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[Frank Koenig]

What Art says, and a note on the Kill-A-Watt meter, which I highly recommend. The Kill-a-Watt has a kWh mode where it will integrate power over as long as you want to get energy. If you run audio program through the system as Art describes and measure the total energy over, say, 30 minutes and divide that by the time you'll get a very good estimate of the average power, which you can use to size the battery. Assume an inverter efficiency of 80% and 80% depth-of-discharge (DOD) for Li batteries and you'll be pretty close. For Pb-acids use 50% DOD.

While you're there, use the Kill-a-Watt to measure the power factor. Divide the average power from above by the power factor to get the average apparent power (in Volt Amperes). This is the number you need to size an inverter or generator. Modern inverters can sustain short-duration peak loads greater than their continuous rating (much like modern power amplifiers) but, even so, I'd use an inverter with a continuous rating at least double the average apparent power. Others with more experience running audio off of inverters near the limits might have some better guidance on this factor.

--Frank

[Russell Ault]

{...} MQ power and their ilk would do well to be developing trailer-mounted 50 - 200 kWh battery-inverter units that could power good-sized outdoor events. {...}

That makes sense to me, especially since the weight-and-size concerns of designing in significant excess capacity (thus allowing access to the longevity benefits of under-charging) start to become a lot less relevant (although of course capital cost will still be an issue).

A large, portable battery-inverter would also open some interesting avenues for shows in places where peak power demand is greater than the available supply, but average power demand isn't. This is especially true in audio, where a lot of our power needs tend to be dictated by the peak demand of highly-peaky loads. Imagine: load-in is at 08:00, and the first thing you do is plug your battery bank into a normal 15 amp circuit. For every hour it's plugged in with no load, an extra ~1.5kW will be available to the system on-demand. Scale that up and perhaps the "stove plug" rig can now be run off a single 20-amp, or maybe even a "three-phase" rig that can be accommodated on a just stove plug. It won't help the sparkies quite as much (and the videots will still need all the power they can get), but for audio-related loads I feel like there might be real potential to significantly reduce venue power requirements by evening out the peaks. Plus, now your whole system is on a UPS!

-Russ

[Brian Jojade]

Part of the challenge in determining the current draw needs and how long the battery will actually last is that speakers will draw power dynamically based on how loud it is, as well as how dynamic the material is you are reproducing.  While you may be able to get a rough idea, you definitely want to make sure you don't cut things too close or you'll be in for a surprise mid show.

If you want to measure the actual max draw to expect, send pink noise to the speaker and measure it with your killawatt.  Using pink noise will give you an accurate reading that simulates typical music, but is consistent instead of varying depending on the song.  Set the speakers to just where the clip lights are starting to flicker and that should give you a good measurement that you can take in just a few seconds.  Typically, you can expect this to be about 1/8 power.  Eg, if you have an 800 watt amp, this should be about 100 watts or so. (not factoring in amp efficiency or any of that stuff) 

Next would be to see what heavy clipping looks like.  Turn it so that the clip lights are on solid.  Typically this represents 1/3 power.  Unless you're a DJ that likes stuff sounding like crap, you'll probably never drive things this hard, but it gives you a likely worst case scenario power draw.  That same 800 watt source now should show around 266 watts of draw.

If you want to get really crazy, you can send a 1khz sinewave to the speaker and turn it up until you see the clip light.  This should get you close to actual full current draw.  The speaker will probably shut down after a bit because they aren't designed to do that, but you can see what absolute worst case scenario might be.

Whatever you come up with, it's a good idea to calculate that you need the system to run for at least twice your expected actual runtime. That gives you plenty of buffer for things like delays in the show, battery wear, random toasters plugged into your system, etc.

[John Lackner]

There are a lot of these lithium battery/inverter/charge controller boxes popping up on places like Ebay. Some of them allow you to plug in solar panels (12 volt)  to extend the range. Two speakers on sticks, a small mixer ,and a laptop should last a long time. Get one with the biggest battery you can afford.  If you start working with a full band with Marshall stacks and SVT bass amps, multiple monitor mixes, things could get problematic.
I have a battery/solar power system with two deep cycle batteries, Morningstar charge controller, and a 24 volt 1100 watt Exceltech inverter, and four 165 watt solar panels. I can run a PA with four powered speakers (class D) all day, even when it's cloudy. But these new lithium power boxes can do the same thing at a fraction of the size and weight.

[MattLeonard]

A large, portable battery-inverter would also open some interesting avenues for shows in places where peak power demand is greater than the available supply, but average power demand isn't. This is especially true in audio, where a lot of our power needs tend to be dictated by the peak demand of highly-peaky loads....

-Russ

This is basically a feature in some higher-end DC->AC inverters. Victron has a "power assist" feature - which basically lets you configure their inverter as a pass-through for shore/gennie power -  but if demand is higher than supply - the battery/inverter steps in to supplement. They also act as a super-fast (~20ms) transfer switch - so if shore power goes down, the battery acts as an automatic backup. This is fast enough that even computers notice a disruption.

A few years back I PM'd an outdoor political rally where we really wanted to run on solar. I had a vendor who could supply 240v/50a with a solar trailer - with something like 12kwh of batteries and 4kw of incoming solar potential. But the sound company (bringing 8 tops/4 subs of powered line-array boxes VRX if I recall) was adamant they needed 3ph/100a service. Not due to their wiring/distros - but because of peak power calculations. We convinced them to work with the solar rig -  but they had me sign on the contract that I wouldn't hold them liable for shortcomings or if they had to keep volume down, kill boxes etc.

Measuring 100dbSPL at FOH (with a DJ), we were averaging something like 3,500 watts off the solar rig across both legs -  with occasional peaks of 5kw. Most sound companies greatly, greatly over-estimate/spec what their power needs actually are.

[Steve-White]

..............Measuring 100dbSPL at FOH (with a DJ), we were averaging something like 3,500 watts off the solar rig across both legs -  with occasional peaks of 5kw. Most sound companies greatly, greatly over-estimate/spec what their power needs actually are.

That's a fact Jack.  Always overstate power demand requirements.  Too many variables to list can go wrong and margin is a good mitigation factor.  Turning down the sound and pulling 1/2 the lighting out isn't particularly well received by client, their customers/guests or the entertainers.

[Chris Hindle]

Measuring 100dbSPL at FOH (with a DJ), we were averaging something like 3,500 watts off the solar rig across both legs -  with occasional peaks of 5kw. Most sound companies greatly, greatly over-estimate/spec what their power needs actually are.

Never spec'd 45Kw and had 25kw delivered because "It was all they had" or "It was the biggest we could afford" or "It's what the last show used" ?
Always order twice what you need, and even if they skimp, you should make out OK.
Not my first Rodeo....
Chris.

[Tim McCulloch]

This is basically a feature in some higher-end DC->AC inverters. Victron has a "power assist" feature - which basically lets you configure their inverter as a pass-through for shore/gennie power -  but if demand is higher than supply - the battery/inverter steps in to supplement. They also act as a super-fast (~20ms) transfer switch - so if shore power goes down, the battery acts as an automatic backup. This is fast enough that even computers notice a disruption.

A few years back I PM'd an outdoor political rally where we really wanted to run on solar. I had a vendor who could supply 240v/50a with a solar trailer - with something like 12kwh of batteries and 4kw of incoming solar potential. But the sound company (bringing 8 tops/4 subs of powered line-array boxes VRX if I recall) was adamant they needed 3ph/100a service. Not due to their wiring/distros - but because of peak power calculations. We convinced them to work with the solar rig -  but they had me sign on the contract that I wouldn't hold them liable for shortcomings or if they had to keep volume down, kill boxes etc.

Measuring 100dbSPL at FOH (with a DJ), we were averaging something like 3,500 watts off the solar rig across both legs -  with occasional peaks of 5kw. Most sound companies greatly, greatly over-estimate/spec what their power needs actually are.

As one of the sound companies that routinely over-spec generators, I can say we never have electricity problems because of not enough genny.  Running close to the edge may be acceptable for some, but my clients will not tolerate or pay for a failed gig.  YMMV.

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