UPS planning
UPS Battery Size Calculator
Calculate the nominal UPS battery energy needed for a load and backup time, then see the equivalent Ah at your selected bus voltage. The model is a transparent energy estimate, not a replacement-compatibility or wiring calculator.
How to size a UPS battery
The calculator multiplies load watts by backup time, accounts for UPS efficiency, usable battery fraction, battery health and planning margin, then reports Wh and Ah.
recommended Wh = load W × runtime hours ÷ UPS efficiency ÷ usable fraction ÷ battery health × (1 + design margin)
UPS battery Wh and Ah
Wh is the primary energy result. Ah depends on the selected bus voltage: Ah = Wh ÷ volts. Changing voltage changes the Ah representation, not the required Wh.
Watts versus VA
Use actual watts when available. If only apparent power is known, the calculator estimates real load as VA × power factor. The default 0.80 power factor is a planning assumption, not a universal UPS or equipment value.
Worked UPS battery sizing example
For 300 W over 30 minutes, load energy is 150 Wh. At 90% UPS efficiency, 50% usable battery fraction, 100% health and a 10% margin, the result is approximately 366.67 Wh, or 15.28 Ah at 24 V.
Energy-equivalent module estimates
If enabled, the module estimate compares nominal stored energy only. It does not determine series or parallel counts, UPS voltage compatibility, wiring or installation layout.
Limitations and methodology
Actual UPS runtime may differ because of manufacturer discharge curves, discharge rate, UPS topology, cutoff behavior, battery age and temperature. This tool does not verify replacement compatibility, cabinet fit, connectors, charging compatibility, BMS/current capability or installation safety.
UPS Battery Sizing & Runtime Buffer Reference Matrix
Recommended nominal battery energy (Wh and 24V Ah) required to sustain IT and telecom loads for specific backup runtime targets:
| Continuous IT Load | 15-Min Safe Shutdown | 30-Min Generator Start | 60-Min Full Outage Buffer | Standard Battery Configuration |
|---|---|---|---|---|
| 100 W (Network Switch + Router) | ~61 Wh (2.5 Ah @ 24V) | ~122 Wh (5.1 Ah @ 24V) | ~244 Wh (10.2 Ah @ 24V) | 2× 12V 5Ah SLA cells |
| 300 W (Workstation + Monitors) | ~183 Wh (7.6 Ah @ 24V) | ~367 Wh (15.3 Ah @ 24V) | ~733 Wh (30.5 Ah @ 24V) | 2× 12V 9Ah SLA cells (1500VA UPS) |
| 600 W (Mid-Tower Server + Storage) | ~367 Wh (15.3 Ah @ 24V) | ~733 Wh (30.5 Ah @ 24V) | ~1,467 Wh (61.1 Ah @ 24V) | 4× 12V 9Ah cells (48V DC bus) |
| 1,200 W (Enterprise Rack Enclosure) | ~733 Wh (15.3 Ah @ 48V) | ~1,467 Wh (30.6 Ah @ 48V) | ~2,933 Wh (61.1 Ah @ 48V) | External Battery Module (EBM 72V/96V) |
UPS Battery Sizing Formulas
Calculates nominal battery energy (Wh and Ah) required inside a UPS chassis or external battery module to sustain critical IT loads for a target duration.
UPS_Wh = [Load_Watts × (Runtime_Min / 60) × (1 + Margin)] / (Inverter_Eff × Usable_Fraction × Health)Variable Definitions
Load_WattsReal Equipment Power(W)- Active power demand (Apparent VA × Power Factor).
Runtime_MinTarget Outage Buffer(minutes)- Desired runtime in minutes for safe shutdown or generator start.
Inverter_EffUPS Inverter Efficiency(fraction)- DC-to-AC conversion efficiency (typically 85%–90%).
Usable_FractionUsable Capacity Share(fraction)- Cutoff depth of discharge (typically 50% for SLA batteries under high discharge rates).
MarginPlanning Design Margin(fraction)- Safety buffer for cell aging and standby losses (typically 10%–20%).
Engineering Notes & Standards
- Amp-Hour equivalent at DC bus voltage V: Ah = UPS_Wh / V.
- For desktop and server loads, typical power factors range between 0.70 and 0.90.