Field calculator

Battery and UPS runtime calculator

When utility power drops, the only thing between the load and a hard outage is the battery, and the first question is always how long it lasts. The runtime in minutes is roughly the usable battery energy times the inverter efficiency, divided by the load, times sixty. Enter the usable battery capacity in kilowatt-hours, the critical load in kilowatts, and the inverter or UPS efficiency as a percent. The result is a planning estimate, and real runtime is almost always shorter for several reasons worth understanding. Battery capacity falls as the discharge rate rises, an effect called Peukert's law that hits VRLA lead-acid hardest, so a battery drained fast delivers less than its slow-rate rating. Usable capacity is also less than the nameplate amp-hours, and capacity fades with age and temperature, which is why designers size around end-of-life numbers rather than a fresh battery. To convert amp-hours to kilowatt-hours, multiply amp-hours by the battery voltage and divide by one thousand. Use this to sanity-check the autonomy and the window to start a generator or transfer power, and confirm the real sizing with the UPS and battery manufacturer.

anvilfield.com/calculators/battery-ups-runtime-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.

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Comparing your options?

VRLA vs Lithium-ionLithium-ion wins most new data center builds on footprint, life, and total cost; VRLA holds on where first cost rules.Online vs line-interactive UPSZero transfer and full isolation for critical IT, or lower cost and high efficiency for small racks and network gear.

UPS runtime FAQ

What is the difference between an online and a line-interactive UPS?

An online (double-conversion) UPS runs the load off its inverter continuously, with zero transfer time and full isolation from utility disturbances. A line-interactive UPS runs the load on utility, regulating voltage with an automatic voltage regulator, and still transfers to battery with a brief break on a true outage. Online suits data centers; line-interactive suits small servers and network gear.

What is a double-conversion UPS?

A double-conversion UPS converts incoming AC to DC and back to clean AC, so the load always runs off the inverter and never touches the utility directly. When the utility fails, the battery on the DC bus feeds the inverter with no transfer break. The result is zero transfer time and full conditioning, which is why data centers use it.

What UPS do data centers use?

Data centers use double-conversion online UPS, classed VFI under IEC 62040, almost without exception. The load runs off the inverter at all times, so there is no transfer break and the IT gear is isolated from every utility sag, swell, and harmonic. Standby and line-interactive types break on a real outage and do not scale to data-hall ratings.

What is eco mode on a UPS?

Eco mode is a high-efficiency mode where the UPS feeds the load through its bypass path, close to straight utility, and switches to full double-conversion only when the power degrades. It saves energy, with bypass efficiency cited in the high 90s, but the load runs exposed to utility disturbances while on it. That is the efficiency-versus-protection tradeoff.

What is the difference between a standby and a line-interactive UPS?

Both run the load on utility and transfer to battery on a failure, with a brief break. The difference is that a line-interactive UPS adds an automatic voltage regulator that corrects sags and swells without using the battery, so it handles dirty power better and saves the battery for true outages. A standby UPS has no such regulation.

More in the UPS types and technologies field guide.