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Generator Education: Backup generators, Power factor, etc

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John Hiemburg:
Hello,


We are looking to have a full-time backup generator installed in our church (In Soweto, Johannesburg, South Africa). The power system in our country leaves much to be desired, and we are on load-shedding (scheduled or not) some portion of nearly every day.


To calculate the size of generator we need, we hired an electrician who specializes in generator installation/etc., he did calculations of our draw with everything powered on, came to the conclusion we need a 35-45 Kw set. This is a 3-phase installation.


I trust them, this is really more for my education: The generators all have a 'power factor' rating, nearly all of the diesel 3ph ones here are 80%. At first I thought this meant that was the max constant load (in other words, only use 80% of the 45kw rating) but from my cursory reading, this appears to be an efficiency rating.


My questions:


1: How oversized should a generator be? If your actual current usage is 35-40kw, will a 45kw generator generally be sufficient or should we be stepping up? Our loads are: Small PA, house and stage lighting (all LED), various office equipment, roof-mounted evaporative coolers. No heaters, no AC. I know bigger will always be better to a point, but two issues: Cost (of course), and from what I've read, if your generator is oversized it will be less efficiently if your regular load is below it's ideal range. Is any of that true, or just mythology?


2: What does the Power Factor rating really mean? Does it matter to me in generator choice?


3: Any other questions that I should be asking?


Thanks in advance!

Frank Koenig:
“Power factor” here refers to an electrical property of the power that is imposed by the load. Specifically, it’s the ratio of power (sometimes called “true power”) to apparent power. Power is the power turned into heat in the load. Apparent power is the product of the RMS voltage and RMS current and, for many purposes, is what is “felt” by the generator, transformer, transmission line, etc. For a sine wave the power factor is the cosine of the phase difference between the current and the voltage and is equal to 1 for a purely resistive load and zero for a purely reactive load.

The 80% may refer to the power factor at which the generator can deliver the rated power, which would mean that the apparent power (in Volt-Amperes)  it can deliver is 125% of the power (in Watts). I would have to see the detailed generator specs to be sure.

In my experience contractors have a way of specifying generators, air conditioners, well pumps and many other things that are larger than what the customer actually needs. (Sound providers, however, never do this ;) ) An over-specified unit not only costs more to begin with but often is less efficient because of its lower utilization. Having said this, engine-driven generators should be sized somewhat generously. There are many factors and it’s a bit of an art. In the case of diesel engines you do not want the generator to be much too large for the steady-state load as the engines become unhappy running for extended periods at very light loads –- see “wet-stacking”. If you’re at all  in doubt get another opinion.

Depending on requirements and budget you might also consider a battery storage system like the Tesla Powerwall.  It would be interesting to compare the life-cycle cost with a generator.

--Frank

Chris Hindle:

--- Quote from: John Hiemburg on June 17, 2022, 08:58:49 AM ---Hello,


We are looking to have a full-time backup generator installed in our church (In Soweto, Johannesburg, South Africa). The power system in our country leaves much to be desired, and we are on load-shedding (scheduled or not) some portion of nearly every day.


To calculate the size of generator we need, we hired an electrician who specializes in generator installation/etc., he did calculations of our draw with everything powered on, came to the conclusion we need a 35-45 Kw set. This is a 3-phase installation.

3: Any other questions that I should be asking?

Thanks in advance!

--- End quote ---
35 to 45 may be good TODAY, but investing like this really demands looking 5 and 10 years down the road...
And YES, sizing a Jenny IS an art...
Chris.

Steve-White:
John, 35-45kw is a fairly large genset.  However, without know detailed specs of both the recommended rig (as Frank stated) and your facility it's not possible to advise.

Do you have the load calculations they used in the recommendation/estimate?  I would want to look at the total load -vs- "Demand Factor" for the facility.

Also, power factor, demand factor and duty cycle are each unique things and not generally understood and often mixed together.

Not to in any way contradict what Frank said about "over-sizing" things.  That's the "safe" way out.  For instance, a manufacturer sizing generator capacity must account for altitude and operating environment ambient temperature averages and extremes.

Sizing a gas engine to drive a pump for a pressure washer comes to mind.  When the manufacturer (CAT Pumps in my case) publishes specs for horsepower requirements for a gasoline engine or electric motor they go WAY conservative.  Line voltage sags must be taken into consideration for electric motor sizing and altitude/.temperature - what racers measure as Density/Altitude to tune racing engines.  A 13-15HP gasoline engine running at 8000' or more above sea level will not perform like it does at 750' altitude or sea level.  They post specifications that will run everywhere reliably anywhere short of Mt. Everest if you will.  They don't even offer tables with ranges nor make mention of this.

So, what you're undertaking can be a pretty complex thing - simple, but many factors must be considered.

Running back-up generators on my house for the past 10+ years I've learned "just enough" isn't where I want to be - as any generator, the higher up the load curve it is operating, the more harmonic distortion there is in the output, which at some point will manifest as issues with electronics.

Battery backup - well that's certainly an option.  I run a combination of both.  Short term, all electronics are on APC UPS's, and for longer term outages which worst case have been 4-5 days during winter storms, the generators reign supreme.

I'd be tempted to have the wiring done and do some testing with rental portable rigs or have the contractor demonstrate various size generators to ensure proper capacity and not buying something you don't need.  There is a "Sweet Spot" that you want to find where budget and performance align.

Bill Meeks:
One reason for "oversizing" a genset is to provide reserve for large load spikes due to HVAC startups. When starting, a load like the compressor in typical residential and commercial heat pumps can draw very large current surges for a short time. If the generator set is not capable of supporting that, the frequency and voltage will sag considerably and the heat pump or other load with that startup in-rush current will fail to start.

Frank has a good point about not going way oversized, but you may not want to size based solely on steady-state loads if you want things like the heating and air conditioning to operate normally while on the generator. Your electrician and generator supplier have hopefully taken this into account when recommending a generator size.

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