UPS Runtime Calculator
Calculate how long a UPS will last on battery. Add usage and rates to estimate energy costs or output.
Battery Configuration
Load & Efficiency
Energy and environment guide · UPS battery runtime estimate
About UPS Runtime Calculator
Estimate backup time from battery amp-hours, voltage, number of batteries, connected load in watts, and an efficiency percentage. The tool reports nominal battery energy, usable energy, and approximate runtime.
This calculator estimates backup time from battery amp-hours, nominal voltage, battery count, load in watts, and efficiency. It converts nominal battery capacity into usable watt-hours and divides that estimate by a continuous load.
How to Use This Tool
- 1.Enter battery capacity in Ah, nominal voltage, and battery quantity.
- 2.Enter the connected load in watts and estimated system efficiency.
- 3.Calculate and compare usable watt-hours and runtime with the UPS manufacturer's runtime chart.
Technical Details and Assumptions
1.Amp-hours need a voltage to express energy
Battery capacity in Ah is a charge measure. Multiplying it by nominal volts estimates watt-hours; the result depends on using the correct battery voltage and compatible capacity rating.
2.Efficiency reduces nominal energy
The calculation multiplies battery capacity, voltage, and quantity, then applies the entered efficiency percentage to estimate usable energy before dividing by the load.
3.Runtime changes with the connected load
Under this simple model, a larger watt load uses the estimated usable energy faster. The tool treats the load as continuous and does not include a changing or startup load profile.
4.Battery behavior is not perfectly linear
Real battery capacity depends on chemistry, age, temperature, discharge rate, cutoff voltage, and manufacturer rating conditions. UPS inverter losses and reserve settings can also affect the time available.
How the Calculation Works
Nominal energy = amp-hours × voltage × quantity. Usable energy multiplies that figure by efficiency. Runtime hours = usable watt-hours ÷ load watts. This ideal calculation treats each battery as contributing the same nominal energy and assumes a constant load.
Worked Example
A 12 V, 100 Ah battery has 1,200 Wh nominal energy. At 85% efficiency, about 1,020 Wh is modeled as usable before dividing by the load.
Where This Tool Can Help
Make an initial comparison of UPS battery capacity and a steady connected load.
Planning basic backup needs
Estimate whether a battery configuration may support a known load for the required duration, then check the UPS and battery specifications.
Comparing loads
Try different continuous watt loads to see how a computer, network setup, or other equipment changes the idealized runtime estimate.
Comparing battery configurations
Check how nominal voltage, capacity, or battery count changes the model, while confirming the actual supported wiring arrangement.
Preparing for an outage
Use the estimate to plan which equipment to keep connected, then perform a real runtime test under the expected load.
Tips for a Useful Estimate
- Use the battery's rated Ah and nominal voltage, and confirm that the batteries are compatible with the UPS.
- Estimate the actual total load in watts rather than relying only on a device's maximum power label.
- Enter efficiency as a percentage, such as 85 for 85%, and account for other derating only once.
- Leave reserve time and test the setup; never rely on the estimate for critical equipment without validation.
Limitations and Review
Lead-acid and lithium batteries have discharge limits, voltage sag, inverter losses, aging, temperature effects, and load-dependent efficiency. Series and parallel battery arrangements must match the UPS input voltage; simply increasing quantity may not be valid. Check the UPS manual and battery curves.
The estimate uses nominal energy = Ah × volts × battery count; usable energy = nominal energy × efficiency fraction; runtime hours = usable watt-hours ÷ load watts. It does not model a battery discharge curve, load surges, inverter cutoff, or changing power draw.
Frequently Asked Questions
Why is usable energy lower than nominal energy?
The efficiency factor reduces the modeled battery energy available to the load.
Does this account for battery chemistry?
No. It uses a simple Ah × V energy estimate.
Can I add batteries to increase runtime?
Only if the UPS supports the battery configuration and voltage; follow manufacturer guidance.
Will the UPS run for exactly the displayed time?
Usually not. Battery age, load behavior, temperature, cutoff voltage, and UPS efficiency affect real runtime.
Can I add battery capacities together?
Only if the batteries and wiring arrangement are supported and the entered voltage and capacity represent the resulting bank correctly.