# UPS Autonomy Philosophy — Generator Bridge The UPS (Uninterruptible Power Supply) in a modular data center isn't designed to run the facility through a long outage. It's designed to bridge the gap between mains failure and generator start — nothing more. **Design parameters:** - **Autonomy:** 5 minutes - **Battery chemistry:** Lead-acid - **Battery lifecycle:** 10 years - **Redundancy:** 2N (two fully independent UPS systems, each capable of carrying the full load) **Why only 5 minutes:** Diesel generators typically start and reach stable output in 10-30 seconds. The 5-minute UPS bridge provides a generous safety margin — roughly 10-30x the actual need. Extended battery autonomy beyond this is expensive weight that adds cost, floor space, and maintenance burden without improving availability. **Why lead-acid:** Lithium-ion batteries are lighter and more energy-dense, but **lead-acid** is cheaper, well-understood, and perfectly adequate for a 5-minute bridge application. The weight penalty doesn't matter in a fixed installation. The 10-year lifecycle means you replace batteries once or twice over a typical MDC deployment life. **The principle:** Minimise stored energy, maximise mechanical generation. Batteries are expensive insurance. Generators are cheap, reliable runtime. Size the batteries to reach the generator, not to replace it. --- See also: [[Modular Data Centers MoC]]