Central battery sizing: energy estimate and discharge-rate checks
A central battery system supplies emergency lighting when the normal source fails. The luminaire load, conversion losses, required duration and end-of-discharge conditions all affect battery selection. A battery's catalogue Ah rating is tied to a discharge rate and endpoint; it cannot be assumed to provide the same useful capacity at every duration.
Prepare the emergency load schedule
Use the actual emergency-mode input of each fitting, not automatically its normal lighting wattage. Identify circuits, monitoring modules and auxiliary loads. Record which loads operate during an outage and confirm the declared autonomy required for the project. Three hours in the example below is a training assumption, not a rule for every location.
Worked example: preliminary energy estimate
Assume 2.4 kW of emergency load for 3 hours, a nominal 216 V battery string and 90% conversion efficiency. Ignoring voltage variation and battery rate effects, Ah ≈ P × t / (V × η) = 2,400 × 3 / (216 × 0.90) = 37.0 Ah.
If a purely illustrative ageing allowance of 1.25 and temperature allowance of 1.10 are multiplied into that estimate, it becomes 50.9 Ah. These factors are not product recommendations. The result is a preliminary comparison, not the final catalogue battery size.
Why a C10 rating is not a three-hour guarantee
C10 describes a capacity measured over ten hours under specified conditions. For a three-hour duty, select using the battery manufacturer's discharge data for that duration, the minimum permitted cell voltage, temperature and loading method. For a constant-power inverter load, manufacturer constant-power tables are particularly relevant. As battery voltage falls, current rises to maintain power, so nominal voltage arithmetic alone can be optimistic.
Check the complete system
- Confirm inverter output capacity and operating power factor, not only battery Ah.
- Assess the furthest luminaire's input voltage near end of discharge, with distributed circuit loads.
- Review charger capacity against recharge requirements while supplying standing loads.
- Check circuit protection, segregation, monitoring, battery ventilation and access.
For final selection, request a discharge calculation identifying battery type, string arrangement, duration, endpoint, temperature and allowances. Commissioning should verify the installed emergency load and the required duration, with results recorded per circuit. Lighting quantity alone does not establish escape-route illuminance: use photometric data and the actual geometry.
Further reference
Yuasa: SWL battery performance and discharge tables. External guidance supports the topic; the illustrative inputs and worked examples above are prepared for this guide and are not manufacturer product selections.
Verify assumptions and results before project use. These worked examples are educational and do not provide authority approval. Report a correction.