UPS Runtime & Sizing Calculator
How big a UPS, and how long will it actually last?
Add up your PC, monitor and network gear, then size a UPS and estimate real battery runtime. Accounts for inverter losses, Peukert capacity fade, battery age and the VA-vs-watts trap — and shows a range instead of a falsely precise minute figure.
A UPS keeps your PC alive long enough to save your work and shut down cleanly. Two numbers decide whether it will: whether your load fits inside the unit's watt rating (not its headline VA figure), and how long the battery really lasts at that load. Runtime falls off faster than the load rises, so a plain "battery watt-hours ÷ watts" sum overstates it badly.
Results
Estimated runtime
6–14 min
Range, not a single figure — see why below.
Total load
405 W
Everything on the UPS.
UPS load
45 %
Of the unit's watt rating.
Minimum UPS to buy
507 W
≈ 845 VA at a conservative 0.6 power factor.
Shutdown goal
Battery age comparison
New pack: min · After ~3 years: min
Recommendations are planning estimates, not electrical or purchasing advice. The hardware figures behind them are vendor-published; your exact board-partner model and build can still differ.
The formula
Runtime is not battery watt-hours divided by load. That ignores conversion losses and, more importantly, the fact that a lead-acid battery delivers less usable capacity the harder you discharge it (Peukert's law). We use:
I = (load W ÷ inverter efficiency) ÷ pack volts
I_rated = rated Ah ÷ 20 h
hours = 20 h × (I_rated ÷ I) ^ k ← Peukert, k ≈ 1.25
runtime = hours × usable fraction × age factorSizing uses your load plus the headroom you choose. VA is converted at a deliberately conservative 0.6 power factor, because many consumer units really do pair 1500 VA with 900 W.
Assumptions & error sources
Why a range? The Peukert exponent (1.15–1.35), the cutoff point, inverter efficiency and the pack's true condition are all genuinely uncertain. The band comes from sweeping those, not from an arbitrary ±%. With a manufacturer runtime chart for your exact model the estimate tightens and we say so.
Example scenarios
All on a 1500 VA / 900 W unit with a 24 V 9 Ah pack — the most common consumer configuration.
| Setup | Load | Estimated runtime |
|---|---|---|
| Office PC + monitor | 120 W | 26–62 min |
| Mid-range gaming PC idle | 165 W | 17–43 min |
| Mid-range gaming PC under load | 395 W | 5–16 min |
| High-end build under load | 660 W | 3–9 min |
Hardware power data last updated 2026-07-22. Runtime figures are model estimates, not manufacturer measurements — see Methodology.
Methodology — Recommendations are planning estimates, not electrical or purchasing advice. The hardware figures behind them are vendor-published; your exact board-partner model and build can still differ.
Frequently asked questions
How long will a 1500VA UPS run a gaming PC?
It depends far more on the load than on the VA number. A 1500 VA / 900 W unit with a typical 24 V 9 Ah pack runs roughly 10-15 minutes at a 300 W draw, and only about 4-7 minutes at 600 W. Runtime falls faster than the load rises, because a battery delivers less usable capacity the harder you discharge it. Enter your own numbers above for a range.
Why is VA not the same as watts?
VA is apparent power; watts is real power. Many consumer UPS units pair a headline VA figure with a much lower watt rating — a 1500 VA model is often rated for only 900 W. Your PC load has to fit inside the WATT rating. Sizing by VA alone is the most common way people end up with a UPS that overloads.
Do I need a pure sine wave UPS for a gaming PC?
Almost certainly yes. Modern PC power supplies use active power factor correction, which can reject the stepped "simulated sine" output of cheaper units — the PC may click, fail to transfer to battery, or shut down at the moment you need it most. Pure sine wave output avoids that class of problem.
Why does this tool show a range instead of an exact runtime?
Because the inputs cannot support minute-level precision. Real runtime depends on the specific battery chemistry, its age and temperature, the inverter, and the manufacturer discharge curve. Sites that print a single confident figure are hiding that uncertainty. If you supply an official manufacturer runtime chart for your exact model, this tool uses it and says so.
How much runtime do I actually need?
For most desktops the goal is a clean shutdown, not working through the outage: two to five minutes covers the majority of brief cuts and gives time to save and power down. Longer runtimes get expensive quickly. Set your target above and the tool tells you whether your configuration clears it comfortably, marginally, or not at all.