Battery/UPS Sizing

The battery and UPS sizing calculator determines the required battery capacity and UPS VA rating to support critical loads during a mains power outage for the specified autonomy period.

Battery sizing follows the IEEE 485-2020 methodology for lead-acid batteries and IEEE 1115-2014 for nickel-cadmium cells. The process begins by establishing the duty cycle — a time-sequenced load profile showing the power demand at each interval during the discharge period. The required cell size is calculated as C = P × t × k_aging × k_temp / (V_cell × n_cells × DOD), where P is the total load power, t is the autonomy time, k_aging is the aging factor (typically 1.25 for end-of-life capacity), k_temp is the temperature correction factor from IEEE 485 Table 2, and DOD is the maximum allowable depth of discharge.

UPS sizing accounts for the load power factor and crest factor. The UPS kVA rating must exceed the total critical load including future growth margin, typically 20–30%. IEC 62040-1 defines UPS classifications (VFI, VI, VFD) and performance requirements. The calculator verifies that the UPS input current does not exceed the upstream protective device rating and that the battery discharge voltage remains within the inverter operating window.

Results include the recommended battery string configuration (cells in series, strings in parallel), Ah capacity at the specified discharge rate, UPS kVA rating, runtime verification at various load levels, battery room ventilation requirements per AS/NZS 3000 Section 7.6, and a discharge curve showing voltage profile over the autonomy period.