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.