UPS Runtime Calculator

Your system load

Total system draw (watts)

Runtime target UPS type

Your UPS (if known)

UPS preset UPS VA rating

UPS watt rating

Runtime estimate

— Est. minutes

— % of capacity

— Headroom (W)

Load vs UPS capacity

0%50%80%100%

Runtime at different load levels (your UPS)

System load	% of UPS watts	Est. runtime	Assessment

Runtime is a rough estimate: consumer sealed-lead-acid UPS hold about 0.14 Wh per VA (a 1500 VA unit ≈ ~210 Wh), adjusted for inverter efficiency and a load-dependent capacity derate. The runtime model is calibrated to typical consumer line-interactive UPS curves; larger-battery online / rackmount units run longer, so it errs low for those. Actual runtime varies with battery age (new batteries outlast aged by 15–30%), temperature, and model, so check your specific UPS model's published runtime chart for exact figures before sizing critical gear.

What size UPS do you need?

Worked examples

These are this calculator’s own outputs, computed with the same formula the tool runs in your browser — so you can see a real answer without touching a single input.

Tower server + NAS + switch drawing 150W, on a CyberPower CP1500PFCLCD (1500VA / 900W, line-interactive). This is the calculator’s own default state

Inputs: Total system draw = 150 W; UPS VA rating = 1500 VA; UPS watt rating = 900 W; UPS type = Line-interactive (PF 0.9); Runtime target = 10 minutes
Result: 32 min | 17% of capacity | +750W headroom. Status: green (“Good load level - below 50% of rated capacity”). Meets the 10-minute target. Recommendation cards at 150W: CP1000PFCLCD 20 min, CP1500PFCLCD 32 min, CP2200PFCLCD 48 min - all three earn the “Fits” badge.

Small homelab - 2x mini PC + NAS + switch drawing 80W, on a CyberPower CP1000PFCLCD (1000VA / 600W, line-interactive)

Inputs: Total system draw = 80 W; UPS VA rating = 1000 VA; UPS watt rating = 600 W; UPS type = Line-interactive (PF 0.9); Runtime target = 10 minutes
Result: 41 min | 13% of capacity | +520W headroom. Status: green. Comfortably meets the 10-minute target. Recommendation cards at 80W: CP1000PFCLCD 41 min, CP1500PFCLCD 62 min, CP2200PFCLCD 93 min.

Tower + GPU + NAS + network gear drawing 350W, on a CyberPower CP1500PFCLCD (1500VA / 900W, line-interactive)

Inputs: Total system draw = 350 W; UPS VA rating = 1500 VA; UPS watt rating = 900 W; UPS type = Line-interactive (PF 0.9); Runtime target = 10 minutes
Result: 12 min | 39% of capacity | +550W headroom. Status: green (still under 50%). Meets the 10-minute target with ~2 min margin. Recommendation cards at 350W: CP1000PFCLCD 7 min (no badge - misses the 10-min target), CP1500PFCLCD 12 min (Fits), CP2200PFCLCD 19 min (Fits).

How to use this calculator

Enter your total system load in watts — use your actual measured draw or the output from the Power & Cost Calculator if you’ve already built your system profile there. Then either select a UPS model from the presets or enter your UPS VA and watt ratings manually.

The runtime estimate assumes a standard sealed lead-acid battery in good condition (under 2 years old). Battery age is the largest variable in real-world UPS performance.

Understanding the results

Est. minutes is calculated using the standard lead-acid energy model: battery capacity (Wh) × inverter efficiency ÷ load (W), adjusted for the Peukert effect at higher load fractions. At light loads (under 30% of rated watts), the battery’s actual capacity is higher than the nominal spec; at heavy loads, it’s lower.

% of capacity is your load divided by the UPS watt rating. This is the number to watch. Stay under 80%; aim for under 50%.

Headroom is watts remaining before you hit the UPS’s rated limit. This matters for inrush current — hard drives and GPUs can draw 2–3× their steady-state wattage at startup. A UPS at 90% capacity during steady-state may trip at startup of a large GPU.

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.

Homelab config	Idle draw	Recommended UPS
N100 mini PC + switch	20–30W	650VA / 390W (CP650E)
2× mini PC + NAS + switch	60–90W	1000VA / 600W (CP1000PFCLCD)
Tower server + NAS + switch	150–220W	1500VA / 900W (CP1500PFCLCD)
Tower + GPU + NAS + network	280–400W	2200VA / 1320W (CP2200PFCLCD)

The CyberPower PFC series (PFCLCD suffix) outputs pure sine wave — required for active PFC power supplies. 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.

For the full sizing methodology including VA/W math, line-interactive vs online double-conversion tradeoffs, NUT configuration, and battery replacement scheduling, see the UPS and Power Sizing guide.

Frequently asked questions

How accurate is the UPS runtime estimate?

Within ±20% for a new battery in normal conditions (68°F / 20°C). Runtime degrades as batteries age — most sealed lead-acid UPS batteries lose 15–30% capacity by year 3 and need replacement around years 3–5. Cold temperatures (below 50°F) can reduce runtime by another 10–20%. Treat these estimates as planning numbers, not guarantees.

What percentage of UPS capacity should my load be?

30–50% of rated watts is the sweet spot — you get good runtime, reasonable battery longevity, and headroom for startup inrush current. Above 80% of rated watts, runtime drops sharply and the battery cycles harder. Below 25%, some UPS units have efficiency drops in the inverter stage.

How long do I actually need a UPS to run?

For a homelab running Proxmox with NUT integration: 5–7 minutes is enough for the UPS to signal low battery, trigger a graceful VM shutdown sequence, and allow the system to power off cleanly. 10–15 minutes gives comfortable margin if you want time to investigate before shutting down. Only target 30+ minutes if you need the homelab to keep running through brief outages without any shutdown.

What's VA vs watts on a UPS?

VA (volt-amperes) is apparent power; watts is real power. The difference is the power factor of your load. Modern PC power supplies have a power factor corrected (PFC) input stage and draw close to unity PF, so watts ≈ VA × 0.95–0.99. The UPS watt rating is the practical limit; VA is the electrical sizing spec. Always size to the watt rating, not VA, when checking if your load fits.

Do I need a pure sine wave UPS for a homelab server?

Yes for any modern UPS. Active PFC power supplies (which all current ATX PSUs use) require a pure sine wave input — they will misbehave, make noise, or shut off on a stepped/simulated sine wave UPS. All the models listed in this calculator output pure sine wave. Avoid older or ultra-budget UPS units that advertise ‘stepped approximated sine wave.’