Warning - long post and not my info,, am just relating it here -

Battery drain is a complicated subject, but here goes, for keyboard busking.

There's a few concepts to get your head around, and then I'll tie it all together with some equations that you'll be able to use to calculate battery sizes.

First up, peak-to-average ratios (aka duty cycle) and how they apply to music.
Peak-to-average ratios describe the ratio between the average power (which is related to how much is being pulled from your battery) and the peak power (which is when your amp clips).
Some examples:
- square waves have a 0dB peak-to-average. All of the signal is at the peak voltage.
- sine waves have a 3dB peak-to-average. If a 100w amplifier is just clipping, the speaker is receiving 50w of continuous power.
- pink noise has a 9dB peak-to-average. That's why PA amplifiers are often rated for power draw at 1/8th power - that's what they'll pull from the wall when they're just clipping pink noise into the rated load.

It's important to note here that the RMS power rating of a driver is found with a pink noise signal that's clipped to 6dB peak-to-average. So a 1000w rated driver will be at its thermal limits with a 1000w amp playing that pink noise, or a 500w amp playing sine waves, or a 250w amp playing square waves. The heating power for all of those is the same, but the peak voltages are different.

Now, someone high up in some organisation decided that pink noise driven just to clipping would be representative of real-world use. Generally speaking, it's not far off. However, we're on diy Audio and we're going to do things properly.
Here's an example piece of music (caution - NSFW lyrics from the start): https://www.youtube.com/watch?v=UnQY3swM4tA
I've chosen this because it's very demanding of subwoofers. When the bass drops, it's a 36Hz compressed sine wave, so the peak to average ratio is less than 3dB below 100Hz. That's evil. The 1000w rated driver earlier would burn if a 500w amp was playing the LF signals from that track at clipping.
This has a knock-on effect for our power draw, which we'll look at soon.
Note that the mid-high range above 100Hz or so would have an nice easy time of things.

Here's a different example track: https://www.youtube.com/watch?v=h0ffIJ7ZO4U
There's not much bass content, and what is there is very peaky (kick drum). The mid-high range is pretty busy in comparison, though.

So, the music you're going to play will have a serious influence on what your battery life will be.
Lets work the examples. We'll take a 100w low-frequency amplifier, and a pair of 50w mid-high, and were going to run at clipping.
The dubstep track above will have around 50w average power from the LF amp (3dB peak-to-average), and probably more like 1w/ch average power (17dB peak-to-average, which is an educated guess) from the mid-high amps. Total average output around 52w.
The other track above will have much lower average power for the bass, but higher for the mid-high range. Lets say it's 10dB peak-to-average for both. The LF amp will be putting out 10w average power, and the HF amps will be doing 5w each. Total average power: 20w.

For most music, good working figure would be 10dB peak-to-average overall. If run-time is critical, bet on a 6dB peak-to-average overall, and you're likely to have some juice left.
If you're going to be playing dustep/EDM, bet on 3dB peak-to-average for the LF amp, and 10dB for the mid-highs.

Remember that speaker sensitivity comes into play - if you gain 3dB in sensitivity, you've halved your power requirements. That's a Very Good Thing. Battery size is usually proportional to the number of watt-hours it'll hold, so if you halve your power requirements, chances are you can cut the battery size in two.


Next up, amplifiers
There's two components to calculating an amplifier's power draw:
- idle draw
- conversion efficiency - how much power going in comes out the other end

This is a little complicated, since the conversion efficiency has the idle draw built-in, and also the efficiency of switch-mode amplifiers (which have a lot of advantages that make them perfect for battery systems. I'm just going to assume you're using one, because you should be) is not linear with output power.
At high power, switching amps will approach 90% efficiency. At lower power, that can go below 50%.
Now, while conversion efficiency ratings will have the idle draw built-in, I usually pretend it doesn't. It makes the calculations easier, resulting in a slightly bigger battery. No big deal. Otherwise, you've have to work out how long music is playing for, how long it's not, etc, resulting in lots of "bits" of equations that you'll have to string together with a stopwatch.

So, you'll need to either do some measurements on your head unit, or take some data from the datasheet.
I'd bet on 75% working efficiency on a class D amp, which is an average over the useful power region.

Power draw = idle power + (power output/efficiency)
You can convert between volts, amps, etc, later.

SPLs and power

As I alluded to earlier, 3dB is a doubling of amplifier power. If I have a speaker running on a 10w amp, and I change out to a 20w amp, I'll gain 3dB of volume, and also increase the draw from the battery according to the equation just above.
10dB is a factor of 10 in amplifier power. +10dB sounds twice as loud.
There's a little misinformation online regarding decibels, so here's the run-down:
1dB difference is noticeable to most listeners.
3dB is obvious
6dB sounds half as loud again
10dB sounds twice as loud

SPL falls off at 6dB per doubling of distance, 20dB for a factor of 10. Most SPLs are calculated at 1m.

Portable speakers often use large PA speakers with tiny amplifiers and batteries. PA speakers often have high sensitivity, so require smaller power inputs for a given SPL.
For example, a 15" 95dB@1w speaker needs 10w to achieve 105dB. A 5" 85dB@1w HiFi speaker will need 100w. The PA speaker will be loafing along comfortably, while the smaller HiFi speaker might well be getting to the end of its rope.

Finally, batteries and electronics

There are lots of different sorts of batteries out there, but they all have something in common - they do not like to be shorted.
As soon as your battery arrives, connect a fuse holder to one side, and then insulate that terminal. Seriously, just do it.
I've seen a single C-cell NiCd battery go off, and it went off like a stun grenade. NiCd isn't particularly energy-dense, either. I wouldn't want to see anything lithium-based, or anything big and lead-acid go off.
Note that if you drop a spanner across a car battery, it will weld to the terminals and the spanner will burn you if you touch it. This isn't some hollow warning, this is serious. You're looking at major injuries if someone happens to drop something that connects the wrong terminals together and the battery explodes, so get a fuse on there and make sure the battery is protected.

Battery storage (in watt-hours) = operating voltage x rating in amp-hours.


Let's do some equations

I'm going to do the maths for an imaginary system so you can do your own maths on yours. I'll happily sanity-check your work, but I'm not going to do it for you. What I've written here should be enough to help you calculate your own system.

I'm going to work out how much power/speaker sensitivity I'll need for a given SPL target. If you've already got your speakers and amplifiers, you can skip ahead.

So, here's the run-down of numbers:
- Target: 90dB at 10m peak SPL from all bands
- I'm going to be playing dubstep and some chart stuff
- Amplifier idle draw will be 20w per channel (you need to measure yours).
- I've got a bluetooth module and some other input electronics that run up 10w total.
- I'm going to call the amplifier at 75% efficiency.
- I want to run at full power for 10 hours.

I can work out that 90dB at 10m means I need 110dB at 1m.
I fire up the simulation software, and find my subwoofer has a sensitivity of 87dB at 1w, so it needs 200w peak power to get there. The mid-high range have 93dB@1w each. You'll get some combining at the lower midrange, but if anything in the music is hard panned left or right, then all the power needs to be delivered to that speaker. Each one, therefore, will need 50w.
My music choice gives me a 3dB peak-to-average ratio in the bass, and I'm going to play it safe and say 6dB peak-to-average ratio for the rest of the range.

Note that all of these decisions lead to an over-built system. There are places you can compromise. It's up to you to work out where you can safely compromise.

So, we can work out the power draw for each range:
Bass: 20+(100/0.75) = 153w. The 100w is taken from the 200w peak power and the peak-to-average ratio.
Mid-high: 2x(20+(12.5/0.75)) = 73w.
Total amplifier power draw: 226w

Add in the 10w total for bluetooth, crossover, whatever, and you've got 236w draw when music is playing.

I'm going to run a 24v system, since the higher voltage often plays nicer with kit amplifiers (the higher rail voltage means they won't need voltage converters to get high power output). You could run 12v, but I want to run 24v.

So, 236w average draw on a 24v system. That's about 10A continuous. I want 10 hours of that, so that's a 100Ah battery. It's gonna be big!
As I mentioned earlier, though, there's a lot of over-building in there. This system I made up will definitely run for 10 hours of continuous dubstep at 90dB at 10m.

Now you can work out your system!

Just pick up an accordion ... arm powered