— 01 Free tool · UPS calculator

UPS calculator: how long a room lasts without power

Add your devices — we add up the load. Then two modes: how many minutes your UPS will give, or which UPS and how many batteries you need to last a given time. We count the real output of lead-acid batteries at fast discharge, not the nameplate amp-hours.

1 · Load
DeviceQtyW each
Load: 0 W
2 · What to calculate

Power in watts — from the UPS label or manual (not to be confused with VA). Batteries — the ones inside; for an inverter, enter the external batteries.

Runtime
Add devices
—
We calculate as soon as there is a load and a UPS
    smartoks.com.ua/kalkuliator-dbzh estimate ±25%
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    — 02 How we calculate

    Why 12 V × 7 Ah is not 84 Wh

    The naive formula divides the battery’s watt-hours by the load’s watts and promises twice what real life delivers. Two corrections bring the numbers back down to earth.

    01the main one

    Fast discharge delivers less

    The nameplate amp-hours of a lead-acid battery are measured over a 20-hour discharge. A UPS drains the battery in 10–30 minutes, and at that current it delivers only 40–60% of its capacity. That is the Peukert effect, and it is what “steals” half the time. The calculator uses an output curve by discharge current:

    Discharge inCurrentOutput
    02

    Inverter and margin

    Converting 12 V DC into 230 V AC costs ~10% (inverter efficiency 0.9). UPS sizing adds another 30% power margin for inrush currents and battery ageing, and the VA-to-W ratio is taken as for line-interactive models (W ≈ 0.6 × VA).

    Online (double-conversion) UPS units have a 0.9 ratio and cost more; for them enter the nameplate watts in the “Power, W” field — the runtime calculation does not depend on the type.

    03

    What is not accounted for

    Battery age (60–70% of new capacity after a year or two), room temperature, the power factor of old PSUs without APFC, monitor inrush currents. So the result is a ±25% planning estimate, not a guarantee. The most accurate test is to unplug the UPS under the working load and time it.

    Typical scenarios — with the same model

    What we powerLoadUPS / batteriesRuntime

    A router with an ONU lives for hours on a single UPS battery because it draws 20 W. A classroom with twelve PCs on the same battery — minutes. That is why “internet during outages” is solved with a small inverter, and “a classroom during outages” — with a generator.

    — 03 On record

    Batteries age —
    the records remember

    A UPS fails quietly: a year ago it held ten minutes, today it holds zero, and you find out during an outage. In SmartOKS a UPS is a device type of its own with specifications, a workstation and a history: when the batteries were replaced and who complained that “it does not hold”.

    • A “UPS” type in the records alongside the PCs, monitors and network of the room
    • An “it does not hold” ticket from a colleague — via the public form or Telegram, no sign-up
    • A report for the committee: how many UPS units, where they are, when they were bought
    • The Free plan is free forever — 30 workstations
    Create an organization for free → PC passport online
    Knowledge base · Server closet · UPS ● batteries 3 years old
    ▸ APC Smart-UPS 1500 · inv. no. 10480077WS: server
    • Power: 1500 VA / 980 W✓
    • Batteries: 2 × 12 V 9 Ah · replaced 03.2023replace
    • Runtime test: 6 min (was 17)↓
    ✓ Battery purchase ticket createdtoday
    UPS units in the fleet: 14 Batteries over 3 years: 5 ● Visible in advance
    — 04 Frequently asked questions

    About batteries, printers and generators

    The nameplate amp-hours of a lead-acid battery are rated for a 20-hour discharge. A UPS drains the battery in 10–30 minutes, and at that current it delivers only 40–60% of its capacity (the Peukert effect). The inverter takes another 10% or so. The calculator accounts for both, so its numbers are closer to reality than simply dividing watt-hours by watts.
    Better not. A laser printer’s fuser draws 500–1000 W in short pulses: even if the average load fits the rating, the UPS trips on overload and shuts down. Power printers and MFPs from the mains separately, and keep the UPS for PCs, network gear and servers.
    A classic UPS on 7–9 Ah batteries is a poor deal for that: expensive per kilowatt-hour and hard on the batteries. For a router, an ONU and a small load, a 12 V inverter with an external 100 Ah battery or a LiFePO4 power station is better; for a room with 10+ PCs — a generator, with the UPS only bridging the gap until it starts.
    Lead-acid UPS batteries live 3–5 years, and with frequent deep discharges during outages — 1–2 years. After a year the real capacity may be 60–70% of new, so keep a margin: if the calculator gives 10 minutes on a new battery, an old one gives 6. The battery replacement date belongs in your records — that is why a UPS is a separate device type in SmartOKS.
    Under the working load, unplug the UPS from the wall and time it until the low-battery alarm. That is the most accurate test: it accounts for your own batteries and their age. Do it every six months and after every battery replacement.