Total load0 W0 VA apparent
UPS class––
min. rated watts–
min. rated VA–
load on recommended unit–
battery energy needed–
equivalent battery–
typical internal runtime–
output waveform–
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VA vs. watts – the number that trips people up
Every UPS has two ratings. VA (volt-amperes) is apparent power – voltage multiplied by current. Watts is real power – the energy your equipment actually consumes. The ratio between them is the power factor. A UPS labelled "1500 VA / 900 W" can never supply more than 900 W, no matter what the VA figure suggests. Always size by watts first and check VA second.
Modern computer power supplies with Active PFC have a power factor of 0.95 – 0.99, so their VA is nearly equal to their watts. Older or cheaper devices may run at 0.6 – 0.7. The calculator assumes 0.9 for a mixed load.
How to measure your real load
Nameplate ratings are maximums. A gaming PC with an 850 W power supply typically draws 350 – 550 W while gaming and 60 – 120 W at the desktop. For the most accurate result plug your equipment into an inexpensive plug-in energy meter and note the peak during your heaviest workload. Then enter that value in the list above.
Typical power draw
| Device | Typical draw | Notes |
|---|
| Office desktop + 24" monitor | 80 – 150 W | A 600 – 850 VA UPS is usually enough |
| Gaming PC (mid/high-end GPU) | 350 – 650 W | Pure sine wave, 1500 VA / 900 W or more |
| Router, modem, ONT | 10 – 30 W | Small UPS can keep internet up for hours |
| 4-bay NAS with HDDs | 35 – 60 W | Use USB/network shutdown signalling |
| 1U rack server | 150 – 400 W | Online UPS for critical workloads |
| PoE switch + 4 cameras | 40 – 90 W | Long runtime → lithium or extended battery |
The three UPS topologies
Standby (offline)
The cheapest design. Equipment runs directly from the mains until power fails, then the UPS switches to battery in 5 – 12 ms. Output on battery is often a stepped "simulated" sine wave. Fine for routers, modems and basic PCs without Active PFC.
Line-interactive
Adds automatic voltage regulation (AVR) that corrects brownouts and small surges without draining the battery. The best value for home offices, gaming rigs and small business equipment. Choose a pure sine wave model for PCs with Active PFC power supplies – with simulated sine wave they may shut down or make a buzzing noise at the switchover.
Online double-conversion
Power is continuously converted AC → DC → AC, so the output is completely isolated from mains problems and the transfer time is zero. It is the standard for servers, medical and laboratory equipment and for any site fed by a generator, whose frequency can drift outside the range a line-interactive UPS accepts.
Battery runtime is not linear
Lead-acid batteries deliver much less of their rated capacity at high discharge rates – the Peukert effect. A battery that would run a 50 W load for 2 hours may run a 500 W load for only 5 – 6 minutes, not 12. That is why the calculator scales the required battery energy with your target runtime. Lithium iron phosphate (LiFePO4) batteries suffer far less from this effect, last 8 – 10 years instead of 3 – 5 and weigh about half as much, at a higher upfront price.
Checklist before buying
- Watt rating ≥ "min. rated watts" above and keep the load under ~80 %.
- Enough battery-backed outlets (some outlets on many UPSs are surge-only).
- USB or network management card for automatic shutdown software (NUT, PowerChute, etc.).
- Extended battery module support if you need more than 15 – 20 minutes.
- Replaceable batteries and the price of replacements.
- Rack or tower form factor and noise level.
- Never plug a laser printer or space heater into the battery outlets – the surge can overload the UPS.
FAQ
What size UPS do I need for a gaming PC?
Measure or estimate the full-load draw of the PC and monitors. A typical gaming setup at 450 – 600 W needs a pure sine wave line-interactive UPS of at least 1500 VA / 900 W; high-end systems above 700 W should use 2000 – 2200 VA models.
How long will a 1500 VA UPS last?
A typical 1500 VA unit with two 12 V 9 Ah batteries runs about 25 – 30 minutes at 150 W, 8 – 10 minutes at 400 W and 3 – 5 minutes at 900 W. Runtime drops sharply as load rises.
Do I need a pure sine wave UPS?
Yes if any device uses an Active PFC power supply, a motor or a sensitive transformer. Simulated sine wave units are acceptable only for simple electronics such as routers and older PCs.
How often should UPS batteries be replaced?
Sealed lead-acid batteries usually last 3 – 5 years, less in warm rooms (every 8 °C above 25 °C roughly halves battery life). LiFePO4 batteries typically last 8 – 10 years.
Can I connect a UPS to a generator?
Yes, but small generators have unstable frequency and voltage. An online double-conversion UPS or a generator with inverter technology is recommended.
About UptimeVA
UptimeVA is an independent, free calculator that helps home users, gamers, small offices and IT staff choose an uninterruptible power supply with the right VA and watt rating, battery capacity and topology. We are not affiliated with any UPS brand or reseller.
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The methodology combines widely published engineering rules of thumb, manufacturer data sheets and the electrical code practices that installers use every day. We review the assumptions regularly and update the calculator when typical product specifications change.
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Last updated: October 28, 2026
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