Introduction #

When this guide fits: You must prove UPS battery minutes cover the bridge window until generator transfer completes, graceful shutdown finishes, or utility returns.

When it is not suitable: You are selecting battery chemistry for BESS arbitrage or utility-scale storage—those designs use different C-rate and grid codes than facility UPS strings.

Ride-through (bridge time) is how long the UPS inverter carries the load from stored DC energy while the upstream source is unavailable or unstable. It is the time-domain partner to kVA sizing—a correctly sized frame still fails if minutes are short by one crank cycle.

Pair this guide with UPS Runtime Calculator, UPS Battery Calculator, Generator UPS Calculator, and the UPS calculator hub.

Bridge time vs runtime sticker #

Concept What it is Common mistake
Bridge time Minutes needed for a defined event (gen transfer, shutdown script) Copying a vendor “typical 10 min” without MOP
Runtime at load Minutes available from V × Ah × strings × η at your kW Ignoring end-of-life or temperature
LCD runtime UPS estimate from measured load Treating as procurement guarantee

Bridge time is a requirement. Runtime is a supply. Supply must exceed requirement at worst-case efficiency and end-of-life battery.

How to define your bridge window #

List every second the critical bus must stay up without stable utility or generator at the UPS input:

  1. Utility outage detect → generator start command
  2. Crank and acceleration
  3. Governor/voltage stable
  4. ATS transfer (include BBM gap if applicable)
  5. Optional margin (cold weather, failed first start, operator verification)

For graceful shutdown only (no generator), bridge time = OS + application + network stop sequence—often 5–15 min for IT, scenario-specific for PLCs.

For generator sites, read UPS ATS Transfer Time and copy the total minutes from your signed MOP—not the shortest step.

Planning formula (screening) #

Required AC energy (approximate):

[
E_{\mathrm{kWh}} \approx P_{\mathrm{kW}} \times \frac{t_{\mathrm{min}}}{60}
]

Battery-side planning (same model as the runtime calculator):

[
t_{\mathrm{min}} \approx \frac{V \times Ah \times strings \times \eta \times safety \times 60}{P_{\mathrm{load,W}}}
]

Where η is inverter efficiency (often 0.85–0.92 for online UPS at partial load) and safety accounts for aging and depth of discharge (often 0.7–0.85 planning).

Use UPS Runtime Calculator for iterations—do not hand-calc once and freeze.

Worked examples #

Example A — Generator bridge (50 kW) #

MOP total bridge: 10 min at 50 kW critical load.

  • AC energy ≈ 50 × (10/60) ≈ 8.3 kWh before efficiency
  • At η = 0.88, DC-side planning ≈ 9.4 kWh usable
  • Cross-check in runtime tool with your 48 V / 240 V string data

If runtime tool shows 8 min at EOL, you are short—add parallel strings or Ah, or fix generator/ATS timing.

Example B — IT shutdown only (no generator) #

Requirement: 15 min for VM cluster + storage flush at 12 kW.

Example C — Verify against Generator UPS calculator #

Inputs: 50 kW, 15 min planning bridge, PF 0.85, 5 kW recharge.

Generator UPS calculator → ~93 kVA required. Runtime calculator must show ≥ 15 min at the same kW with documented η and safety—or the integrated design is inconsistent.

Efficiency and topology change minutes #

Online UPS at 40% load may run η ≈ 0.92; at 90% load perhaps 0.88. Eco mode can raise average η but adds transfer-out behavior during disturbances—re-run bridge verification whenever operations toggles modes.

See UPS efficiency reference on the runtime page and Online vs Offline UPS for topology trade-offs.

Acceptance testing checklist #

Test Pass criteria
Battery discharge at design kW Minutes ≥ bridge table + margin
Loaded generator transfer No unexpected UPS overload or bypass
Failed first start (simulated) Minutes cover second crank attempt if MOP requires
EOL battery scenario Use aged impedance or OEM EOL curve

Log results next to UPS Battery Maintenance impedance trends.

Common mistakes #

  1. Sizing minutes for nameplate load when steady load is lower—but transfer test runs at higher kW.
  2. Forgetting rectifier recharge surges generator load after bridge (generator topic, but affects whether minutes were conservative enough).
  3. Using new battery curves only—year-4 strings fail first real outage.
  4. Mixing minutes targets from IT (15 min) with seconds ATS steps without summing.

Next steps #

  1. Freeze bridge minutes from MOP or shutdown script.
  2. Iterate V, Ah, strings, η, safety in the runtime calculator until minutes ≥ bridge.
  3. If generator-backed, align the same minutes in the Generator UPS calculator and schedule a loaded transfer test.
Is 15 minutes enough UPS ride-through?

Only if your documented sequence fits inside 15 minutes at design kW with end-of-life batteries. Many generator sites need 10–15 min; IT shutdown-only sites often target 5–15 min.

How does bridge time relate to Ah sizing?

Longer bridge at the same kW requires more stored energy—use the battery calculator after you freeze minutes from this guide.

Should I include UPS efficiency in bridge calculations?

Yes—lower η means more DC energy for the same AC minutes. Use measured or OEM η at your load point, not brochure peak efficiency.

What if my UPS LCD shows more minutes than the calculator?

LCD estimates use present load and internal models. Use the calculator for procurement and EOL—then align policy with OEM acceptance tests.