Standby batteries · Series, part 6

A 1200 W voice alarm's battery is sized by its quietest 24 hours

We ran a 1200 W, eight-zone system through the British Standard BS 5839-8 Annex C method. Tripling the amplifier power moved the answer less than doubling the standby current.

By Nigel Williams · Published 16 October 2026

Think of a security guard on a twelve-hour night shift. Most of the shift is spent sitting in the gatehouse. A few minutes, if any, are spent running.

Pack food only for the running and the guard is hungry by three in the morning. Voice alarm batteries have the same problem, and the numbers below show how big it is.

What the battery has to do

The standby battery behind a Public Address Voice Alarm (PAVA) system is sized for two jobs in a row. First, 24 hours of keeping the system running with the mains off (6 hours where an automatically started standby generator serves the system, Clause 25.3.5 c)). Then, at the end of those 24 hours, at least 30 minutes of evacuation messages in every alarm zone (BS 5839-8:2023, Clause 25.3.5 a)). Where evacuation is likely to take longer than 20 minutes, the 30 minutes is extended by an agreed margin (Clause 25.3.5 b)).

That is 24 hours against 30 minutes: the quiet period is 48 times longer than the loud one. The calculation method is in Annex C of the standard.

The system we ran

We used Level 1 of the ProAudium Battery Calculator, which implements the Annex C method. We ran the calculator's own code, unchanged, on 3 October 2026. These are the inputs:

And the result, with capacities in ampere-hours (Ah):

Where the ampere-hours go

Look at the two shares. The 24 hours of standby takes 14.16 Ah. The 30 minutes of evacuation messages, on 1200 W of amplifiers, takes 8.01 Ah. Nearly two thirds of the battery is there for the quiet day.

Two things keep the alarm share small. Speech is mostly gaps between syllables, so the 2023 edition assumes an announcement draws one eighth of full amplifier current (Annex C, C.2.1). And 30 minutes is a short time against 24 hours.

That is why a battery sized from the amplifier wattage alone is commonly overstated on the alarm side. A quick sum at full amplifier current, 62.5 A here, puts eight times the speech current into the evacuation half hour.

The test: change one thing at a time

One result could be a quirk of the example. So we ran the same building three more ways, changing one input each time.

Read it the other way round. Going from 400 W to 1200 W, three times the amplifier power, adds 29 % to the battery. Doubling the standby current adds 67 %. Every extra 0.1 A of standby current costs about 3 Ah.

For completeness: the same 1200 W system on older Class A/B amplifiers (50 % efficient) needs 33.2 Ah, and on a 48 V supply it needs 23.1 Ah per battery in the string.

The number nobody checks

That makes the quiescent current the most important figure in the calculation. It is also the one most often guessed.

Our Level 1 figure of 0.59 A is an estimate. It is ProAudium's own model, not yet checked against a survey of real systems, and the calculator says so beside the result. A real control panel, with its network switches, routers and monitoring, can draw more or less. The fix is to use the manufacturer's quiescent current for your equipment, or a measured one, at Level 2 or Level 3 of the calculator.

What a real panel draws

Installed eight-zone panels rarely draw as little as 0.59 A in standby once the network equipment, monitoring and every amplifier channel are counted. So we ran the same 1200 W system at three higher standby currents, changing nothing else:

At 1.7 A the minimum is already 62.5 Ah, and the next standard size up is 65 Ah. That is the size most installed systems of this scale carry, and the arithmetic shows why: it is the standby current, not the amplifier wattage, that puts it there. A small 12 V block of the kind used in fire alarm panels is nowhere near enough for a voice alarm system of this size.

So the two errors run in opposite directions. Sizing on full amplifier power overstates the alarm share. Guessing the standby current understates the larger share. A measured standby current fixes both.

"Surely the amplifiers are what drain a battery?"

At full power, they are: 62.5 A in this example, against 0.59 A in standby. But full power is not what the standard asks the battery to supply. It asks for speech, which averages one eighth of full current, for 30 minutes. Standby asks for a small current for 24 hours, and small times long beats large times short.

There is a knock-on effect. A bigger battery takes longer to recharge, and the code recommends that a 24 h charge restores the capacity Clause 25.3.5 calls for (Clause 25.3.4). Levels 2 and 3 of the calculator run that check when you enter the charger's output current.

In short

• The battery covers 24 hours of standby, then at least 30 minutes of evacuation messages (Clause 25.3.5 a)).
• For a 1200 W, eight-zone system with our Level 1 assumptions, Annex C gives 27.7 Ah, and the standby share is nearly two thirds of it.
• Tripling the amplifier load added 29 %. Doubling the standby current added 67 %, and at 1.7 A the minimum reaches 62.5 Ah.
• So the quiescent current deserves a real figure, not a guess.

The guard does not get hungry running. The guard gets hungry waiting.

Next step. Find the quiescent current for your control equipment on the manufacturer's data sheet, or measure it, then run your system at Level 2 of the ProAudium Battery Calculator: proaudium.com/battery-calculator. No email, no form. If you would like the calculation checked against what is installed, contactus@proaudium.com or 01273 034630.