UPS Runtime For Rack Calculator

Work out ups runtime for rack instantly with clear inputs, formula shown and shareable results.

About UPS Runtime For Rack Calculator

UPS runtime is usable battery energy divided by load, after inverter conversion losses. Runtime falls faster than linearly with load because of the Peukert effect on lead-acid batteries, so a UPS rated for thirty minutes at half load may deliver under ten at full load. The critical check is that runtime comfortably exceeds generator start and transfer time.

Formula

UPS runtime

usable energy = battery capacity x inverter efficiency; runtime = usable energy / load; margin = runtime - generator start time

Worked example

With the default values below, UPS Runtime For Rack Calculator returns UPS runtime at this load of 45.1. Change any input above and every figure here is recalculated in your browser.

Inputs used

Load to supportminimum 0.01 kW: 6 kW
Usable battery capacityminimum 0.01 kWh: 4.8 kWh
Inverter efficiencyaccepts 50–100 %: 94 %
Generator start and transfer timeminimum 0 s: 45 s

Results

UPS runtime at this load: 45.1 min
Usable energy after conversion losses: 4.51 kWh
Margin beyond generator start: 44.4 min
Assessment: Adequate runtime

Step by step

1Values used
2UPS runtime
3UPS runtime at this load= 45.1 min
4Usable energy after conversion losses= 4.51 kWh
5Margin beyond generator start= 44.4 min
6Assessment= Adequate runtime

Assumptions and limits

The result is derived only from the 4 inputs listed above. Anything not among them is not part of the calculation.
In particular, this calculator does not take inflation, tax or compounding frequency into account.
Figures are rounded for display to 2 decimal places, so totals built from several rounded parts can differ slightly from the unrounded arithmetic.

How these calculators are built and tested is described on the Methodology page. If a figure looks wrong, please report it.

Frequently Asked Questions

Why is real runtime shorter than the rating?

Ratings are quoted at a specific load and with new batteries. Capacity declines with age and temperature, and the Peukert effect reduces effective capacity at high discharge rates.

How often should batteries be tested?

At least annually with a load test. Impedance monitoring catches degradation earlier, and lead-acid strings typically need replacement every three to five years.