UPS planning
UPS Battery Size Calculator
Calculate the nominal UPS battery capacity (Wh and Ah) needed to sustain critical equipment for a desired shutdown or generator-start buffer duration.
How to Size a UPS Battery Bank
- Enter Equipment Load (Watts or VA): Enter the active power consumption or apparent power with power factor of your equipment.
- Set Target Backup Duration: Enter desired runtime in minutes (e.g., 15 minutes for automated graceful shutdown or 30 minutes for generator start).
- Select UPS DC Bus Voltage: Select internal system voltage (12V, 24V, 48V, etc.).
- Review Illustrative Battery Configurations: Review calculated nominal Wh, required Ah, and illustrative module string layouts.
UPS Battery Sizing & Runtime Buffer Reference Matrix
Illustrative nominal battery energy (Wh and 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 | Illustrative UPS 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 9Ah SLA modules in series (24V string, 216 Wh nominal covers up to ~30 min) |
| 300 W (Workstation + Monitors) | ~183 Wh (7.6 Ah @ 24V) | ~367 Wh (15.3 Ah @ 24V) | ~733 Wh (30.5 Ah @ 24V) | 4× 12V 9Ah SLA modules (2S2P at 24V, 432 Wh nominal covers up to ~30 min; 60 min requires ~733 Wh) |
| 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 modules at 48V (432 Wh covers ~15 min; 60 min requires ~1,467 Wh / 16 modules) |
| 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 sized for required nominal Wh) |
Illustrative configuration only — actual UPS battery configuration must be verified against the specific UPS model and required capacity.
Calculation Formulas
Calculates nominal battery energy (Wh and Ah) required inside a UPS chassis or external battery module to sustain critical equipment loads for a target duration.
Variable Definitions
Load_WReal Equipment Power(W)- Active power demand in watts (VA × Power Factor if entered in VA).
Runtime_MinTarget Outage Buffer(minutes)- Desired runtime in minutes for safe shutdown or generator start.
Planning_MarginPlanning Margin(fraction)- Additional capacity buffer applied to the simplified calculation (typically 10%–20%).
UPS_EfficiencyUPS Inverter Efficiency(fraction)- DC-to-AC conversion and inverter efficiency (typically 85%–92%).
Usable_Capacity_FractionUsable Capacity Fraction(fraction)- Illustrative planning depth of discharge assumption (typically 50% for high-rate lead-acid discharge).
Battery_HealthBattery Health / Available Capacity Factor(fraction)- Available capacity factor relative to nominal nameplate (1.00 = 100% nominal).
Calculation Notes
- Required Battery Ah at DC bus voltage V: Required_Battery_Ah = Required_Battery_Wh / Bus_Voltage.
- This calculator uses a simplified energy-based model and does not explicitly model Peukert effects, cell aging, or temperature derating.
- Actual usable capacity depends on UPS cutoff voltage, chemistry, discharge rate, and manufacturer curves.
Frequently Asked Questions (FAQ)
How do I calculate what size UPS battery I need?
Use the canonical formula: Required Battery Wh = (Load_Watts × Runtime_Hours × (1 + Planning_Margin)) ÷ (UPS_Efficiency × Usable_Capacity_Fraction × Battery_Health). For example, supporting a 300W load for 30 minutes (0.5 hr) with 10% planning margin, 88% efficiency, 50% usable fraction planning assumption, and 100% battery health requires: (300 × 0.5 × 1.10) ÷ (0.88 × 0.50 × 1.00) = 375 Wh of nominal battery energy (~31.3 Ah at a 12V bus or ~15.6 Ah at a 24V bus).
What battery capacity is inside a standard 1500VA UPS?
Some illustrative 1500VA UPS configurations use two 12V 9Ah batteries in series, providing 216 Wh nominal energy. Actual UPS battery voltage, quantity and capacity vary by model.
Why is a 50% usable battery fraction used in UPS sizing examples?
A 50% usable fraction is an illustrative planning assumption for rapid discharge scenarios. Actual usable capacity depends on the UPS low-voltage cutoff, battery chemistry, battery model, discharge rate, ambient temperature, manufacturer discharge curves, and required service-life assumptions. This calculator uses a simplified energy-based model and does not explicitly model Peukert effects.
Can I add an External Battery Pack (EBM) to extend UPS runtime?
Yes, many expandable UPS models feature an external DC port allowing daisy-chained external battery enclosures (EBMs) that expand total nominal Wh and support extended runtimes.
Technical References & Model Basis
- IEEE Std 1184: IEEE Guide for Selection and Sizing of Batteries for Uninterruptible Power Supply Systems.
- UL 1778: Standard for Uninterruptible Power Supply Equipment.
- IEC 62040-3: Uninterruptible Power Systems (UPS) — Method of Specifying Performance and Test Requirements.