Enter your real load and battery details to estimate how long a UPS will last before your shutdown rule takes over. The defaults reproduce my 55W home-server test, but every assumption is editable. Treat the result as a planning range, then check it against the runtime chart or a supervised test for your exact UPS.
Model details: 128 Wh rated, 90% chemistry-usable depth, 88% rate factor, 100% health, 70% conversion efficiency, and 10% reserve.
How this calculator works
The short version is rated battery energy, adjusted for chemistry, health, discharge rate, and inverter loss, divided by the connected load. The shutdown reserve then removes the part you do not plan to use. If your battery label gives volts and amp-hours, the calculator first multiplies volts × amp-hours × battery count to estimate watt-hours.
Lead-acid capacity is normally rated at a slow discharge. The Yuasa REC10-12 data sheet, for example, specifies 10Ah at the 20-hour rate to 10.5V for a 12V block, but only 9.3Ah at the 10-hour rate to 10.8V. That is why the model starts at a 0.05C reference and applies Peukert’s law as load rises.
The calculator uses a Peukert exponent of 1.25 for sealed lead-acid/AGM and 1.05 for LiFePO4. Those are the fallback values in Victron Energy’s battery-monitor documentation. Victron also warns that Peukert’s equation is only a rough approximation, especially at very high current. A battery-specific data sheet should replace the defaults when it provides better values.
For chemistry-usable depth, the model treats an SLA rating as capacity down to its published cutoff, then applies your software reserve separately. That is not a recommendation to deep-cycle an SLA battery in normal service. LiFePO4 starts at 90% usable depth. Victron’s SmartShunt manual suggests a 10% to 20% discharge floor for LiFePO4 unless the battery maker specifies otherwise.
The 70% inverter-efficiency default is deliberately conservative because the measured UPS was carrying only about 9% of its 600W rating. UPS efficiency is not fixed. Vertiv’s efficiency guidance shows that it falls outside the load range for which a UPS is optimized, including at light load. Change this input if the manufacturer publishes a battery-mode efficiency curve for your unit.
The displayed band sweeps the estimate by 10% in either direction for LiFePO4 and by minus 20% to plus 15% for SLA/AGM. This is a practical uncertainty range for temperature, cutoff behavior, condition, and conversion loss. It is not a statistical confidence interval or a runtime guarantee.
Checked against a real outage and test
I calibrated the defaults against my APC BX1000M with a ROCKSOLAR 12V 10Ah LiFePO4 battery. The battery is rated at 128Wh. My estimated load was 54.9W, calculated from the UPS load percentage times its 600W rating, so it was not measured by a laboratory power analyzer.
During the March 14 test, the server began shutting down at 70 minutes while the battery still measured 12.4V. Logging continued to 77 minutes and 31 seconds, ending at 12.3V. The samples add up to about 71Wh delivered to the load during the complete logged window. With 55W, 128Wh, 70% efficiency, 90% chemistry-usable depth, and a 10% shutdown reserve, this calculator estimates 69.8 minutes to shutdown and 77.6 minutes to cutoff. Read the full LiFePO4 UPS test and six-month record for the raw-data caveats.
A June 29 outage provided a second check. The same UPS carried the server for 75 minutes and 50 seconds before a clean automatic shutdown. One measured system cannot validate every UPS, but it is a better calibration anchor than choosing an efficiency number only because it is common online.
Why lead-acid and LiFePO4 give different answers
Lead-acid loses more apparent capacity as discharge current rises, which is the Peukert effect. Its terminal voltage also declines through the discharge, giving an SLA-designed UPS useful information for its charge gauge and cutoff. Age, cold, and time spent discharged can shorten runtime further.
LiFePO4 keeps a flatter voltage for most of its discharge and has much less rate penalty at ordinary UPS loads. Its battery management system can disconnect abruptly at a low-cell limit. The flat curve is useful for delivering energy, but it can fool firmware designed around lead-acid voltage. My APC reported 10% and began shutdown at 12.4V even though the battery was not empty. A tested voltage or elapsed-time shutdown is safer than waiting for the BMS cutoff.
How to get more runtime
- Measure the wall load and remove devices that do not need battery backup, especially monitors and idle desktop PCs.
- Replace a weak battery with the exact type and size approved by the UPS manufacturer.
- Use the maker’s runtime chart to choose a UPS that operates efficiently at your normal load.
- Keep enough reserve for the operating system and storage to shut down cleanly.
- Run a supervised test after any battery, load, or shutdown-setting change.
Do not add an external battery, change chemistry, or alter charging wiring unless the UPS and battery manufacturers support that configuration. Capacity is only useful if the charger, current limit, case, terminals, temperature range, and safety protections are compatible.
UPS runtime calculator FAQ
How long will a UPS last?
It depends on usable battery energy and the actual watt load, not the VA number alone. A small UPS may provide only minutes at a desktop-computer load but much longer for a modem and router. Enter measured watts and the battery rating above, then confirm with the manufacturer’s runtime chart.
Can I calculate runtime from the UPS VA rating?
No. VA describes the apparent-power load a UPS can support. It does not say how many watt-hours are stored in the battery. Use VA times load power factor to estimate watts, then find the battery’s Wh or its voltage and Ah rating.
How do I use this as a UPS sizing calculator?
Add the watts for every protected device and make sure that total is below the UPS watt rating as well as its VA limit. Then use the battery inputs to check whether the runtime range clears your shutdown target. Leave load headroom for startup peaks and future additions.
Why is the real runtime shorter than watt-hours divided by watts?
The inverter consumes energy, lead-acid capacity falls at high discharge rates, batteries age, temperature matters, and the UPS stops at a voltage or software threshold before every theoretical watt-hour is used.
Should I trust a UPS battery runtime calculator for a server?
Use it for planning, not as the only safety control. Set a conservative automatic shutdown rule and perform a supervised unplug test while watching load, voltage, and shutdown completion. For critical equipment, use the UPS maker’s measured runtime data and professional guidance.
Method note: This calculator assumes a steady load and room-temperature battery performance. It does not model startup surge, temperature curves, cell imbalance, charger compatibility, or an exact manufacturer’s runtime curve. Values stay in your browser and are not submitted.