UPS & Battery Backup Calculator
Size fire alarm standby batteries (BS 5839-1 / NFPA 72) and estimate CCTV & network UPS runtime.
System & standard
Load currents
Use the panel's quiescent current plus all field devices in standby; alarm current includes sounders, beacons and relays energised.
| Device | Qty | Standby mA | Alarm mA | |
|---|---|---|---|---|
| Total | 0 | 0 |
Enter per-device currents from datasheets in mA; totals are converted to A. Include the panel itself as a row.
Battery capacity gauge
Standby vs alarm contribution
Inputs
📘 Worked example
Load
| Device | Qty | W each | Sub-total | |
|---|---|---|---|---|
| Total | 0 W |
Use real power (W). For PoE cameras, count the camera's PoE draw and the switch's own consumption, or the switch's total input power — not both.
UPS
Typical 85–92 % on battery. Default 90 %.
Battery bank
Runtime vs target
Runtime curve (runtime vs load)
Blue curve: estimated runtime of this battery bank at any load. Dot: your load. Dashed green: target runtime. Amber / red verticals: 80 % / 100 % of the UPS W rating.
Inputs
📘 Worked example
🎓 How to use this tool
A · Fire alarm standby battery
- Choose the standard (BS 5839-1 or NFPA 72) and the panel battery voltage (usually 24 V = two 12 V batteries in series).
- Select the standby period (normally 24 h) and check the alarm duration: 30 min BS, 5 min NFPA, 15 min NFPA with EVACS / voice.
- Enter the total standby and alarm currents from the panel datasheet, or switch to Device table and list every device with its mA figures.
- Keep the 1.25 ageing factor unless your specification says otherwise; read the required Ah and the next standard battery size.
B · CCTV / network UPS runtime
- List the loads (cameras, NVR, switches, routers) in watts, or enter a total.
- Enter the UPS VA and power factor, then the battery bank: DC voltage, number of 12 V blocks, Ah each, chemistry and usable %.
- Set a target runtime: the tool shows the estimated runtime, the bank Ah needed to reach the target, and warns if the UPS is loaded above 80 %.
Tips & common mistakes
- Don't mix mA and A: 350 mA = 0.35 A. The device table takes mA, the totals take A.
- Never pick a battery larger than the panel charger is rated to recharge (BS 5839-1 expects recharge to 80 % within 24 h) — check the panel's maximum battery size.
- Battery runtime is not linear: doubling the load more than halves runtime on lead-acid (Peukert effect).
- VA is not W. A 1000 VA UPS at 0.6 PF only delivers 600 W.
- Batteries in series must be the same type, capacity and age; replace them as a set (typically every 4–5 years for VRLA).
Standards & assumptions
Fire mode: C = (Is × Ts + Ia × Ta) × F, following the BS 5839-1 annex method (F = 1.25) and common NFPA 72 secondary-supply practice (20–25 % safety factor, typical). Standard sizes are matched conservatively (17/18 Ah is treated as 17 Ah, etc.). UPS mode: battery current I = P ÷ (η × V); runtime t = H × (C ÷ (I × H))^k × usable %, Peukert's law at constant power; values marked typical are not normative. Results are for guidance only and must be verified against the panel / UPS / battery manufacturer's data and the applicable local code and authority having jurisdiction.
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