How to Use the UPS Battery Capacity Calculator
How long a UPS (uninterruptible power supply) can carry your load during an outage depends directly on battery capacity. Enter your load power (W), the backup time you want (minutes), and your battery bank voltage (V) to get the required capacity in amp-hours (Ah).
The calculation assumes a 90% inverter efficiency and an 80% depth of discharge (DoD) for the battery. In other words, it conservatively assumes only 80% of rated battery capacity is usable, protecting battery life while still meeting your target runtime.
Real-world capacity also degrades with battery age and cold temperatures, so it's good practice to size up from the calculated result. Confirm the final battery specification with your UPS manufacturer or a qualified electrical professional.
Frequently Asked Questions
Repeatedly draining a lead-acid battery to 100% shortens its lifespan significantly. Sizing calculations typically assume only about 80% of rated capacity is usable, leaving a safety margin for battery life.
Small UPS systems commonly use a 12V bank, mid-size systems 24V, and larger systems 48V. Check your UPS spec sheet for the battery bank voltage it's designed for.