Power & UPS

UPS runtime estimator

Estimate UPS runtime with chemistry-aware capacity math, SoC headroom, LFP cold-lockout warnings, and annual idle electricity cost.

Use this to plan outage-shutdown sequencing for routers, modems, NAS, desktops, and small network gear — and to see how much your idle load is costing per year. The estimator picks up LFP retrofits, SLA aging curves, cold-environment lockout risk, and the SoC misreport that catches SLA-firmware UPSes running LFP cells.

Medium priorityAbout 26 minutes

Runtime looks usable: ~26 min total, ~13 min before 50% shutdown trigger.

Estimated runtime: ~26 minutes (usable battery 52.7 Wh after age and inverter losses). Safety headroom before 50% shutdown trigger: ~13 min. Annual electricity cost of the idle load (90 W at $0.16/kWh): ~$126/year. SLA packs over 3 years on float duty have already lost meaningful capacity; the estimate is conservative on that.

First checks

Confirm actual watt load from the UPS display, a smart plug, or device labels (sum of devices on battery-backed outlets).
Check battery age, replacement date, and any health warning LEDs / app status.
Verify only critical gear is on battery-backed outlets — monitors / printers / speakers belong on surge-only.
If running an LFP retrofit, confirm the UPS firmware has an LFP-aware profile (CyberPower CP1500PFCLCDa, APC Smart-UPS Ultra Li-ion).

Step-by-step troubleshooting

1
Confirm the load
Add only battery-backed devices: router, modem/ONT, NAS, desktop, switch, and critical small gear. Exclude surge-only outlets.
Expected: The entered load should roughly match the UPS display, a smart plug reading, or the sum of device labels.
Next: If the number is a guess, treat the runtime as optimistic until measured.
2
Trim the idle wattage
Drop idle load with the tactics in /guides/home-server-power-budget-idle-watts (PCIe ASPM, HDD spin-down, C-states, PSU rightsizing). The savings stretch both UPS runtime AND the annual electricity bill.
Expected: Runtime should increase, overload warnings should clear, and the SoC reading should track reality past the shutdown trigger.
Next: Retest with the new watt load before trusting the shutdown plan.
3
Write the outage behavior
Decide what happens at 2 minutes, 5 minutes, and low battery: keep network alive, shut down NAS, or shut down desktop. Wire the shutdown trigger to the chosen threshold.
Expected: A written shutdown rule prevents guessing during the outage; the NAS receives the shutdown signal within seconds of the threshold firing.
Next: Configure NAS/desktop UPS shutdown (APC PowerChute, NUT, vendor app), then test by pulling the UPS plug while not actively writing.

What your answers suggest

Effective usable battery: 52.7 Wh (label 108 Wh × 0.65 age factor × 0.75 inverter loss).
Estimated runtime: 26 minutes at 120 W load.
Safety headroom before 50% shutdown: ~13 min (the rest is reserved for the shutdown sequence itself).
Chemistry: SLA, 3+ years on float — capacity reduced below label.
Annual electricity cost of the 90 W idle load at $0.16/kWh: ~$126/year.
No high-draw devices were reported.

Likely cause area

Conservative estimate: about 26 minutes under the entered load.
SLA battery age is likely reducing capacity below the label number.
The entered load is within a typical small-network planning range.

Safe actions

Annual idle cost of ~$126/year is meaningful — see /guides/home-server-power-budget-idle-watts for the optimization tactics ranked by impact (PCIe ASPM, HDD spin-down, C-states, PSU sizing) and the typical $140-280/year delta between a tuned N100 and an untuned tower.
Use the estimate for shutdown planning, not as a guarantee — compare against the UPS manufacturer's runtime chart when available.
Configure UPS-to-NAS signalling: APC PowerChute, NUT (Network UPS Tools), or QNAP/Synology native UPS integration. Test by pulling the UPS plug and confirming the NAS shutdown signal arrives within seconds.

When to stop

Stop immediately for smell, heat, swelling, leaking, sparking, or overload alarms.
Stop before opening battery compartments beyond the manual's safe replacement steps.
Get qualified electrical help for outlet, breaker, or wiring concerns.

Assumptions

This is a conservative planning estimate, not a manufacturer runtime chart.
Age factors are coarse: SLA halves capacity by ~5 years on float duty; LFP retains 70-90% through 8-10 years. Real curves vary by depth-of-discharge and temperature history.
Inverter loss assumes 75% efficiency (typical consumer-tier double-conversion UPS). Online/dual-conversion UPSes lose slightly more; line-interactive units lose slightly less.
Annual cost uses idle wattage × 24h × 365d at the entered electricity rate. Peak draw during transcoding / backup jobs is not included.
Chemistry detection requires the user to read the spec sheet — many consumer UPSes don't label LFP retrofits prominently.

What should I check first?

Sum the watts of every device on battery-backed outlets (UPS display, smart plug, or device labels).
Note idle (steady-state) load separately from peak — idle drives the annual electricity bill; peak drives outage runtime.
Check battery chemistry (SLA on the original UPS, LFP if retrofitted or on Smart-UPS Ultra Li-ion / CyberPower CP1500PFCLCDa).
Note battery age, the UPS environment temperature (garage / closet / heated room), and any LED / app health warnings.
Confirm the NAS / desktop shutdown trigger threshold — 20% is risky for LFP, 50% is the conservative default.

What is likely wrong?

Idle wattage is much higher than expected because the NAS isn't using PCIe ASPM, HDD spin-down, or CPU C-states.
The UPS is overloaded for what's plugged into the battery outlets, including monitors / printers / speakers that should be on surge-only.
Battery age has crossed the 3-year (SLA) or 8-year (LFP) curve and capacity is well below the label.
LFP retrofit is running on SLA-firmware UPS — the SoC reading is unreliable and the shutdown trigger fires late.
UPS lives in a cold garage / unheated basement / attic — LFP packs refuse to charge below 0°C / 32°F.

What is safe to try?

Move monitors, printers, speakers, and high-draw accessories to surge-only outlets.
Raise the NAS shutdown trigger to 50% if the UPS firmware doesn't have an LFP-aware profile.
Cut idle wattage with the tactics in /guides/home-server-power-budget-idle-watts (PCIe ASPM, HDD spin-down, C-states, PSU rightsizing) — savings stretch UPS runtime AND cut the annual bill.
Configure UPS-to-NAS signalling: APC PowerChute, NUT (Network UPS Tools), or QNAP/Synology native integration. Test with a real plug-pull.
Compare the estimate against the manufacturer's runtime chart for the exact UPS model and battery age.
Replace batteries only with compatible cells from a trusted source; for SLA-to-LFP retrofits, confirm the UPS firmware supports the swap.

When should I stop?

Stop immediately for smell, heat, swelling, leaking, sparking batteries, or persistent overload alarms.
Stop before opening battery compartments beyond the manual's safe replacement steps.
Stop before relying on a UPS in a cold garage when the pack is LFP — the runtime works but the recharge doesn't.
Outlet, breaker, or wiring concerns need qualified electrical help — not a UPS swap.

Source-backed checks

HomeTechOps turns official docs and conservative safety rules into a shorter runbook. These links are the source trail for the page direction.

APC Support: UPS battery and runtime supportUsed as the vendor-support baseline for UPS runtime, battery replacement, and safety escalation.

FAQ

Is this an exact UPS runtime calculator?

No. It is a planning estimator. Manufacturer runtime charts and real load measurements are more accurate, but the chemistry-aware curves here are conservative — runtime should always meet or beat the estimate for a healthy battery.

What's the difference between idle watts and load watts?

Idle watts is the steady-state draw 24/7 (drives annual electricity cost). Load watts is the actual draw during the outage (drives runtime). For a typical NAS + router + small switch, idle is often 60-100 W; peak during a Plex transcode or backup window can hit 200-400 W. Plan UPS runtime against the peak — and reduce idle to cut the annual bill.

Why does the chemistry input change my runtime estimate so much?

SLA on float duty loses capacity quickly — roughly halved at 5 years even without deep cycling. LFP retains 80-90% through 3000-4000 cycles (~8-10 years on a UPS). At 5 years, an SLA pack contributes ~45% of its label Wh to runtime; an LFP pack still contributes ~85%. The age factor is the biggest single variable in the estimate.

Why does the tool warn about LFP retrofits with the 20% shutdown trigger?

SLA-firmware UPSes (most CyberPower / APC Back-UPS / Eaton 5E units pre-2024) calibrate their SoC curve for lead-acid. LFP has a flat discharge curve and SLA-firmware UPSes hold ~100% indicator for most of the discharge, then drop fast to 0% — skipping the low-battery shutdown warning. Setting the NAS trigger to 50% (not 20%) buys back the safety headroom that SLA firmware can't measure. Newer LFP-aware models (APC Smart-UPS Ultra Li-ion, CyberPower CP1500PFCLCDa) don't have this problem.

How is the annual electricity cost computed?

Idle wattage × 24 hours × 365 days / 1000 = annual kWh, multiplied by your entered $/kWh rate. Defaults to $0.16/kWh (US average 2025). UK / EU rates of $0.30-0.40/kWh roughly double the number. The cost is the AC draw of the gear plugged into the UPS, not the battery — you pay for it whether or not there's an outage. See /guides/home-server-power-budget-idle-watts for tactics that cut idle wattage by 40-60% on typical tower NAS hardware.

When should I replace the SLA pack vs swap to LFP?

Replace SLA-with-SLA when the UPS is recent, the firmware is SLA-only, and the pack is under 3 years old (just past warranty). Swap to LFP when the UPS firmware has an LFP profile (Smart-UPS Ultra Li-ion, CP1500PFCLCDa) and you'd otherwise be replacing batteries every 3-4 years. LFP retrofits into SLA-firmware UPSes work but require the 50% shutdown trigger workaround. APC's Trade-UPS program (older units → Smart-UPS Ultra Li-ion trade-in) ended August 31, 2025.