Generator Decisions - How big/what kind?

[Oonrahnjay]

OK, I've got most of my things figgered out.  Here's what I know.  At max (full air/con on, battery charging, water heater, and small stuff), I'll need 5400 - 5800 watts.  There are a couple of Onan "QD's" at 6KW and 8KW but they're both the "variable engine speed/3 cylinder models".  I'm really leery of the variable engine speed gennies.  But Onan has a OD at 10KW that's 1800 RPM/fixed speed and *very* quiet.  Is it best to "really load down" a generator -- i.e., if I need about 5500 watts specify a 6KW gen or is it better to add in some excess capacity.  And specifically, is the 10KW just way too big for my needs.

If a 7.5 or 8 KW is about the best for my needs, does any company make an 1800 RPM/fixed speed 7.5 or 8KW generator?  (I'm assuming water cooled and all the extras like overload/overheat protection, auto generator start with low batteries, etc. -- also, low noise level is important to me.)  I don't have $$$ to throw away but I'm hoping to not spoil the whole thing by "cheap out" specs.

Hope you can understand what I'm asking here.  Thanks,  BH NC USA

[Sean]

Bruce,

The thing you have to watch out for here is the start-up demands of your air conditioners, and refrigerator if you have a household model.

If you have a load-supporting inverter such as the Xantrex (Trace) SW series, or many Mastervolt or Victron models, you can get away with sizing the genny for the maximum continuous load.  We have such an inverter and could get by with a 6 kW generator, and many days I wish that's what we had instead of the 15 kW monster that came with the bus.

If you don't have such a mechanism to deal with the start-up demand, then you need to oversize the genny to compensate.  The rule of thumb here is that a one-ton air conditioner, whether roof or basement, takes from 1,300-1,800 watts while running, but easily double that while starting up.  If you have three roof airs, you'd need around 5 kW to keep them all running, but at least 6 kW and possibly more to get them started -- one at a time.  If you expect that more than one of them may start simultaneously, then add accordingly.

If you think your max continuous load will be 5,500 watts, I would add in another 1,500 watts, minimum, to account for motor starting if you don't have an inverter to handle it.

Variable-speed "inverter" type generators are more efficient than fixed-speed units, especially at or below half load.  I can tell you from experience that your generator will run at or below half load a great deal of the time, so bear this in mind.  That said, I, too, favor fixed-speed 1800 rpm units.  For that matter, for starting motors I also favor the older transformer-regulated type.  These things run forever if taken care of, and there are plenty to be had on the used market in this power range.

HTH,

-Sean
http://OurOdyssey.Blogspot.com

[chev49]

Same topic someone called and asked me about this afternoon. If i were needing 5000 or so watts of power i would consider my load to be 8000, for reasons mentioned in the post above, as well as the fact that you never know when you are going to be adding something later that you didn't count on... I think  overkill in generating capacity is a good idea, and it is also possible that someone one else might be out of power some night...

[luvrbus]

I would go for the 10kw but then I am not into the power management deal I want a generator large enough to power everything at once and do it at 3/4 of the load capacity of the unit has worked good for me over the years JMW a under sized generator will drive you nuts on a hot day

good luck

[chev49]

Or higher...

[Lin]

We have an ancient 7.5kw Kohler.  I think it was originally put in service by Washington during the revolution (this is not confirmed).  It is quiet adequate for our needs, but then again, we only have two AC's and it is very rare that we actually run both at once.  I think we could probably make it with a 6kw.  A 5.5kw would probably work most of the time with a lot of power management and prayer.  However, if I am going to be praying, I would rather it be for other things.

[eagle19952]

Generator efficiency is about 80% of capacity.
Alot less makes a lot of carbon,not fuel efficient....

[gus]

Diesel engines need to be worked hard and they thrive on it. A loafing diesel is not a happy diesel.

My little Honda 6KW handles two 13.5 ACs and frige with no strain. However, when we use the MW we turn off the frige just in case.

[rv_safetyman]

Someone touched on it, but you need to make sure that your calculations take into account all of the efficiency losses.

Folks seem to loose sight of the fact that generators are rated at sea level or standardized conditions.  Try getting the full output when you are at 6K + altitude.

If you are going to wire the generator for 240V (most don't, but I did to accommodate my dryer), you need to have some extra capacity to account for unbalanced loads on each leg.

Another factor is how you install your generator.  If you use a remote radiator and a big 110 fan to cool it, the fan reduces the capacity.  In my case I use a large squirrel cage fan that draws about 11 amps at 120V.  Kind of overkill, but my generator never overheats.  BTW, it is a fan that was recommended and supplied by Dick Wright.  

There have been many of these thread that mention variable speed generators.  I always, emphatically, say that they sound **TERRIBLE** and drive me crazy (short drive ).  When you get two or three of those units in the same area, the sound is really obnoxious.  Typical human hearing can handle (and tend to ignore) a steady sound (if not above acceptable sound pressure level), but the continuous rise and fall of the engine RPM will offend most people who are around it for any time.

Folks always bring up running your generator at less that max load, warning that it will do all sorts of terrible things to the engine.  I don't buy into that.  Modern diesel engines manage fuel delivery pretty well.  One only has to look at the duty cycle of over-the-road trucks with huge HP engines.  They run at a pretty small percent of max capacity for hours on end and then sit and idle all night.

Bottom line, I would be very careful about trying to select a generator with a capacity that is just slightly greater than the needs.  Someone like Sean has the systems that would let him manage with a small generator, but most do not.

Jim

[luvrbus]

I am with you Jim on this one price one of the inverters Sean made reference too that will shock you

good luck

[TomC]

Jim-I was going to use a squirrel cage blower, like I do now. But Dick Wright has found a high efficiency blade fan made in Europe that only pulls 4.5amps.  It is so strong that if you're holding the fan and crank it up to full speed, it will push you back.  Dick also provided an electronic speed control so you can reduce the speed to produce no sound, yet have it cool at night.  I haven't installed it yet, but maybe call and talk to Dick about it.

As to generators-look at the Powertech 7.5 or 8kw lay down.  It is compact, brushless and was the unit a friend of mine had over 23,000 hours on it when he sold his big rig.  So those small Kubota engines running at 1800rpm are very long lived engines.  Good Luck TomC

[Lin]

I have a couple of Vostermann blade fans.  They were designed for greenhouse ventilation and were very expensive.  However, they put out a lot of quiet air and have relatively low energy usage.  Is that the type of fan you are referring too?

[Charles in SC]

I have an Onan 8kw Quiet Diesel and have never regretted the choice. I just serviced it yesterday at 1200 hours in 7 years. It runs 3 roof airs, electric water heater and a small household fridge with ease. I like the variable speed feature because you do not have the extra noise and fuel usage unless you need it.

[TomC]

I have three roof airs (all with heat strips that do work), 2-10gal elec water heaters, electric heat in the bathroom, microwave, toaster oven, electric refrigrator and chest freezer-the only propane run is the furnace and stove.  I run a 10kw Powertech gen that I can over load. Has been very reliable (mainly because it is a non electronic model) and runs my A/C going down the road (I have no dash air).  On my truck I'm going with a 12kw Wrico but using a 3 cylinder compared to the 4 cylinder on the 10kw-hence my 12kw is actually smaller then my 10kw.  With prechamber engines, carboning up isn't as much a problem as a direct injected engine since the prechamber engine has higher compression ratio.  But-you should periodically load up a Diesel genset to full output for at least 15 minutes to heat it up and burn off the carbon if it has built up.  Good Luck TomC

[luvrbus]

You guys running 3 roof tops on 8kw you can forget about in the AZ heat a 10,000w won't run 2- 15,000 btu very long here at 115 degrees with out shutting down I size my generator and factor in the heat rule of thumb here is 3000w for each 13,000 btu roof top got that from my local Cummins dealer at Kingman and altitude plays a big factor when you get over 3000 ft
He told me that is the biggest problem with Onan they have are the manufactures undersize the unit 


good luck