Battery & UPS Sizing Calculator

Battery & UPS Sizing for AS 62040 and AS/NZS 3000

Uninterruptible power supplies protect critical loads from mains failures, voltage sags, and transient disturbances. Sizing a UPS correctly requires matching the UPS kVA rating to the connected load, then calculating the battery bank capacity needed for the required backup runtime. Undersizing results in insufficient runtime or UPS overload during a mains failure. Oversizing wastes capital and floor space. This calculator takes your load profile, desired runtime, battery voltage, and chemistry, then outputs the required UPS rating and battery bank capacity in Ah.

The calculation follows the principles in AS 62040.1 (general and safety), AS 62040.2 (EMC), and AS 62040.3 (performance and test), alongside the installation wiring rules in AS/NZS 3000:2018. Battery sizing accounts for inverter efficiency losses and depth of discharge limits to protect battery cycle life.

Key concepts

• Load rating: VA vs watts. UPS units are rated in both VA (apparent power) and watts (real power). The VA rating accounts for reactive power drawn by the load. A typical UPS has a power factor of 0.8 to 0.9, meaning a 10 kVA UPS delivers 8 to 9 kW of real power. Always check both the VA and watt rating when selecting a UPS to ensure neither is exceeded.
• Depth of discharge (DOD). The percentage of total battery capacity used during each discharge cycle. Lead-acid batteries should not be discharged below 80% DOD (leaving 20% remaining) to preserve cycle life. Lithium-ion batteries tolerate 90 to 100% DOD. Using a shallower DOD extends battery life but requires a larger, more expensive battery bank.
• Inverter efficiency. The UPS inverter converts DC battery power to AC output. Typical efficiency is 90 to 95% for modern double-conversion units. The battery bank must supply extra energy to compensate for these losses, so the required Ah increases as efficiency decreases.
• Battery chemistry. Valve-regulated lead-acid (VRLA) batteries are the most common in UPS applications: lower upfront cost, 3 to 5 year lifespan, and well-understood behaviour. Lithium-ion batteries last 8 to 15 years, tolerate deeper discharge, and weigh less, but cost significantly more. Temperature affects both chemistries; every 10 degrees Celsius above 25 degrees roughly halves lead-acid battery life.

Common scenarios

1. Server room backup. A small business server room has 4 servers, 2 network switches, and a NAS drawing a total of 3.2 kW (4.0 kVA at 0.8 PF). The client requires 30 minutes of runtime to allow a clean shutdown. Using a 48 V lead-acid battery bank at 90% inverter efficiency and 80% DOD, the calculator determines a minimum battery capacity of approximately 58 Ah, leading to a 5 kVA UPS with a matched battery cabinet.
2. Medical facility essential services. A pathology lab needs 15 minutes of UPS backup for 8 kW of analytical instruments while the diesel generator starts. The load is sensitive to power quality, requiring a double-conversion (online) UPS. The calculator sizes a 10 kVA online UPS with a 96 V battery string, factoring in the higher ambient temperature (28 degrees Celsius) of the plant room.
3. Retail point-of-sale protection. A retail store wants 10 minutes of backup for 4 POS terminals and a router (total 600 W). A small 1 kVA line-interactive UPS with an internal 12 V sealed lead-acid battery is sufficient. The calculator confirms the standard internal battery provides adequate runtime without an external battery pack.