How to size an UPS battery backup?
Sizing a UPS battery backup requires calculating load requirements, desired runtime, and battery specifications. Key steps involve converting load wattage to VA (considering power factor), determining total energy needs in watt-hours (Wh), then calculating battery capacity (Ah) based on system voltage and discharge efficiency. Always incorporate 20–30% buffer capacity for aging and temperature effects.
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How to calculate required battery capacity?
Fundamental formula: Battery capacity (Ah) = (Load power (W) × Runtime (h)) ÷ (Battery voltage (V) × Discharge efficiency). For example, a 500W load needing 2hrs on 48V UPS with 90% efficiency requires (500×2)/(48×0.9)=23.15Ah.
Start by converting equipment wattage to volt-amps (VA) using VA = W ÷ Power Factor. Most IT equipment uses 0.7–0.9 power factor—a 800W server with 0.8 PF becomes 1000VA. Next, calculate total energy: 1000VA × 2hrs = 2000VAh. Since 1VAh ≈ 1Wh for DC systems, this equals 2000Wh. Now factor in battery voltage: 2000Wh ÷ 48V = 41.67Ah. But wait—have we considered discharge limits? Lead-acid batteries shouldn’t exceed 50% depth of discharge (DoD), effectively doubling capacity needs to 83.34Ah. Pro Tip: Use lithium batteries (LiFePO4) for 80–100% usable capacity, reducing physical space by 60% versus lead-acid equivalents.
| Battery Type | DoD | Efficiency |
|---|---|---|
| Lead-Acid | 50% | 80–85% |
| LiFePO4 | 80–100% | 95–98% |
What voltage configuration works best?
48V systems dominate modern UPS installations, balancing cable thickness and efficiency. A 10kVA UPS typically uses 16×12V batteries in series (192V DC bus), but lithium alternatives achieve this with 15–16 cells (48V nominal).
Voltage selection directly impacts battery quantity and wiring complexity. For 5kW loads requiring 1hr runtime: At 24V, you’d need ≈208Ah ((5000×1)/(24×0.9)). At 48V, only ≈104Ah—halving cable costs while improving efficiency. Why does this matter? Higher voltages reduce current (Amps = Watts/Volts), minimizing energy loss as heat. Real-world example: A data center using 48V lithium racks instead of 12V lead-acid saves 35% floor space and 22% cooling costs. Transitional Note: Always verify your UPS’s DC input voltage range—mismatched configurations can trigger protection circuits.
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FAQs
Below 25°C, lead-acid loses 20–40% capacity—size up 1.5× for 0°C environments. Lithium handles -20°C to 60°C with ≤15% derating.
Can I expand runtime after initial installation?
Yes—modular UPS batteries allow parallel connections. For RackBattery’s 48V 100Ah units, each added rack extends runtime by ≈1hr per 5kW load.