When forced with the option to use portable generators for live production, there are aspects that most sound engineers do not understand. While install power connections are reasonably lightly loaded for the audio watts provided, the nature of the current demands will vary with the music intensity of the performance and needs to be considered with portable power.
Yes, there are stories of success using small generators designed for the RV and household emergency power markets; these scenarios are very conscious of the limitations and the gear is minimal to do the gig. A typical rock ‘n’ roll show, even with local talent production standards and modest audience, can draw tens of thousands of “peak” watts during the course of the event. No genny-cart I know of would survive that kind of show; and if the constant breaker tripping doesn’t get you, the possibility of equipment damage due to under voltage and under frequency is likely.
Understanding the Load
So, how do you size a generator? Well, a fair and conservative assessment of the power “loading” needs to be done. Audio equipment like mixing consoles, signal processing and wireless equipment are the easy items because they mostly draw consistent current at 120 volts AC. But audio power amplifiers are where things get difficult. The good news is that the higher frequency audio signals are filtered by the power supplies within the amplifiers and just the amplitudes of the signals are brought back as power supply demand. So, the amplifiers will draw based on the loudness demands throughout the show.
For instance, subwoofer power amplifiers will draw power in impulses as each kick drum hit occurs and bass guitar note attack sounds. The power distribution will see an ampere or two continuously with 10 ampere or more impulse draws to replenish in the internal amplifier power supply as the source material demands. The midrange and high frequency power amplifiers also have the same crescendo demands, but typically the impulses are wider and less intense. Given all the amplification with each amp’s internal power supply needing replenishment with the source audio, the power distribution and portable generator peak/average demands vary a lot.
Genny Design
Generator designers are not dumb engineers, and they do understand that some load tasks will require a lot of peak current demand with lower average demand. To accomplish the provision for peaks, the electric AC generator transfers the electrical demand back as resistance to shaft rotation of the generator’s armature. To counter this variable mechanical problem, a beefy “flywheel” is employed. The flywheel stores rotational energy and keeps the generator shaft moving at constant speed (nearly) as the generator varies its torque demands. There are two ways to deal with kind of loading. One is to design AC generator systems for “show power,” in which variable loading is a given and the sufficiently oversized flywheel is provided to ensure the voltage and frequency are stable for the event. The other way is to just choose a larger-than-required AC generator to oversize the flywheel and everything else.
A portable generator also uses a diesel or gas engine as the energy source. But just like driving a vehicle using the accelerator pedal, engine response can be hundreds of milliseconds, which is slow compared to the load demands. That is why the flywheel effect is essential to power regulation while waiting for the engine to sense shaft speed slowing and to add fuel to speed up back to normal.
To be honest, the best practice on genny selection is to do both items when specifying a generator for a gig. Ask for both a “showpower” generator, and more power capacity than is required to produce the show. Besides, you may get stage lighting and concession loading if the promoter is not aware.
Practice
In practice, a good rule of thumb is to compute your audio loading at maximum capability and then double that value. For example, my rig offers about 10 kW at FOH and a similar value to the stage monitors, but I will specify a 45 kW to 50 kW showpower generator or more for outdoor gigs. Not only do most showpower generators have extra engine muffling for quiet backstage operation, but the design is conservative for live audio work.
Stage lighting can work with you or against you if tacked on to the same generator. Today’s actively lit shows routinely go from very dim to very intense lighting (e.g. audience blinders) to accent the performance. If you are on the edge on either generator capacity or using a generator with not much ability to keep voltage or frequency stable, then adding some constant stage wash lighting will help lower the peak loading with respect to average loading. My advice is to always error on the side of plenty of genny power.
