Practical tools for measured work
AV Rack UPS Runtime Estimate
Estimate runtime from total battery-pack capacity and measured rack load, while checking both UPS watt and VA ratings.
Use the voltage of the complete battery string, not one battery.
Series batteries add voltage, not amp-hour capacity.
Check both VA and watt ratings. Runtime uses the entered derating and a linear energy model; the UPS manufacturer's runtime chart is more authoritative when available.
How it works
What to enter: copy both UPS ratings and the total voltage and amp-hour capacity of the complete battery pack. Measure the rack load if possible, then use conservative loss factors.
Calculation. Runtime = pack voltage × pack amp-hours × usable fraction × efficiency ÷ (load + UPS idle draw).
Example. A 24 V, 9 Ah pack contains 216 nominal Wh before discharge-rate, age and conversion losses.
Current-input example
The result above uses these exact values. This snapshot is included when the page is printed so the output can be checked against the original measurements.
- UPS VA rating
- 1500 VA
- UPS watt rating
- 900 W
- Total battery-pack voltage
- 24 V
- Pack capacity at that voltage
- 9 Ah
- Measured AV rack load
- 350 W
- Load power factor
- 0.9
- Inverter efficiency
- 85 %
- Usable battery capacity
- 65 %
- UPS internal draw
- 15 W
Primary result: Estimated runtime under entered assumptions: 20 minutes.
Before using the result
- Measure from the datum or reference edge described by this tool, and do not mix inside, outside and centerline dimensions.
- Keep inputs in the displayed units and preserve more precision than the final cutting or purchasing tolerance requires.
- When the result is close to a limit, verify it with a test piece, field measurement, manufacturer drawing or qualified project professional.
Limitations
Real UPS runtime is non-linear. Discharge rate, chemistry, age, temperature, inverter cut-off, startup surge and firmware matter. Confirm the pack specification and use the manufacturer runtime chart for safety-critical loads.
Frequently asked questions
Why can this estimate exceed the UPS runtime chart?
The usable-capacity factor may be too optimistic. Small lead-acid packs deliver less rated capacity at high current, and ageing reduces it further.
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