or 6. Above 6kW, they only supply 3ph 400V, normally meant for industrial usage.

Fwiw, I never had any problem with the 6.5kW genset in my previous boat, and the 11kW I've got now sounds like sheer opulent luxury to me...

 

[jrudge]

Uk houses have a 60 amp fuse on the main supply as standard. You can ask for an upgrade.

So in KW terms 14kw.

Given most homes dont have Air Con ( I do!) and most heating is gas not electric i would suspect that the average house never comes close to popping the fuse.

 

[MapisM]

Blimey, 14kW? And here I was, thinking that only over the Pond they are used to take electricity for granted, and use it as if it were FoC…
I've got 6kW at home, with 3 external a/c compressors and 7 internal fancoils, on top of all the usual other stuff (heating aside, for which I'm not aware of anything but gas used down here).
And I never had the supply tripping off due to overload!

 

[Portofino]

Porto

I simply don't believe that a 42,000 but AC system pulls 2 amps!

We can all agree the gen is a bit small but it is manageable. It is really no different to managing load on shore power when you are limited to 32amps etc which is a typical supply for a mooring the size of the OPs boat.

Something has to be turned off - but it is not hard to do, and you dont use all the rings of the hob for long.

Chargers off is pretty common for me as it takes a big draw if you have been out for a while - but again it is no hardship - just flick a breaker and flick it back on when you are done.

Big the seller!

I did quote “ Frigomar “ in the expectation folks perhaps would click about .
Never mind sorry for that in your case .
If it’s of any help ps read this link - hopefully to field your concerns .

http://www.frigomar.com/catalogo-frigomar/FRIGOMAR_Air_Conditioning.pdf

 

[jrudge]

Says 10.5 amps which I do grant you I probably half other systems - but more than 2 amps !

 

[Portofino]

Says 10.5 amps which I do grant you I probably half other systems - but more than 2 amps !

607 NT is the one ( 42 ooo btu )
Start max 2 A , others like yours 60/70 ? - running max 3-4 , but when settled 2A .

Shed loads of other benefit s as the blurb says .
like eco functions etc etc ,

Not cheap thus rarely OEM .
How ever huge retro fit trade when folks junk the std power hungry stuff for exactly the issues the OP alludes correctly imho to .

But for us the usp , wasn’t,t the Amps technobabble stuff it was the silence .
Was more than happy like the OP is thinking ? To upspec the geny toss another €10|15 K at that if needs be .
But not necessary.

Yup silence it’s great compared to the Domtec ( sp ?) stuff in the Sunseeker we had ,

So we run it all the time , there’s no contradictions at all .

 

[John100156]

In the old days, as a general and very rough rule-of-thumb, I use to assume say a 3:1 CoP on AC Chiller Units, so to estimate running current:

42,000BTU divide by 3412 to convert BTU to kWc = 12.3
Divide by an estimated CoP of 3:1 = 12.3 / 3= 4.1 kWe
4.1 kWe/230V gives a single phase running current in the order of 17.8A, lots and lots of variables of course.....

I expect nowadays units have greater CoP/efficiencies and 'starting' currents are less inductive/low with inverter driven compressors. However, at maximum load 2A even if this was a three phase current, would not be able to produce 42,000BTU cooling IMHO. 2A is more likely the running current of the AC fans but not the compressor.

 

[Bouba]

I did quote “ Frigomar “ in the expectation folks perhaps would click about .
Never mind sorry for that in your case .
If it’s of any help ps read this link - hopefully to field your concerns .

http://www.frigomar.com/catalogo-frigomar/FRIGOMAR_Air_Conditioning.pdf

This is very interesting Porto, can you give a rough guide to the costs?

 

[John100156]

Without the compressor running, the unit may draw 2A and would be quiet.....

 

[Portofino]

Without the compressor running, the unit may draw 2A and would be quiet.....

John “old days “

Welcome to new days .

U tube Frigomar to see a vid of the amps on the comp and the running amps etc + the sound it makes the comp .

Comp unit is actually under a bunk in the aft cabin .
You can’t hear it running and it’s pushing out eq of 42 000 btu .
They do a 62000 btu as well so it’s poss to modula up a bigger boat .

Our maths using online vol calcs was between 19/30 ,000 btu depending on which website method you used to do the calcs .

So I doubt being on top of the job it’s ever running at max anyhow .

I did not spec this I commissioned a boat Aircon tech to come up with the best solution .
He did so ps don,t shoot the messenger.
Silence was numero uno the tech low amp stuff secondary .

Trad domtech / marinair self contained units under €10 K fitted 3 units one in each space .This was a typical yard quote.
Another quote from a different yard circa €12 K for similar kit .


This stuff was €18 K —— no noise .No need to upgrade the geny
Which is in effect what the OP above is eluding to ?


Then of course let lift the stone and see what’s underneath of dodgy shore power in expensive visiting marinas slap bang in school hols August when the temp are 30 + degrees ( you have already forked out hundreds of € for the night - ) to find the Aircon trips so a terrible nights sleep ahead .

Mine still works on the fractional less than perfect shore power .

It’s great watching guys all on the pontoon unplugging and replugging the lines in , starring at our AC water outlet trickling away while there’s are silent , scratching there heads wondering why ours works and there’s doesn’t .

 

[Bouba]

John “old days “

Welcome to new days .

U tube Frigomar to see a vid of the amps on the comp and the running amps etc + the sound it makes the comp .

Comp unit is actually under a bunk in the aft cabin .
You can’t hear it running and it’s pushing out eq of 42 000 btu .
They do a 62000 btu as well so it’s poss to modula up a bigger boat .

Our maths using online vol calcs was between 19/30 ,000 btu depending on which website method you used to do the calcs .

So I doubt being on top of the job it’s ever running at max anyhow .

I did not spec this I commissioned a boat Aircon tech to come up with the best solution .
He did so ps don,t shoot the messenger.
Silence was numero uno the tech low amp stuff secondary .

Trad domtech / marinair self contained units under €10 K fitted 3 units one in each space .This was a typical yard quote.
Another quote from a different yard circa €12 K for similar kit .


This stuff was €18 K —— no noise .No need to upgrade the geny
Which is in effect what the OP above is eluding to ?


Then of course let lift the stone and see what’s underneath of dodgy shore power in expensive visiting marinas slap bang in school hols August when the temp are 30 + degrees ( you have already forked out hundreds of € for the night - ) to find the Aircon trips so a terrible nights sleep ahead .

Mine still works on the fractional less than perfect shore power .

It’s great watching guys all on the pontoon unplugging and replugging the lines in , starring at our AC water outlet trickling away while there’s are silent , scratching there heads wondering why ours works and there’s doesn’t .

Thanks for the info:encouragement:

 

[tom2018]

Thanks all for the info. When on the test drive we tested the aircon everywhere but the main engines were running (!), it never occurred to me to check how the aircon runs on generator. Managed to get a better photo though and some googling reveals they are RX10CK units, so 10000btu and 4.6 amps full load cool each x 4 = 18.4amps total with all of them on. If you plan a 5x startup spike then I guess that's where I'm going to get in trouble with a 29.2 full generator capacity? The 4 units are labelled stbd + port cabin, fwd cabin, dinette + saloon, and cockpit, so depending on which rooms/areas are being used there may not be a need to run all 4 units. And as you all mention, careful management and make sure not to turn them all on at the same time.

I've been reading several people's build threads on these forums and how they set themselves up to be able to run the a/c off the batteries for a totally silent option. Assuming ~5amps AC per unit x 11 (?) through the inverter would give 55ah AC - is this about right? I have 4x 120AH house batteries, so would this in theory give a little over 8hrs running time if we just had the a/c on in the fwd cabin only for example? Assuming the inverter was up to the challenge, a full charge, and nothing else drawing from the batteries?

Pardon my ignorance, I really should have paid more attention in science classes!

(and our offer on the boat hasn't even been accepted yet so it's all moot!)






Thanks

 

[jrudge]

Do not worry to much about start up spikes.

Firstly they are spikes no more, but more importantly the units all have different start delay times ( you set it when you install ) so that by design they cannot all start at the same time as that would be an issue worth worrying about.

Re inverter running non starter I am afraid. 5 amps at 240 volts is 50 amps at 24 volts or 100 amps at 12 volts assuming the inverter is 100 % efficient.

Your batteries will only be good for half their rated amp hours ( all lead acid are beyond which they become unhappy ) you will have refrigeration running which will take its toll so there is never a nil load.

So whilst in an ideal world you will have 240ah of usable capacity that will run your ac on one unit for 2 hours in practice with that scale of draw you will get less.

You then have the issue of running the gen for x hours ( 4-5?) to charge them up again !

The only person who did this I believe is medmilo and his huge battery bank needed replacement. He also has huge chargers.

Run the gen !

 

[jfm]

Tom, I agree with j that your maths is out. It is meaningless to state "4x 120ah" unless you also specify the voltage. The label on battery assumes 12v as do most people in conversations on this topic. So you have 4x120 ah at 12v. Which is 240 ah at 24v. And 120 ah at 24v if you build in the 50% that j mentions. You therefore have a bit under 12amps for one hour at 240v. You will struggle to run fridges on that overnight in the med, and a/c is non starter. But just run the gen if you want overnight a/c- that's what it's for!
Back to main Q: a gen of 7kva is very light for your (proposed) boat and standard spec from thr main builders would be 13.5. I wd therefore be suspicious that the airco has been retrofitted so that's an extra thing to check when buying (ie is it well fitted; is the electrical side done perfectly, etc) but it will work if you take care over what is switched on. Which frankly might become a PITA in day to day use. People get hacked off if they have to ask permission to make a cup of tea and you will get hacked off resetting trip switches.
At least you don't need cockpit a/c so that helps.
If genset is in engine room sucking hot air after a decent run of the engines it might be more like 6kva, depending on spec and hp headroom of the genset engine.