Working out required power capacity

[Jakester]

Right, apologies in advance, I am VERY crap when it comes to electrical engineering. 
 

I have been asked whether a portable battery with an “1800w output” will be sufficient to power a PA and bass and guitar amps for a gig. 

The spec suggests it has a 1024Wh capacity and 1800w output with a ‘surge’ capability of up to 2700w. 

 

The PA comprises 2 Yamaha DXR12s and a DXR10 monitor. We might also use a Stagepas200 as a monitor too. 
 

My amp is a CMD121P and guitar amp is  50w valve amp. 

The Yamaha specs suggest a power draw of 110w per speaker. I can’t find equivalents for the bass or guitar amps. There’s a digital mixer too. 
 

My instinctive reaction is it isn’t going to be enough for 2h of performance at sensible (I.e not loud) levels but I don’t know how to work it out - can anyone assist?

[itu]

I am doing several roundings, and without testing the set my results are only guesstimations. My text is long to clarify details - and if there are grave mistakes, anyone can fix them. 

 

I checked one battery, named as Fossibot. It just came up, but the important thing is to understand the specs. Yours claims 1 kWh capacity, and 1.8 kW max output. Don't care about that 2.7 kW, that is probably marketing. We are not welding anything.

 

PA consumes something like 400 W in total at full speed + monitor.

Markbass is 300 W, and therefore consumes maybe 500 W max.

A 50 W valve amp needs 150 W.

(Hopefully no one will connect lights to the system!)

 

The numbers should be rounded upwards: this system can draw up to 1500 W. Your battery claims to be able to push 1.8 kW, so we are just on the safe side.

 

An audio system rarely draws this much energy for a long time. Usually the peaks are measured in milliseconds (ms). Overall power consumption could be in the ballpark of 20 - 30 % of the total power. This would mean some 300 - 500 W continuous.

 

lower power: 1 kWh capacity should be divided with 300 W continuous, and round the result downwards: 1 kWh / 0.3 kW = 3 hours of continuous operation.

 

higher power: Divide the capacity with 500 W continuous, and round the result downwards: 1 kWh / 0.5 kW = 1.5 - 2 hours of continuous operation.

 

This is because the claimed battery capacity is not linear, and is less than its maximum at the end of its drain cycle. If there's some extra power drawn, the capacity is used much faster. An outdoor gig will draw far more energy than a low volume pub gig. I would try the unit at the rehearsal to get an idea of the capacity, and the unit's functionality if there are possible noises, or any artefacts. Shorter cables are better, position the power bank in between you all.

 

To repeat myself, my guesstimations are only ballpark figures, and have to be checked by a pro. @agedhorse, or @Bill Fitzmaurice, please comment.

[agedhorse]

First of all, kWh is a nebulous term because there are some assumptions about discharge rates and depth of discharge that are hard to get accurate data on from marketing brochures.

 

1. Assume that whatever kWh that is advertised is total capacity, only 80% should be used under most situations. This leaves you with 0.8kWh (800Wh)

 

2. The power consumption numbers on the PA cabinets are at 1/8 rated maximum output with is similar to driving your PA UNDISTORTED to maximum output.

 

3. Your MarkBass amp's 300 watts power consumption is calculated (almost always) based on the same or similar metrics, so that number is a good one also. (In reality, it may be quite a bit less, which model is it?)

 

4. The digital mixer will be right around 100 watts (assuming it's not a "bigger boy" console)

 

The calculations would be:

 

PA cabinets: 3 x 110 = 330 watts

Bass amp: 1 x 300 watts = 300 watts

Digital mixer: 1 x 100 watts = 100 watts

Tube gtr amp: 1 x 150 watts = 150 watts

 

Total = 880 watts which will last about an hour in your application.

 

The maximum power draw of 1800 watts isn't going to be as much of an issue here.

 

 

 

[Jakester]

Thanks, that’s all really useful. 

[BigRedX]

Make sure your portable power supply produces a "pure sine wave" AC output. This will be almost essential for getting noise-free performance from your amplification. From what I have read most budget systems are not suitable in this regard. There was another thread on here recently about this and several over on the SOS forums and the general consensus amongst those with more electrical knowledge than me and also from actual experience was that without pure sine wave AC you will run into problems, either with obvious unwanted noise or some equipment not even powering up.