Estimate a UPS power rating in watts and VA, then calculate rough battery backup runtime for a PC, NAS or router.
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UPS and load parameters
Use rated power and battery data from manufacturer documentation.
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How does it work?
Recommended UPS W = load × (1 + headroom %). Recommended VA = load W / power factor × (1 + headroom %). Check both W and VA against the UPS nameplate.
Nominal battery energy ≈ battery voltage × Ah × number of parallel battery strings. Runtime in hours ≈ nominal energy × usable fraction × efficiency / load W. Lead-acid batteries can deliver much less usable energy under high discharge loads.
Important: Guidance only, NOT a replacement for the manufacturer runtime chart. Lead-acid discharge rate, battery aging and UPS overhead can substantially change real runtime.
Frequently asked questions
Does 1000 VA mean 1000 watts?
No. UPS units have separate active power (W) and apparent power (VA) limits; both matter.
Is the battery runtime exact?
No. Lead-acid discharge and inverter efficiency vary with load and conditions.
USEFUL FOR THIS ESTIMATE
Useful equipment for this calculation
Practical buying criteria — no invented offers or prices.
02 SUGGESTIONS
🔌BACKUP POWER
UPS battery backup
Compare real output watts, VA, waveform, switchover time and replaceable batteries.
✓ What to check
🔋ENERGY STORAGE
Portable power station
Check UPS/EPS support, transfer time, usable Wh and continuous output rating.
✓ What to check
These are general buying tips, not brand advertisements. Affiliate links will be labeled only after programme approval.
UPS sizing example for a gaming PC
Examples explain the formulas and do not represent current retail prices.
Example: 300 W system
At 300 W load, power factor 0.8 and 25% headroom, the minimum indicative ratings are 375 W and 469 VA. Verify the UPS model meets both.
Battery capacity versus runtime
A 24 V, 9 Ah battery holds approximately 216 Wh nominally. At 55% usable and 82% conversion efficiency, about 97 Wh remains for the load; actual runtime needs a manufacturer chart.
