Formula: Runtime (hours) = (Battery Capacity (Ah) × Voltage (V) × Efficiency) ÷ Load (W). This gives a first-pass estimate — real-world runtime is also affected by battery age, temperature and discharge-rate derating, so always apply a safety margin and confirm against the battery manufacturer's discharge curve.
Typical Standby Requirements by System
| System | Typical Standby Requirement |
|---|---|
| Fire alarm panel (NFPA 72) | 24 hours standby + 5 min alarm |
| Fire alarm panel (BS 5839) | 24 hours standby + 30 min alarm (or 72h + 30min for some systems) |
| PAVA system (EN 54-4) | 24 hours standby minimum |
| Data centre UPS (Tier 3+) | Runtime to generator start, typically 10–15 min |