FAQ: Lithium Battery UPS Capacity Calculation & Runtime Estimation
Last updated 2026-08-25
Correctly sizing the battery is the difference between a UPS that protects your load and one that shuts down too early. This FAQ explains the runtime formula, LD48 series combination examples, ET6KRS/ET10KRS 192 VDC configurations, load matching, power-factor conversion, and how runtime changes when you expand capacity.
1. The runtime formula for a Lithium Battery UPS?
Runtime (hours) = (Battery capacity in Ah × Battery voltage in V × System efficiency) ÷ Load power in W. A 48 V pack with 50 Ah stores 2400 Wh. At 800 W with 90% efficiency: 2400 × 0.9 ÷ 800 = 2.7 hours. Always use real watts, not VA, and include inverter efficiency — the formula is a planning estimate, not an exact guarantee.
2. Combining LD48 packs for a target runtime?
The LD48 series offers 48 V LiFePO4 packs in 10 Ah (LD4810), 20 Ah (LD4820), 30 Ah (LD4830) and 50 Ah (LD4850). A single LD4850 stores 2400 Wh; two in parallel store 4800 Wh and double runtime at the same load. For 1600 W, one LD4850 gives ~1.35 hours, two give ~2.7 hours. Choose pack count so stored energy covers required runtime plus safety margin.
3. Runtime for ET6KRS / ET10KRS at 192 VDC?
The ET6KRS (6000 VA / 4800 W) and ET10KRS (10000 VA / 8000 W) run on an external 192 VDC string. A 192 V 20 Ah pack stores 3840 Wh. At 4800 W full load with 90% efficiency: 3840 × 0.9 ÷ 4800 = 0.72 h (~43 min); at half load (2400 W) it roughly doubles to ~1.4 h. With a fixed 192 V bus, runtime scales directly with the external pack’s Ah rating.
4. Matching load power to battery capacity?
Convert the load to watts, add 20–30% headroom for inrush and growth, multiply required runtime by load watts and divide by efficiency to get minimum stored energy in Wh, then select the LD48 combination or 192 VDC string that meets it. Slight oversizing is cheap insurance; undersizing causes premature shutdown and deep discharges that shorten cycle life.
5. How the 0.8 power factor affects the calculation?
The ET series is rated at 0.8 power factor, so real power is 80% of VA: 1000 VA ET1K → 800 W, 3000 VA ET3K → 2400 W, 10000 VA ET10KRS → 8000 W. Runtime must be computed from watts, not VA. For reactive loads, size the UPS in VA and the battery in watts.
6. Runtime after capacity expansion?
Adding a parallel pack adds its energy directly: two identical packs double the Wh and double runtime at the same load. The LD48-50PRO supports parallel expansion, and the ET6KRS/ET10KRS accept larger external 192 VDC strings. After expansion, re-verify charge current and breaker rating — a larger pack needs longer charge time and the DC wiring must handle the same current at the same voltage.