How long will a 900 watt UPS run?
The runtime of a 900W UPS depends on its battery capacity, load percentage, and efficiency. For example, a 48V 100Ah lithium battery pack (4.8kWh) at 80% load (720W) with 90% inverter efficiency provides ≈5.3 hours. Lead-acid variants typically deliver 50-60% less runtime due to lower energy density and stricter discharge limits. Always check the UPS manufacturer’s discharge curve for precise estimates based on your specific battery bank configuration.
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What factors determine UPS runtime?
Battery capacity, load percentage, and inverter efficiency dictate runtime. A 48V 200Ah lithium battery (9.6kWh) sustains 900W loads for ≈9.5 hours at 90% efficiency. Pro Tip: For critical systems, design runtime with a 25% capacity buffer to account for battery aging.
Runtime calculations use the formula: (Battery Voltage × Amp-hours × Efficiency) ÷ Load Watts. A 24V 150Ah lead-acid system with 85% efficiency running at 720W (80% load) lasts (24×150×0.85)/720=4.25 hours. However, lead-acid batteries shouldn’t discharge below 50%, effectively halving usable capacity. Lithium-ion alternatives avoid this limitation with 80-90% depth of discharge. Real-world example: Data centers often use modular lithium UPS systems where adding battery cabinets extends runtime incrementally—like adding fuel tanks to a generator.
How does battery chemistry affect runtime?
Lithium-ion batteries provide 2-3× longer runtime than lead-acid at equal weight. LiFePO4 cells maintain stable voltage during discharge, while lead-acid experiences voltage sag that reduces effective capacity. Pro Tip: For lithium UPS, enable deep discharge cycles (80-90% DoD) to maximize runtime.
Consider a 5kVA UPS: With VRLA batteries, 10 minutes at full load requires eight 12V 100Ah batteries (96V system). Lithium achieves equivalent runtime with four 48V 50Ah modules at half the weight. Nickel-based chemistries like NiCd offer intermediate performance but require complex maintenance. Thermal management also impacts runtime—lithium batteries lose <2% capacity per month vs. lead-acid’s 5-8% monthly self-discharge. For perspective, lithium UPS systems powering MRI machines often achieve 30+ minute runtimes where lead-acid would require impractical battery racks.
| Chemistry | Energy Density (Wh/kg) | Cycle Life |
|---|---|---|
| LiFePO4 | 90-120 | 2,000+ |
| Lead-Acid | 30-50 | 300-500 |
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FAQs
Yes, by reducing connected load—cutting from 900W to 600W increases runtime by 50%. Use energy-efficient devices and enable UPS eco-mode where available.
Why does my 900W UPS show shorter runtime than calculated?
Actual runtime often underperforms specs due to aging batteries, high ambient temperatures (>35°C), or cumulative inverter losses. Test batteries annually and replace at 80% original capacity.