Sizes a central emergency lighting battery system to BS 5266-1 / EN 50171 from a
luminaire/circuit load schedule: required system rating, stored battery energy, a recommended
standard battery block, and an optional candidate-battery verdict. Self-contained luminaires get a
simple rated-duration check instead.
Tender-stage only. Verify final design against BS 5266-1/EN 50171 and manufacturer guidance.
BS 5266-1 SELF-CONTAINED: luminaire rated duration checked against the required duration (3 h standard; 1 h only where evacuation is immediate and the premises are not re-occupied until batteries are fully recharged). For design guidance only. Always verify with a qualified engineer.
About this emergency lighting battery calculator
This free emergency lighting battery calculator sizes a central battery system to BS 5266-1 / EN 50171 from a luminaire or circuit load schedule, the required system rating, the stored battery energy, a recommended standard battery block and an optional candidate-battery verdict, or performs a simple duration check on self-contained luminaires. It is aimed at UK electrical and fire-safety engineers, designers and contractors specifying emergency lighting at tender stage. Build the schedule row by row and switch between central and self-contained modes. Everything runs in your browser. Nothing is uploaded.
How the battery sizing is calculated
For a central battery system (CBS/CPSS) the required system rating is total load × 1.2 (the EN 50171 120% load-capability requirement), and the required stored energy is (total load ÷ inverter efficiency) × duration × ageing factor, with the default 1.25 ageing factor giving a 25% end-of-life margin. Given a DC bus voltage, capacity is energy ÷ voltage in Ah, rounded up to the nearest standard 10-year-design-life battery block, and an optional candidate battery is checked against it. Self-contained luminaires instead get a straight BS 5266-1 duration check: the luminaire's rated duration against the required duration (3 h standard; 1 h only where evacuation is immediate and the premises are not re-occupied until batteries are fully recharged).
Frequently asked questions
Should the duration be one hour or three?
BS 5266-1 takes 3 hours as standard, for sleeping risk, phased or entertainment premises, or where the building is re-occupied before batteries recharge. One hour applies only where evacuation is immediate and the premises are not re-occupied until the batteries are fully recharged.
What are the 1.2 and 1.25 factors?
The 1.2 factor is the EN 50171 requirement that a central system supply 120% of the design load. The 1.25 factor is a 25% end-of-life battery design margin, so the battery still meets duration near the end of its service life. Both are editable.
Do I need to enter the DC bus voltage?
Only to convert the required energy (Wh) into a battery capacity (Ah) and pick a standard block. Without it you still get the total load, the required system rating and the stored energy, enough to specify the inverter and confirm the schedule.