VA isn’t the watts you get
A 1 kVA UPS with pf 0.8 only delivers 800 W of real power. Plugging in 900 W will trip overload long before batteries drain.
kVA vs kW
UPS Runtime & Sizing Calculator – Battery Backup, Load, kVA
Calculator
Nominal DC bus voltage of the UPS battery string.
Nameplate ampere-hours at the specified rate (approx).
Use % (e.g., 90%) or ratio (0.9).
Total real power drawn by connected equipment.
To account for age/temperature/Peukert effects (e.g., 80%).
Runtime results will appear here.
Real power of the load.
Typical UPSes are rated at pf 0.9–1.0.
Recommended 20–30% to allow growth and transients.
Sizing results will appear here.
Target backup time.
Real power drawn by equipment.
UPS DC bus / battery string nominal voltage.
Use % or ratio.
Accounts for aging/temperature/high-rate effects.
Battery capacity results will appear here.
How UPS Runtime & Sizing Work
Runtime (hours) is approximated by \(\; t \approx \dfrac{V \times Ah \times \eta \times d}{P_{\text{load}}} \;\), where \(V\) is battery voltage, \(Ah\) battery capacity, \(\eta\) efficiency (0–1), \(d\) is any derating factor (0–1), and \(P_{\text{load}}\) is the real load in watts.
UPS sizing uses \( \, \text{kW} = \text{kVA} \times \text{pf}\, \) so \( \, \text{kVA} = \dfrac{\text{kW}}{\text{pf}} \, \). Add headroom (e.g., 20–30%) to handle growth and inrush.
This tool provides engineering approximations. Actual runtime depends on battery chemistry, temperature, discharge rate (Peukert), age, and manufacturer curves.
Frequently Asked Questions
What values should I use for efficiency and derating?
Modern online UPS efficiency is often 0.90–0.95. Derating (0.7–0.9) accounts for aging and high-rate discharge.
Why are UPSes in kVA, not just kW?
kVA is apparent power; kW is real power. They relate by power factor: \(\text{kW} = \text{kVA} \times \text{pf}\).
Does power factor affect runtime?
Runtime is driven by real load power (W). If your meter shows kVA, multiply by pf to get kW before calculating.
Understanding UPS Runtime & Sizing
A Uninterruptible Power Supply (UPS) is designed to provide backup power during outages and protect sensitive equipment against power disturbances. Correct UPS sizing ensures your devices receive enough runtime to safely shut down or continue operating during an outage.
Key Concepts
- Load (Watts): The total power drawn by connected equipment. Add up the wattage ratings of all devices you plan to connect to the UPS.
- Apparent Power (VA): UPS systems are usually rated in Volt-Amps (VA). Because of the power factor, a 1000 VA UPS does not always deliver 1000 W. Most small UPS units assume a power factor of 0.6–0.8.
- Battery Capacity (Ah): The amount of energy stored in the UPS battery, measured in ampere-hours. Larger capacity = longer runtime.
- Runtime (Minutes): How long the UPS can support the load before the battery is depleted. Runtime depends on load size, UPS efficiency, and battery condition.
Basic Formula
UPS battery energy can be approximated by:
$$ \text{Energy (Wh)} = \text{Battery Voltage (V)} \times \text{Capacity (Ah)} $$
The expected runtime is then:
$$ \text{Runtime (hours)} = \frac{\text{Battery Energy (Wh)} \times \text{Efficiency}}{\text{Load Power (W)}} $$
For example, a 24 V battery pack rated at 9 Ah has about 216 Wh of stored energy. If your equipment uses 100 W, and UPS efficiency is 0.9, the runtime is roughly: $$ 216 \times 0.9 / 100 \approx 1.94 \text{ hours} $$
Why Correct Sizing Matters
Choosing a UPS that is too small may result in very short runtimes or even overload shutdowns. Oversizing wastes money and space. A good rule of thumb is to size your UPS at 20–30% above your expected load to account for future growth and to ensure stable performance.
Practical Tips
- Use the equipment’s actual watt ratings (not just nameplate maximums).
- Account for power factor when comparing W and VA ratings.
- Remember that battery health degrades over time; a 3-year-old UPS may not deliver the same runtime as when new.
- For critical systems, consider redundancy with multiple UPS units or extended battery packs.
With the UPS Runtime & Sizing Calculator, you can quickly estimate the right unit for your setup, compare runtimes, and avoid under- or over-provisioning. All calculations are performed in your browser for complete privacy.
5 Fun Facts about UPS Runtime
Runtime isn’t linear
Halving the load often gives more than double the runtime because UPS efficiency usually improves at lighter loads.
Efficiency curve
Cold batteries sag
Lead-acid capacity can drop 20–40% below 0 °C. A UPS in a chilly closet may run far shorter than its spec sheet.
Temperature tax
Fan noise = wasted watts
That hum is energy leaving as heat. UPS overhead (fans, inverter losses) counts against runtime, especially on small loads.
Overhead matters
Headroom helps batteries
Running at 70–80% of rated load keeps voltage higher, reduces heat, and can extend both runtime and battery lifespan.
Plan buffer