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Sizing and Selection: the Big Picture of the Small End of Power Generation

Buc continues his exploration of small power generators

In the first part of this series (“The Good, the Bad and the Noisy,” RWEE, Feb. 7, 2018), we outlined the scope of small power generators, how they do what they do, and the general factors (plus and minus) that affect their performance.

In this installment, we’ll touch somewhat on how to select an appropriate generator and the factors that lead you to a determination for the peak and average power output requirement.

Engineering decisions are always dictated by a myriad of effecting factors, large and small. Most significant to our purchase decision in regard to small generators are the loads (and any related unique qualifications) and the duration that must be sustained.

In the present state of radio, no longer is there a “typical” station. Unique formats; clustering of facilities; the elimination of the main studio; programming from multiple sources such as satellite, internet, over-the-air pickup from a correspondent station, etc. — all this and more make each operating situation a singular consideration. For this reason, we must start our evaluation by identifying the critical signal path from sources to air for the station.

Quite possibly, this may be the moment to firmly ask and establish: What is the station’s mission? What gear and amenities do we absolutely need to keep broadcasting seamlessly? Do we need all the paths to “air,” including the transmitter(s), cable, internet, Twitter, Snapchat, Facebook, mobile alerts et al?

As our focus is small generators, a suitable example would be an FM station with offices, air and production facilities all in one location. Regardless of source, the air product passes through a file server frame, which also generates the programming when not live.

With just the file server, the processor and the STL transmitter; the station programming could continue providing at least the entertainment section to their audience.

Add in a simple production room with LED lighting and on just-a-watt basis, you’ve used up the capability of a small generator but maintained a local origination and production update capability. A minimally flexible circumstance most useful in a longer outage as well as in time of emergency or disaster.

Obviously, if in a power failure you want to continue seamlessly doing business as usual, you’ll have to replicate the power that you would have gotten from the power utility. For this level of luxury, you are committing to a major investment and also to reading a future series of articles in RWEE on BIG generators.

Again, another tier of decisions. What is the ROI to your station, since outages represent a very small percentage of the station’s on-air time? Generator selection is a time for hard choices.

Tables 1 and 2 have the annotation of the power consumption, power factor and a semi-subjective rating for each of these loads to less than perfect power … related to each of the items noted above. As noted, we made it under 5 kW.

Why is power factor (PF) important to us? PF is a decimal or percent value of the real power consumed divided by the apparent power flowing. Earlier, we mentioned mainly resistive loads, such as an incandescent light bulb or a portable heater, both of which have a PF of very close to 1 or 100 percent. 500 watts of incandescent bulbs takes 500 watts of generator power. In the present environment of switching power supplies and other reactive loads, the generator is looking into something of a load more complicated than a resistor. The charging current flows in and out of these inductive or capacitive components, but is never consumed (turned into heat or work), after which this reactive power is returned back to the generator.

The total current flow, though, is real and must be produced by the generator. So, if you have a 0.9 PF load you’ll need a 10 percent greater power output and you need to add that to your power requirement calculation.

To twist a quote from quote Roy Schneider’s character in the movie “Jaws” … “I think we’re going to need a bigger generator!”

Finicky index? In the first part of this series, waveform purity, which is an additional critical factor, was addressed. The net distillation of that review was that for a plethora of reasons, the sinewave waveform from these small generators varies greatly from manufacturer to manufacturer and with the load and the operation of that load.

The difficulty that certain UPS units have operating properly on generator are well recognized. Additionally, many switching power supplies, particularly those with high reservoir issues or sensitive control loops, do not do well on less than perfect sinewave waveforms.

Can you name one computer vendor that does not utilize a switching power supply?

Sometimes, a load in a station may be either so sensitive or so critical to operation that you must provide a perfect waveform.

In the small power world, the “inverter” generator meets the challenge. Sort of like a motor drive UPS, our small engine generates DC instead of AC. The DC is then made into AC using a rather sophisticated solid-state switching activity with an exact 60 Hz sine wave as its model. Not all that efficient, but definitely clean!

So, if at your AM transmitter site, your trusty 1960s-vintage tube transmitter is your backup, running on a “classic” (make to read: CONELRAD-era) generator, you may be running your local file server and internet STL on an inverter generator.

Before we wrap up this time, we should touch back on one other item broached above: operational duration.

Most of power outages directly related to utility failures are short. If most in your area never exceed 3 hours, the 5-gallon fuel tank on most gasoline-powered generators or a couple of 20-pound LP cans should be adequate.

If you’re thinking days unsupported, fuel supply becomes far more complicated and we’ll address these and more in the next installments.


1. Any discussion of operational duration needs to consider whether if real living personnel will be in the facility as well. If your staff is there, necessary creature comforts such as the well pump to allow the use of the washrooms have to be added to the loads.

2. Facilities that have gas forced air heating systems, the only real power penalty is the fan. For the relatively small demand of the fan, normal heat levels can be maintained.

3. Important: For facilities that have a sump pump, the pump should be powered by the generator. Many outages involve excessive rain, and for this small power penalty, one can avoid what can be severe water damage.

Charles “Buc” Fitch, P.E. is a registered professional consultant engineer, senior member of the SBE, lifetime CPBE with AMD, licensed electrical contractor, former station owner and former director of engineering.