What You Need to Know
Summer is coming up fast, and warmer weather means outdoor events — often in venues where power will come from a generator. Here’s the skinny on what you need to know about running a small P.A. using generator power.
How Much Power Do You Want?
The amount of power consumed by electrical equipment is typically expressed as watts; the amount of electricity produced by a generator is expressed as kVa (kiloVolt-amps); one kVa produces 1,000 watts.
Figuring out what sort of generator you’ll need first requires determining how much power your P.A. will consume (don’t confuse this with the audio output of a power amp or powered speaker). A visit to the manual or rear panel of your gear can provide a lot of information, and what you are looking for is “maximum power consumption.” This information can be listed in amps or watts; if you need to convert one to the other, use the formula (above right) in Fig. 1.
Power amps or powered speakers often show the power consumption spec at “1/8 power while running pink noise, all channels” which is a standard test condition indicating what the amp draws when playing music at levels just below clipping. When you’re driving the amp hard, allow approximately twice that value for power consumption. (See Fig. 2.)
Once you’ve determined the power requirements for each piece of gear (including powered speakers such as the QSC LA1121 in Fig. 3), add them to get the total power requirement of the system. Then add at least 25% for “headroom” and you’ll have an idea of how much generator power you need. A mid-size, ground-stacked P.A. consisting of six compact line array tops, four subs, six wedges, a digital console, a couple of laptops and some processing gear requires approximately 5,000 watts or 5 kVa. To be on the safe side, I’d spec a generator for at least 7.5 kVa. If there’s backline to be powered, adjust the requirements accordingly.
If anyone asks if they can connect their electrical equipment (lighting, deep frying machines, etc.) to the generator, respond with a polite but firm “no.” Motors or lighting gear (dimmers in particular) can cause noise in the audio system when they’re running from the same generator, not to mention the fact that they might cause an overload.
Is Your Generator Upside Down?
Portable generators fall into two broad categories: inverter and non-inverter. A non-inverter “construction grade” generator produces dirty power, meaning power with a high amount of total harmonic distortion (THD), and the line frequency can vary significantly from the standard (60 Hz in North America, 50 Hz in the UK and Europe). Computers and digital devices don’t like dirty power, and since almost every piece of gear used in a modern P.A. employs a microprocessor of some sort (don’t forget the DSP built into most powered speakers), it’s a smart move to stay away from non-inverter generators whenever possible.
An inverter generator (such as the Honda EU7000iS in Fig. 4) regulates frequency and stabilizes output power by converting AC to DC and then back to AC, resulting in a cleaner 60 Hz sine wave with THD typically under 6% — which is about as clean as shore power. Inverter generators tend to be more quiet than non-inverter types: 50 to 60 dB for an inverter generator versus 60 to 70 dB for a non-inverter generator of similar size. Keep in mind, however, that there is no standard for measuring the noise level of a portable generator, so YMMV.
The flip side of inverter generators is that they are more expensive than non-inverter types. Portable generators run either on diesel or gas, and one of the attributes of an inverter ‘gen is that they are usually more efficient than non-inverter types. But — and this is a big but — this efficiency is often achieved by running the generator in some sort of “eco mode,” whereby the engine idles down when demand for power is low. This can be a problem in live sound because transients (especially low-frequency transients) require a burst of power to be delivered, pronto. Running in eco mode can prevent generator “headroom” from being available when it’s needed by the P.A. system.
Regardless, it’s a good idea to use power conditioners and a UPS (uninterrupted power supply) for the most sensitive gear such as computer-based mixing systems, system processors and laptops.
Generator Specs
Two important specifications to look at when choosing a generator are running (or “rated”) power, and surge power. Running power is the amount of power that the unit can deliver continuously. Surge or peak power (sometimes called “starting power”) is the peak amount that the generator can deliver to accommodate a short-term surge in demand, usually a few seconds. Beware of generators that show specs only for peak power because that spec can be deceiving — running power will always be lower.
It’s important to respect the starting power by not firing up the entire P.A. system or amp rack all at once. Power amps draw a lot of power on startup, so stagger the starting process, firing up the biggest amps first and allowing 30 to 60 seconds between turning on each amp. While you’re doing this, have someone keep an eye on the generator’s meter for variations in voltage. It should remain stable as the load increases. If the voltage drops as the load increases, tweak the gen’s voltage adjuster accordingly.
It should go without saying, but here I am, saying it: generators should be used only outside in unenclosed areas, at least 20 feet from any other structures and… please point the exhaust port away from the stage! Never cover a generator with any kind of enclosure in an effort to reduce noise. And — though it can be done under certain circumstances — it’s a generally bad idea to run some of your gear from shore power while also running some of it from a generator. Ditto for using two generators simultaneously. Most generators can run 10 to 12 hours on a full tank of fuel, but some may reduce performance when the fuel level drops below a quarter-tank, so make sure to monitor the fuel level, and have extra fuel on site in case a refill is necessary.
Respect the Grounding Requirements
Smaller portable generators (under say… 10 kVa) use their metal frame as a ground, so it may not be necessary to drive a ground rod in situations where gear is connected directly to the AC receptacles on the generator using cord-and-plug (i.e., extension cables). This arrangement is not going to fly when using commercial-grade generators with a power distro to power a large-scale P.A. system. If you’re not familiar with the grounding requirements as well as the local electrical code, hire someone who is. And even for a small event, it’s worthwhile having a qualified electrician check the connections before you fire up the gear. Safety is always the first concern.
Steve “Woody” La Cerra is the tour manager and front of house engineer for Blue Öyster Cult.
