So – you’re being asked to handle a gig, and the event organizer wants to know a couple of things. First, do you need a generator? Second, how big does the generator have to be?

I always feel that I need a generator if the power situation gives me any pause at all. There’s one outlet available from a building we don’t have access to? That needs a generator. We’ve got power available, but every other vendor at the event might be hooking into it? That needs a generator. We have to run more than 100 feet of cable from regular-ol’ outlets to even reach the stage? That needs a generator. The event organizer is offering to rent a generator because of “prior experiences?” That needs a generator.

So, needing that generator for reasons including, but not limited to, anything or everything above – how much capacity is required? There’s a relatively easy way to get a good number on that front.

On the audio amplification side, tally up all your peak (not continuous – peak) draws. For powered speakers, you can do this by taking the manufacturers peak-power claim at face value. If they tell you it’s a 2000 watt loudspeaker, call it 2000 watts (within limits, because that little HF driver isn’t going to have a 700 watt peak applied to it). For passive speakers, the manufacturer’s claim for the peak wattage at the nominal load you’ve hooked up is the number you want.

Got all that? Great. Now multiply by 1.1 to account for life not being 100% efficient. This is the power needed for your audio output, which is variable over time. After that, total up your other power draws, like mixing consoles and processing. Those units require a fixed amount of power at all times, so you don’t need to account for momentary spikes of demand. For lights, use the highest power-draw number you can find. (For instance, an LED lamp might pull 100 watts, but if you’ve got the lamp on full AND are panning and tilting like crazy you’ll use more power. The motors require energy too, right?)

Here’s the “why” of my being so focused on peaks and other “highest case” draws. When you’re running on a generator, the capacity of the unit is all you have to handle everything that can possibly happen. This is in contrast to when you’re on the municipal grid. When connected to the grid, there is generally a huge power delivery capacity for momentary draw. If you try to pull 10,000+ watts all of a sudden, that’s likely not much trouble for the “megawatts to spare” municipal supply. With a generator, though, there’s no extra capacity. If the unit has a maximum load of 5000 watts, trying to pull 10,000 is futile. The power doesn’t exist. As such, the safe thing to do is to take a good stab at figuring your highest possible momentary draw, and ensuring that instantaneous load can be handled.

By way of example, let’s take a system that I was involved in deploying recently. There were two, double 18 subs that could draw 2000 watts each, plus two tops handling the same amount of power. Add three monitor wedges with 2000 watt peak ratings, and you get 14 kW. Multiply by 1.1 and you have 15.4 kW for loudspeakers. There was a bass amp that could probably deliver a 1kW peak, and a 100 watt guitar amp, so that gets us to 16.5 kW. There were also two mixing consoles needing 120 watts each, and a processing rack that consumed about 25 watts. At this point, we need 16.8 kW. Add about 500 watts of LED lighting, and we’ve reached 17.3 kilowatts.

What the event gave us was a 20 kW diesel generator, which was perfect. The peak power available was in excess of what we needed, and our continuous draw (per the unit’s ammeter) was about 10 amps. We had no power problems with the generator itself, running as loudly as we pleased for several hours.

The conclusion here is, there’s no need to finagle and guess what you can get away with. Instead, spec comfortably and then work with confidence. It’s much better.