Free lighting tool
Central battery sizing calculator (BS EN 50171)
This calculator sizes the battery for a central battery emergency lighting system to BS 5266-1. Enter each luminaire type with its quantity and emergency-mode wattage, choose the DC system voltage and a 1-hour or 3-hour duration, and it returns the total connected load, the system current and the battery capacity in ampere-hours with a 1.25 ageing factor applied.
By Nicholas RedicanLast reviewed
What is a central battery emergency lighting system?
A central battery system supplies a building's emergency luminaires from one battery bank and charger in a protected enclosure, instead of a battery inside every fitting. When the mains fails, the central unit feeds the escape-route, open-area and exit-sign luminaires through fire-resistant cabling for the rated duration, usually three hours in the UK.
BS EN 50171 is the product standard for central power supply systems (CPS) and the smaller low-power systems (LPS); BS 5266-1 governs how they are applied: duration, location, wiring, monitoring and testing. Sizing the battery comes down to the total emergency load, the system voltage, the duration and an allowance for ageing.
How to use the central battery calculator
- 01Choose the system voltage: 24 V, 50 V, 110 V or 230 V DC. Larger buildings normally use 110 V or 230 V to keep current and cable sizes down.
- 02Choose the rated duration: 1 hour or 3 hours. Three hours is the usual UK requirement under BS 5266-1 and is the default.
- 03Add a row for each luminaire type with a description, the quantity and the wattage each draws in emergency mode, taken from the manufacturer's data. The totals update as you type.
- 04Press Calculate battery capacity. The schedule is sent to NR Lighting's servers for the calculation and is not published. The cards show total load in W, system current in A and battery capacity in Ah, with the duration and the 1.25 ageing factor listed underneath.
- 05Above 200 Ah the tool warns you to consider more than one battery unit or a higher system voltage.
- 06Pro subscribers can label rows by circuit for a per-circuit breakdown, add project details, apply N+1 redundancy (which doubles the capacity) and export a PDF report.
The method: from watts to ampere-hours
Battery capacity (Ah) = (Total load W ÷ System voltage V) × Duration h × 1.25 ageing factor
The tool sums quantity × watts for every row, divides by the nominal DC voltage to get the discharge current, multiplies by the duration to get the ampere-hours drawn, then multiplies by 1.25. That factor specifies the battery at 125 % of the calculated demand, so a battery that has aged to 80 % of its rated capacity still delivers the full duration. Manufacturers apply the same allowance for BS EN 50171 systems, which must support the rated load for the rated duration throughout the battery's design life.
Duration is fixed by BS 5266-1 according to how the building is used.
| Situation | Minimum duration |
|---|---|
| Evacuated immediately on supply failure and not reoccupied until the batteries have recharged | 1 hour |
| Sleeping accommodation: hotels, hospitals, care homes | 3 hours |
| Not evacuated immediately, or reoccupied as soon as the supply returns: most shops, offices, restaurants and places of assembly | 3 hours |
Worked example: a 120-bedroom hotel on a 110 V DC central battery
A hotel has sleeping accommodation, so the duration is 3 hours, and the M&E engineer has chosen 110 V DC. Emergency-mode wattages come from the luminaire data sheets.
| Luminaire | Qty | W each | Total |
|---|---|---|---|
| Exit signs | 40 | 3.0 | 120 W |
| Corridor and stair downlights (emergency mode) | 80 | 6.0 | 480 W |
| Back-of-house and plant room luminaires | 28 | 8.0 | 224 W |
| Bulkheads at final exits | 14 | 4.0 | 56 W |
| Total connected load | 880 W |
Reading the result
System current = 880 W ÷ 110 V = 8.00 A. Battery capacity = 8.00 A × 3 h × 1.25 = 30.0 Ah. The tool reports 880 W, 8.0 A and 30 Ah with no warning; N+1 in Pro doubles it to 60 Ah. At 24 V the same load would draw 36.7 A and need 137.5 Ah, with far heavier cabling and greater sensitivity to voltage drop, which is why larger loads move to higher voltages.
From the 30 Ah figure the manufacturer selects a string rated at the 3-hour discharge rate and the correct end voltage, normally a larger nominal Ah than the raw calculation.
Central battery vs self-contained emergency luminaires
Self-contained luminaires carry their own battery, charger and test circuit; a central battery system moves all of that to one cabinet.
| Central battery | Self-contained | |
|---|---|---|
| Battery life | Ten-year design life for a CPS to BS EN 50171 | Typically four years, replaced luminaire by luminaire |
| Luminaire choice | Standard mains luminaires can act as slaves, keeping the ceiling consistent | Emergency versions or conversion kits only |
| Wiring | Fire-resistant cabling to every emergency luminaire | Ordinary final circuit wiring |
| Testing | One point of test, monitored at the cabinet | Each fitting tested and its battery changed individually, often at height |
| Cost | Higher capital cost; lower whole-life cost on large or high-ceiling schemes | Lower capital cost; suits small or dispersed installations |
Limitations, and when to involve a lighting designer
The tool gives indicative design values for early sizing and for checking a manufacturer's proposal; it is not compliance sign-off. It applies a single 1.25 ageing factor and nothing else: no temperature correction, no inverter or converter losses on AC-output systems, no discharge-rate or end-voltage correction and no voltage-drop check on the final circuits. Battery data sheets rate capacity at a 10-hour or 20-hour discharge; at 1 or 3 hours the same block delivers less, so the manufacturer's selection will differ from the figure here. Enter emergency-mode wattages, not mains wattages: many LED luminaires run at reduced output on a DC supply.
In my own work the decision to go central battery is usually architectural: a hotel corridor or showroom ceiling where the emergency luminaire should be the same downlight as everything else. That decision belongs at developed design, with cable routes, the unit's location and ventilation, monitoring and the BS 5266-1 test regime agreed with the M&E engineer and the fire strategy. At that point I can carry out the BS EN 1838 illuminance calculations and produce the emergency luminaire schedule and system selection the electrical designer needs.
Standards and guidance referenced
- BS 5266-1: code of practice for the emergency lighting of premises, including rated duration, system selection, wiring and testing
- BS EN 1838: illuminance requirements for escape routes, open areas and high-risk task areas
- BS EN 50171: central power supply systems for emergency lighting, including battery design life, recharge time and load rating
- SLL Lighting Guide 12 (LG12): emergency lighting design guide
Frequently asked questions
How do I size a central battery for emergency lighting?
Add up the emergency-mode wattage of every luminaire, divide by the nominal DC voltage to get the current, multiply by the rated duration (usually 3 hours) for ampere-hours, then multiply by a 1.25 ageing factor. For 880 W at 110 V over 3 hours: 8 A × 3 h × 1.25 = 30 Ah. The manufacturer then selects blocks rated at that discharge rate.
Should the duration be 1 hour or 3 hours?
Three hours in almost every UK building. BS 5266-1 allows 1 hour only where premises are evacuated immediately on supply failure and not reoccupied until the batteries have recharged. Sleeping accommodation, licensed premises, places of assembly and any building reoccupied as soon as the supply returns need 3 hours, so the calculator defaults to it.
What is the ageing factor and why is it 1.25?
Batteries lose capacity through their service life. BS EN 50171 requires a central battery system to deliver its rated load for the rated duration throughout the battery's design life, so the battery is specified at 125 % of the calculated demand: at 80 % of its nameplate capacity it still meets the full duration. The calculator applies this factor automatically.
Central battery or self-contained emergency luminaires: which should I choose?
Self-contained luminaires suit small, dispersed or low-ceiling installations where capital cost matters and four-yearly battery changes are manageable. A central battery suits larger buildings, high ceilings and architecturally sensitive spaces where mains luminaires should double as emergency slaves, and where single-point testing and a ten-year battery life offset the cost of fire-resistant cabling and an enclosure.
What does BS EN 50171 require of a central battery system?
BS EN 50171 covers the central power supply unit itself: a battery design life of ten years at 20 °C for a CPS (five for a low-power LPS), the ability to supply the rated load for the rated duration through that life, recharge to 80 % of capacity within 12 hours, deep-discharge protection, and charger and battery monitoring. BS 5266-1 governs how the system is applied in the building.