UPS Battery Runtime Calculator — How Long Will Your UPS Last Under Load?
Our free UPS battery runtime calculator estimates how long your uninterruptible power supply (UPS) will run your equipment during a power outage, based on the UPS battery capacity (Ah), voltage, efficiency rating and connected load (watts). Plan your UPS correctly — and ensure you have enough runtime to gracefully shut down servers or wait for a generator to start.
How UPS Runtime is Calculated
UPS runtime is calculated from the battery's energy capacity in watt-hours (Wh): Wh = Battery Voltage × Amp-Hours × Efficiency. Dividing Wh by the load in watts gives runtime in hours. A 12V 7Ah battery has 84Wh of energy. Running a 50W load at 95% UPS efficiency gives approximately 1 hour 35 minutes of runtime. Load calculation must include all connected equipment — servers, network switches, storage and any ancillary devices.
UPS Types Explained
- Standby (Offline) UPS — Lowest cost. Switches to battery on power failure (transfer time 5–25ms). Suitable for desktop PCs and basic network equipment.
- Line-Interactive UPS — Voltage regulation without switching to battery. Transfer time 2–8ms. Best for servers and network equipment in areas with frequent voltage fluctuations.
- Online Double Conversion UPS — Always running on battery (zero transfer time). Highest protection, highest cost. Required for critical servers, SAN storage and sensitive medical or telecoms equipment.
UPS Sizing Best Practices
- Size your UPS for 70–80% of rated VA — running at 100% load dramatically reduces battery life and increases heat
- Always provide enough runtime for a graceful shutdown — typically 10–15 minutes for servers
- For generator-backed facilities, size for generator start time + transfer time + 20% margin — typically 5–10 minutes
- Replace UPS batteries every 3–5 years — batteries degrade significantly with age and may not provide rated capacity during an actual power event
- Test your UPS annually by simulating a power failure during a planned maintenance window