how big of a gen do we need?

[ruthi]

If we are running an entire elec. bus, with 2 coleman  basement units hooked up to 2 20 amp breakers, extra large fridge, 20 gall.  110 volt ge elec water heater, small insta heat water heater that runs on 240. Kenmore clothes dryer, small, runs on 240 volt, that will run occasionally. Other than that, coffee pot, microwave, tvs, sound system. Occasional one burner electric induction cook top.  Can anyone just estimate the size gen we would need? The biggest one Onan makes is a 12.5.  I know there are ways to do this to know exactly, just want an estimate by someone who knows this stuff. Thanks

[Frank @ TX]

Hi Ruthi ,
I'm sure you won't be using all the systems at the same time.
Come up with a normal "heavy day" useage of which ones will be used at the same time.
For example: the AC running , clothes dryer running , water heater running.
From a list like that a total AMP draw could be found.
I would get that total heavy day and add another 30% to cover coffee and TV.
Thinking how you'll use the systems will help you use the systems better.
Remember the fridge runs about 30% of the time , the Ac will draw full AMPs
  for and hour or two when just turned on in a hot day ( but then cycle ).

We have a 10KW gen and watch the AMP meters to see what else we can turn on.
You'll get the hang of load shedding when your meters near the top.
Frank

[luvrbus]

Ruthi, the Marathon runs a 20kw Kohler with about the same equipment you are talking about guess you could do a little power management and get by with a smaller unit but me I would go with the big unit as I don't like to give up comfort for a little in fuel savings 


good luck

[ruthi]

Normal day would consist of fridge, 2 airs, hot water heaters, tvs, sound system coffee pot and microwave. dryer, maybe one day a week.

[ruthi]

luvrbus, we feel the same way. We have the opportunity to buy a nice gen with a roll out tray, 20 k for 7,500. Only prob. is it is not in Tn, but in Ga. So, maybe the gen we have now will keep running for 2 weeks. Hubby says it needs a  seal where the oil is leaking and a filter. It has to be took apart to fix, so, that cant happen now, plus the fact we are not close enough for anyone to work on it. We keep putting oil in it, so, we will see..........

[Len Silva]

I think you can do just fine with 12.5 with a little power management. It could be automated with a few relays so, for instance, one A/C is disabled when the dryer is running or the water heater is off when the cooktop is on.  It's fairly easy to come up with a plan that greatly limits any inconvenience.

[Barn Owl]

I would PM Sean on this or wait for him to respond. 20k will be way to big and you could find yourself in a condition called "wet-stacking". Also it is more efficient and better to run a smaller gen near capacity than a large one at partial. If I wasn't at work I would try to explain better, but I will leave that up to the real experts.

[rip]

  Ruthi; I also have an all electric coach except for gas cooktop and oven and we have no problems running everything we need. Of course you can't run everything at once,but with a little management it's not a problem.
       Don

[luvrbus]

Ruthi, "wet stacking" doesn't happen with the Japan made engines they run at 1800 rpm loaded or not.
Wet stacking is a term used for unburned diesel fuel in a engine doesn't happen in todays engines unless they are running cold. 
Bill Harris in Virgina a well respected generator guy in the marine and RV world and he tells me that a generator should be large enough to carry a full load at 80 to 90% capacity for a trouble free unit.
I have followed his advice for years and have never had a problem with a generator but some of these guys may know different.  


good luck

[James77MCI8]

Basement airs     2200watts each Based on 20 amp breaker x 110V
large Fridge        1100 watts
110V WH            1500 watts
Instant Hot WH    1500 watts
Small Dryer           4500 watts
Coffee pot             900 watts
Microwave            1500.00 watts
TV plasma               400watts
Sound system            80 watts
Inductive Burner       1500 ?
Lights                       300
Misc                          800
Total                        18480  If you had everything going at the same time!

I think you will be fine with a 15KW, 20 KW if you don't want to load manage

[WEC4104]

If you had just two large appliances, I think the chances would be pretty good that you would have them both running at the same time.  But if you have four large appliances, the chances drop significantly that you would have them all running at once.  When you start getting up to James' list with 7 or 8 appliances that draw 1000 watts or more, I really start to doubt that you will EVER see these all running at once. Also, some of these appliances only draw their maximum wattage for a brief period (compressor kicking in) and then drop to a more modest power draw.

I'm thinking that a 15kw unit is all that is needed, and maybe just a 12.5kw with some power management.

[Melbo]

I have a 13.5 and I have run the generator for little of nothing and as clifford said no problem with "wet stacking" it also has handled everything I have ever wanted to run --- the only thing I watch is that each leg is balanced but that really is pretty basic. My generator came from wrico.

HTH

Melbo

[ruthi]

Update, Ken took the gen out of the bus in the walmart parking lot, after work tonight. Found a filter that was plugged like we thought, dont know about the oil leak yet though. It is running smooth now, and the air in the back room is working now, not sure what this would have to do with the gen, if it does. He will run it tonight and see if it runs all night. If not, we have a plan for Rusty to send us a part. Thanks to all of you for the advice, you guys always come through! Still thinking of getting the larger gen when we get one.

[Sean]

FWIW, a "50-amp shore service" is 12kW.  So if you can run everything you need or want on the rig when you are plugged in to a 50-amp pedestal, then 12kW should be sufficient for your needs.

Moreover, most RV accessories such as Automatic Transfer Switches, and, of course, plugs and receptacles, are rated at 50 amps or 12,000 watts.  To use a generator large than that requires larger wiring, a larger transfer switch, and some way to "manage down" when on shore power instead of the generator.

For this reason I recommend setting up your system such that a 12kW generator would be sufficient to meet your needs.

As Clifford says, Marathon (and others) uses a 20kW unit.  But the mentality there is that if you can afford a Marathon, then you don't ask how much the generator costs or how much fuel it uses.  Marathons also come with 8kW of inverter capacity, which few people would need.

Load management is the name of the game when it comes to "right-sizing" a generator.  If you have a huge load such as a washer/dryer and you want to be able to run it irrespective of whatever else might already be running, then you will need a bunch of extra generator capacity that you won't (and can't) use any other time.  You'll burn extra fuel every time you run the genny just to have this capability available.

If, OTOH, you can plan your laundry for a time when you don't need, say, all three air conditioners, or the microwave, or whatever, then you can get by with a smaller unit, and save money all the way around.

Another factor is whether you have a load-supporting inverter, which can "make up the difference" between generator output and total load to handle those part-time loads.  This can make a huge difference in the size generator you need.

Only you can make these decisions, and so it is hard to advise you on "proper" size for your generator.  That said, since it appears to be a replacement situation, the sizing of existing wiring, transfer switches, and panels will dictate an upper limit to the size of the generator.  So, for example, if what you have now is 12kW, and you have 50-amp switchgear, then the largest you can safely go is 12kW without changing out all the wiring, switchgear, and main breakers.

To give you a calibration, we have three roof airs (and a fourth air for the cockpit, but this is seldom run while parked), electric water heater, induction cooktop, Advantium oven, and the usual "small appliances" like a coffee maker, LCD TV, stereo, laptops, etc..  We can run absolutely everything all at the same time on about 7.5 kW, but because we have a load-supporting inverter, we could easily get by with a 6kW unit.  If I were shopping for one today, it would be a 6.5 kW model, because that's a common size.  (The one we have is 15kW, because it came with the bus.  And, yes, it is wet-stacking, and costing us about 2x the fuel.)

HTH,

-Sean
http://OurOdyssey.BlogSpot.com

[luvrbus]

Maybe time for a little work on your Kubota Sean  the only thing that will cause that on a Kubota is a bad injector pump,bad injectors,or low compression.If you are sure it is from carbon buildup run a water through the intake with a spray bottle under a load and it will cleanup.I personally have never seen it and I had 10kw generators for years that never ran anything larger than a 7 1/4 saw same with my light towers.
FWIW reading my manuals last night a 20kw Kubota will burn less fuel at 80% capacity than a 12.5kw under full load .
I can pm you the fuel useage charts on the Kubota engines and generators if you like