UPS Runtime Calculator

Worked examples

A 500W load on the APC SMT1500 tower preset

• Inputs: preset = SMT1500 tower (1500VA / 1000W), system draw = 500W, target = 10 minutes.
• Result: 23 minutes from the published point, 50% of the watt rating and +500W headroom. The point comes from Schneider Electric’s tower specification table. It describes the manufacturer’s test conditions; an aged battery may not achieve it.

A 150W load with manual consumer ratings of 1500VA / 1000W

• Inputs: generic/manual mode, draw = 150W, VA = 1500, watts = 1000, type = consumer line-interactive.
• Result: about 32 minutes from the generic model, 15% of the watt rating and +850W headroom. This is an unvalidated planning estimate for those ratings. It is not a CP1500PFCLCD runtime claim.

A 150W load on the SMT1500 tower, or a named preset without a verified curve here

The runtime card reads Unavailable. No extrapolation is made. The SMT1500 evidence starts at 200W. Other named presets supply power ratings only until a model curve is verified. Watts, VA overload checks and headroom still calculate; the runtime target remains unverified. Comparison cards no longer award a Fits badge from the generic model.

How to use this calculator

Enter the total AC load in watts that your UPS must support during the intended workload. Use a wall measurement for the connected equipment, then select a UPS model or enter its nameplate VA and watt ratings. The Power & Cost Calculator supports electricity budgeting; its combined profile draw is an estimate for energy planning and does not establish the peak AC load needed for UPS sizing.

Select a named preset to use its verified curve, if one is available. Editing a preset rating returns to manual mode. The generic model assumes a consumer sealed lead-acid battery in good condition at room temperature. Online units and other battery systems need their own runtime data.

Understanding the results

Est. minutes in manual consumer mode is (VA × 0.14 × 0.88 × (0.469 − 0.225 × load / rated watts)) / load × 60. The battery energy, efficiency and usable-capacity terms are assumptions. This empirical derate is not a measured discharge curve for the selected model. At 500W and 1500VA / 1000W it produces 7.9 minutes; the SMT1500 tower specification at those same ratings gives 23 minutes. A VA-to-energy ratio cannot capture that difference. Selecting SMT1500 therefore uses its published points, and selecting a model without a verified curve returns Unavailable. Manual mode keeps the formula available for rough comparison while stating that its accuracy for your hardware is unknown. Buying or configuring shutdown around a specific duration requires the exact model curve and a battery check.

W/VA capacity used is the greater of load watts divided by the UPS watt rating and estimated load VA divided by the UPS VA rating. The load VA estimate uses a 0.95 power factor. At 100%, one rating has no remaining power margin; above 100%, the load exceeds a rating.

Headroom to first limit is the smaller of the remaining watt margin and the VA margin expressed as watts at the assumed 0.95 load power factor. It is a power-rating margin, not a runtime guarantee. Startup load may consume that margin; confirm the peak draw and the manufacturer’s runtime curve.

Sizing for a Proxmox homelab

The rule I use: load the UPS to 50% of rated watts, target 10 minutes of runtime. That gives a comfortable shutdown window when combined with NUT (Network UPS Tools) configured for automatic VM shutdown.

The CyberPower PFC Sinewave series (PFCLCD suffix) and the Smart App Sinewave series (PR prefix) are both listed by CyberPower with “Waveform: Sine Wave” — required for active PFC power supplies. Check the waveform line on the spec sheet before buying: CyberPower’s own catalog mixes true-sinewave and “Simulated Sine Wave” units within the same VA class (the 2100VA OR2200LCDRTXL2U is simulated, the 2150VA PR2200LCDRT2U is not). Do not buy a simulated-sine UPS for homelab hardware.

Battery replacement math

The battery is a consumable. CyberPower and APC replacement batteries run $25–50 for the 1500VA class and typically need replacement every 3–5 years. Factor that into your TCO: a $150 UPS plus one battery replacement over 5 years is a $175–200 5-year cost, not $150.

You can verify battery condition any time with upsc (NUT) or your UPS’s self-test button. A UPS that fails its self-test or reports low battery capacity is overdue for a replacement cell.

What this calculator does not cover

This is a runtime estimator for standard sealed lead-acid (VRLA) UPS units. Lithium-ion UPS units (CyberPower’s LX series, APC’s BX-Li series) have different discharge curves — flatter, higher capacity-to-weight — and will outperform the estimates here at the same VA rating. Extended-runtime models with external battery packs (APC SMT1500RM2U with APCRBC) are also outside the scope of this calculator’s model.

Corrections

2026-08-14 — two recommended UPS models were not real, and one watt rating was wrong. This page previously recommended a “CyberPower CP2200PFCLCD (2200VA / 1320W)” in the preset list, the recommendation cards, the sizing table and all three worked examples, and a “CP650E” in the sizing table. Neither model exists: CyberPower’s own site search returns 0 results for each, and the PFC Sinewave line ends at the CP1500PFCLCD. They are now the PR2200LCDRT2U (2150VA / 1980W) and the CP850PFCLCD (850VA / 510W), both listed by CyberPower with “Waveform: Sine Wave.” The CP1500PFCLCD was also listed here at 900W; CyberPower publishes 1500VA / 1000W. Every figure that depended on that rating - the calculator default, the percent-of-capacity and headroom numbers in the worked examples, the sizing table - has been recomputed. If you sized a UPS from this page before 2026-08-14 using the CP2200PFCLCD row, that purchase decision was based on a product you cannot buy; re-run the calculator.

Assumptions and sources

• This calculator starts with nominal battery energy, applies a fixed 88% inverter-efficiency assumption, and reduces usable capacity as load rises. Treat the result as a planning estimate rather than the UPS’s own runtime curve. Schneider Electric’s Smart-UPS runtime FAQ says the UPS calculates its own estimate from the connected load and the runtime of its last discharge, and that the estimate is least accurate at very low load; it recommends at least 10% load for a better estimate. Use the manufacturer’s model-specific runtime graph when one is available.

• The UPS preset ratings (for example the CyberPower CP1500PFCLCD’s 1500 VA / 1000 W) are the manufacturer’s published specifications; see CyberPower’s CP1500PFCLCD product page, which also publishes that unit’s own runtime figures (10 minutes at half load, 2.5 minutes at full load). The earlier generic model was checked against those two points and did not pass both: it reproduces the full-load figure (2.7 minutes modelled against 2.5 published) and reads about 21% low at half load (7.9 against 10). The curve the coefficients were originally fitted to was retracted on 2026-08-25 as not vendor-published, and they were left as they are rather than refitted to these two points. That history does not establish a safe lower bound. Named CP1500PFCLCD results now withhold runtime until the model evidence can be reverified; the generic formula remains available only as a labeled manual estimate.

• The battery-capacity figure behind the generic manual mode (0.14 Wh per VA) is a TechFuelHQ modeling assumption for a healthy battery at room temperature. It does not establish the battery capacity or runtime of a named model. The 0.14 Wh/VA ratio is read off consumer line-interactive towers (APC BR1500MS2 216 Wh / 1500 VA, APC BE650 84 Wh / 650 VA, CyberPower CP1500PFCLCD 216 Wh / 1500 VA). Larger banks can run longer, and other configurations can run shorter. The model is not offered for online or other battery systems.

• The overload test applies a load power factor of 0.95 - the conservative end of the 0.95-0.99 that the FAQ above describes for PFC computer supplies - to convert your watts into the VA the UPS has to supply, and rejects a load that exceeds either the unit’s watt rating or its VA rating. It previously divided by the UPS’s own output power factor instead, which is a property of the UPS rather than of your load, and that rejected loads at and just below some units’ rated watts.

• The Schneider Electric Smart-UPS catalogue, page 8, lists the SMT1500 tower at 1500VA / 1000W and 84 / 23 / 12 / 7 minutes at 200 / 500 / 700 / 1000W. Checked 2026-10-12. The tool interpolates logarithmically between those points and withholds loads outside them. This curve must not be applied to an SMT1500 rackmount, SMT1500C or an external battery configuration.

• CyberPower’s product pages could not be retrieved in the 2026-10-12 primary check. Their earlier nameplate presets remain labeled ratings only, with runtime withheld. The generic model no longer certifies a model against your target. Unverified Eaton and online model presets were removed; their nameplate ratings can still be entered manually.