UPS and Battery Backup Calculator
Size a UPS or inverter backup system from first principles: list the equipment it must carry, set the backup duration, battery voltage, efficiency and depth of discharge, and the calculator returns the UPS rating in kVA, the battery bank in amp-hours, the exact number of batteries in series and parallel, and the estimated runtime of the bank you would actually install.
Sizing for solar instead of the grid? Pair this with the Solar PV Calculator — and if a generator backs the batteries, the Generator Sizing Calculator completes the picture.
| Equipment | Qty | Watts each |
|---|
Backup system selection CALC-07 · REV 2026-07
| Total equipment load | – |
|---|---|
| Required UPS capacity (with 25% headroom) | – |
| Recommended UPS rating | – |
| Energy required from battery | – |
| Required bank capacity at DC bus | – |
| Battery configuration | – |
| Installed bank | – |
| Estimated runtime of installed bank | – |
| Average DC discharge current | – |
Guidance tool only. Runtime uses a linear energy model at the battery’s rated capacity — real lead-acid banks deliver less at high discharge rates (Peukert effect) and in heat, and UPS runtime charts from the manufacturer take precedence. Loads with motors or compressors need surge capacity about three times their running watts and a pure sine wave output.
How this calculator works
- Load and UPS rating. The equipment list totals the running watts, converted to VA through the UPS output power factor (0.8 typical). A 25% headroom is added and the next standard UPS frame (0.65–200 kVA) is recommended.
- Energy balance. The battery must supply load watts × hours, grossed up for inverter efficiency: Wh = W × h ÷ η.
- Bank capacity. Ah = Wh ÷ (Vbus × DoD), plus a design margin for aging and temperature. Depth-of-discharge defaults follow the chemistry — 50% for flooded and VRLA lead-acid (deeper discharge collapses cycle life), about 85% for LiFePO4.
- Configuration. Series count = bus voltage ÷ unit voltage; parallel strings = required Ah ÷ unit Ah, rounded up. The tool reports the installed bank and recomputes the honest runtime it delivers — usually longer than requested, since strings round up.
- Reality checks. Discharging lead-acid faster than C/5 triggers a Peukert warning (rated Ah is not fully available at high rates); more than four parallel strings triggers a balancing warning; and the notes include a C/10 charger rule of thumb.
Assumptions and limits
- Runtime is a linear model at rated capacity and 25 °C — manufacturer runtime charts take precedence for a specific UPS.
- Motor, pump and compressor loads need surge capacity about 3× their running watts and a pure sine wave inverter.
- Battery-room ventilation, DC protection and cable sizing at battery currents (which are large at 12/24 V) are part of the full design.
Need a complete backup power design — UPS, batteries, changeover and generator — engineered and documented? See our consultancy services or learn the method in a training program.