UPS runtime refers to how long your uninterruptible power supply can keep your equipment running when mains power cuts out. It's measured in minutes and depends on three factors: battery capacity, the power draw of your connected devices, and the UPS model's efficiency.
Higher runtime means you get more time to save work, shut down servers gracefully, or wait for power to return. A typical desktop UPS offers 5-20 minutes of runtime, whilst enterprise systems can run for hours.
Why it matters: If your runtime is too short, you won't have enough time to safely power down. If it's excessive for your needs, you're paying for battery capacity you don't use.
Common gotchas:
- Manufacturers often specify runtime at 25% load (the lightest realistic usage), not at your actual load. Full-load runtime is typically 30-50% less.
- Battery age reduces runtime. A three-year-old battery may deliver 80% of original runtime.
- Runtime degrades in hot environments. A 25°C room gives better runtime than a 40°C server room.
- Connecting more devices reduces runtime for all of them.
What to check: Calculate your actual power draw in watts, then use the UPS manufacturer's load tables (not the headline figures) to find real-world runtime. Add a 20% safety margin for battery ageing. If you need runtime beyond what battery allows, consider a generator or larger UPS model with extended battery modules.