Calculate how long a UPS will power your equipment. Enter battery watt-hours, efficiency, and load to find backup runtime in minutes and hours.
A UPS (Uninterruptible Power Supply) provides emergency power when the main supply fails. The critical question is always: how long will it last? Runtime depends on three factors: the battery's energy capacity (in watt-hours), the efficiency of the DC-to-AC inverter, and the power draw of connected equipment.
UPS batteries are typically rated in VA/Wh or Ah at a specific voltage. The inverter converts DC battery power to AC at 85–95% efficiency, meaning some energy is lost as heat. Your runtime equals the usable battery energy divided by the load power.
This calculator estimates UPS runtime from battery capacity, inverter efficiency, and connected load. It helps you determine if your UPS provides enough time for a graceful server shutdown, whether you need additional battery packs, or how reducing the load extends runtime.
Quantifying this parameter enables systematic comparison across facilities, time periods, and equipment configurations, revealing optimization opportunities that reduce both costs and emissions.
Knowing your UPS runtime lets you plan shutdown procedures, determine if additional batteries are needed, and ensure critical equipment stays powered long enough for a generator to start. Data-driven tracking enables proactive energy management, helping organizations reduce operational costs while progressing toward environmental sustainability goals and carbon reduction targets. This quantitative approach replaces rough estimates with precise figures, enabling facility managers to identify the most cost-effective opportunities for reducing energy consumption.
Runtime (hours) = Battery Wh × Efficiency ÷ Load (W)
Result: 2.7 hours (162 min)
Runtime = 1,500 Wh × 0.90 ÷ 500 W = 2.7 hours (162 minutes). A 1,500 Wh UPS at 90% efficiency powers a 500W load for nearly 3 hours — well beyond the typical 10–30 seconds for a generator to start.
Standby UPS: Switches to battery on outage. Efficient (97%+) in normal mode but has a brief transfer time (5–12ms). Best for home PCs. Line-Interactive: Regulates voltage without battery. 95–97% efficient. Good for small servers. Online Double-Conversion: Continuously runs through inverter. 88–94% efficient. Best for critical servers.
Battery capacity decreases at higher discharge rates. A battery rated 100 Ah at a 20-hour rate (5A) may only deliver 80 Ah at a 1-hour rate (80A). This means runtime drops faster than linearly as load increases. Manufacturer runtime charts account for this.
For a server room: Sum all server + switch + router watts. Add 20% margin. Select UPS with sufficient VA and watt ratings. Verify runtime at your load meets your shutdown requirements. Configure UPS software for automatic graceful shutdown.
Check the UPS specification sheet for Wh or Ah. If only Ah and voltage are listed: Wh = Ah × V. For example, a 9 Ah battery at 48V = 432 Wh. Some UPS models publish runtime charts instead of raw Wh.
UPS efficiency is the ratio of AC output power to DC battery power. A 90% efficient UPS loses 10% of battery energy to heat in the inverter. Online (double-conversion) UPS typically has 88–94% efficiency. Line-interactive: 95–98%.
Several factors reduce actual runtime: battery degradation over time, high ambient temperature, higher-than-expected load, and Peukert's effect (high discharge rates reduce effective capacity). Manufacturer runtime charts account for these factors.
Many enterprise UPS models support external battery modules (EBMs). Adding a battery pack proportionally extends runtime. A UPS with 30 minutes runtime gains another 30 minutes from one additional battery module of the same size.
For servers: 5–15 minutes minimum (for graceful OS shutdown). For home use: 5–10 minutes (to save work and shut down). For generator-backed systems: 30–60 seconds minimum to bridge the gap until the generator starts and stabilizes.
Replace UPS batteries every 3–5 years (lead-acid) or when the UPS reports low battery or fails a self-test. Some UPS models display remaining battery health as a percentage. Don't wait until a power outage reveals a dead battery.