When to use: Size standby batteries for a security panel, NVR, and access-control gear. Battery Ah = (load watts × runtime hours / efficiency / system voltage) / usable depth-of-discharge. UL 294 / NFPA 72 typically require 4 hours standby for access control and fire-integrated systems; verify the AHJ requirement for your occupancy.
This calculator sizes the battery capacity required to keep security system components — NVRs, access control panels, and network switches — operational for a specified duration during a power outage. It is a required step in designing compliant physical security installations.
Battery capacity in ampere-hours (Ah) is calculated from: Ah = (Total Watts × Runtime Hours) / (System Efficiency × Battery Voltage × Usable Depth of Discharge). Total watts is the sum of all backed-up loads. System efficiency accounts for UPS inverter losses and battery internal resistance (typically 85–90% for sealed lead-acid systems). Depth of discharge for SLA (VRLA) batteries is limited to 80% to preserve battery life — regularly discharging below 80% significantly reduces cycle count.
The result in Ah is compared against standard SLA battery sizes (7, 12, 18, 26, 35, 55, 75, 100 Ah and above) to select the next available size that meets or exceeds the requirement. Multiple batteries may be wired in series (to increase voltage) or parallel (to increase Ah capacity).
The Peukert effect reduces effective SLA capacity at high discharge rates — a 35 Ah battery discharged at a high rate over two hours delivers less than 35 Ah. For shorter runtimes with high loads, apply an additional 10–15% derating.
UL 294 (Access Control System Units) requires Level II standby power — a minimum of 4 hours of standby operation from the battery backup. NFPA 72 (National Fire Alarm Code) requires 24-hour standby for fire alarm and notification systems. NFPA 70 (NEC) Article 700 covers emergency systems and their power supply requirements. For access-controlled doors on life-safety egress paths, the fire alarm interface and access control power supply must both meet the applicable standby duration requirements of the AHJ.
Temperature significantly affects SLA battery capacity — capacity decreases approximately 0.5% per degree Fahrenheit below 77°F (25°C). Equipment rooms that drop below 50°F in winter may require a 20–30% upward derating of the battery capacity. SLA batteries must be replaced every 3–5 years; include battery replacement in the maintenance schedule.
Distinguish between UPS runtime (keeping equipment operational) and emergency egress power (releasing maglocks on power failure). Maglocks must release on power loss per NFPA 101 — this is handled by the fail-safe maglock design itself, not the UPS. The UPS maintains the access control panel, readers, and recording equipment for the required standby duration.
Select the battery voltage (12 VDC for most access control PSUs, 24 or 48 VDC for larger systems), enter the required runtime in minutes, enter system efficiency (85% is typical for SLA UPS), and set the usable depth of discharge (80% for SLA). Add each backed-up load by name and wattage — NVR, access control panel, PoE switch, monitors. The tool calculates total energy in watt-hours, required battery capacity in Ah, and recommends the next standard SLA battery size.
UL 294 Level II requires 4 hours of standby power. Level III requires 24 hours and Level IV requires 24 hours plus additional endurance testing. Most commercial installations specify Level II (4-hour) as a minimum. Verify with the AHJ for the specific occupancy type.
Lithium iron phosphate (LiFePO4) batteries offer higher cycle count, lighter weight, and better temperature performance than SLA, and are increasingly used in security UPS applications. They require a compatible charger and BMS. The calculation method is the same, but usable depth of discharge increases to 90–95%, reducing the required Ah by approximately 15%.
Sealed lead-acid (SLA) UPS systems typically operate at 85–90% efficiency (including battery charging and inverter losses). Lithium-based systems achieve 92–95%. Use 85% as the conservative planning value for SLA installations.
At 85% efficiency, 80% DoD, and 12 VDC: Ah = (250 W × 24 h) / (0.85 × 12 V × 0.80) = 6000 / 8.16 = 735 Ah. This would require multiple 100 Ah batteries in parallel, or a 48 VDC battery bank with proportionally fewer cells. Large standby requirements typically use 24 or 48 VDC battery systems.
SLA (VRLA) batteries in access control panels should be replaced every 3–5 years, or when capacity falls below 80% of rated capacity on an annual load test. High-temperature environments accelerate degradation — at 95°F ambient, battery life may be reduced to 2–3 years. Include battery replacement as a mandatory line item in preventive maintenance contracts.
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