UPS Runtime Calculator

What this tool will do

• Honest runtime curves — what your UPS actually delivers, not the vendor headline number
• Battery aging modeled in: year-1 / year-2 / year-3 runtime so you plan for replacement, not surprise
• Multi-vendor coverage — APC, Eaton, CyberPower, Tripp Lite, Vertiv, Schneider, with per-model battery curves
• Load-profile aware: 30% / 50% / 75% / 100% load at every age point, not just the spec-sheet number
• External battery pack sizing — when adding more amp-hours beats buying a bigger UPS

Frequently asked questions

How long will my UPS run on battery?

It depends on three things: rated VA / watts of the UPS, your actual load (often much lower than rated), and battery age. A 1500 VA UPS at 30% load with year-1 batteries typically runs ~25–30 minutes; the same UPS at the same load with year-3 batteries runs ~15 minutes. Plan for the year-3 number unless you're disciplined about battery replacement at the 2–3 year mark.

How do I calculate UPS runtime?

Runtime in minutes ≈ (battery watt-hours × inverter efficiency) ÷ load watts. For a 1500 VA UPS with ~280 Wh of battery (typical) at 50% load (~525 W) and 90% efficient inverter: 280 × 0.9 ÷ 525 ≈ 0.48 hours ≈ 29 minutes — at year 1. Subtract 30–50% for year-3 degradation. The math is direct; the inputs (true load, current battery health) are what most calculators get wrong.

How much UPS do I need for a server room?

Size for actual load (not nameplate), aim for 50–60% of UPS rated VA at peak (gives headroom for spikes and runtime), and target ride-through past your generator start time + buffer (typically 5–15 minutes). For a small server room drawing ~2 kW peak, a 5 kVA UPS sized for 8–10 minute year-3 ride-through gets you to a generator transfer or graceful shutdown reliably. Never spec to nameplate watts — the runtime falls off a cliff above 75% load.

How long do UPS batteries last?

Sealed lead-acid batteries (SLA) in most consumer/SMB UPSes: 3–5 years under typical conditions, 18–30 months under high temperature or frequent deep discharge. Lithium UPSes: 8–10 years and barely degrade — but cost 2–3× upfront. Most fleets replace SLA at year 3 by policy because runtime has dropped to ~50–60% of rated by then; waiting longer means the UPS is unreliable when it actually matters.

What's the difference between VA and watts for a UPS?

VA (volt-amperes) is apparent power; watts is real power. For modern IT loads (servers, switches with active PFC power supplies) the ratio is close to 1:1 — a 1500 VA UPS delivers ~1500 W. For older or non-PFC loads, the ratio is ~0.6:1 — a 1500 VA UPS only delivers ~900 W. Vendor spec sheets quote both; pick the lower number when sizing for runtime, since runtime is bounded by watts not VA.