Because VA and watts are different numbers, and a UPS has a limit on both. A 1500VA UPS is rated for 1,500 volt-amperes of apparent power, but its real-power limit in watts is often much lower: APC's own 1500VA models are rated anywhere from 865 W to 1,500 W. And a "1500W PC" usually isn't drawing 1,500 W. That figure is normally the power supply's maximum output, not what the PC pulls from the wall. Size a UPS from the PC's actual input watts, add headroom, and keep the load at or below 80% of the UPS's watt rating. A typical office PC fits any 1500VA unit easily. A fully loaded workstation may need a 1,500 W-rated online model or a 2–3 kVA unit.
How much load can a 1500VA UPS really carry?
| Scenario | Wall draw (assumed) | UPS watts needed at ≤80% load | Does a 1500VA UPS fit? |
|---|---|---|---|
| Office PC with a 300 W supply | About 150 W | 190 W or more | Yes, any 1500VA model |
| Gaming PC with a "1500 W" supply, typical load | About 600 W | 750 W or more | BR1500GI (865 W) is tight; a 1,000 W-rated model is comfortable |
| Workstation at full load | About 1,000 W | 1,250 W or more | Only a 1,500 W-rated model, such as the SRT1500XLI |
| 1500 W supply run at its full rated output | About 1,650 W (assuming ~90% efficiency) | 2,060 W or more | No — needs a 2–3 kVA class UPS |
| APC's rule for supplies with active PFC | 600 W supply rating × 1.25 | 750 W input to plan for | Depends on the model's watt rating |
Source: CALC (illustrative) — wall-draw figures are assumptions, not measurements; PSU efficiency assumed at about 90%; 80% load limit and the 1.25× PFC rule from Schneider Electric FAQs FA158934 and FA158939; UPS watt ratings from Schneider's Singapore product pages, checked 8 Nov 2026. Measure your own PC with a plug-in power meter or the UPS's load display. SourceIT has not measured these systems.
What is the difference between VA and watts?
Watts measure real power: the energy your equipment actually turns into work and heat. Volt-amperes (VA) measure apparent power: the voltage multiplied by the current flowing in the circuit. In equipment with a power factor below 1, the current and voltage are out of step, so VA is higher than watts. The ratio between them is the power factor.
A UPS has to handle both. Schneider Electric puts it plainly: "neither the watt nor the VA rating of a UPS may be exceeded by the attached equipment". Manufacturers design lower-cost UPS units with a lower watt rating for each VA, which is why the VA number on the box can mislead. APC's 1500VA range shows the spread:
- Back-UPS Pro BR1500GI (line-interactive with AVR): 1500VA / 865 W, a ratio of about 0.58.
- Smart-UPS SMT1500IC (line-interactive, pure sine wave): 1500VA / 1,000 W, about 0.67.
- Smart-UPS On-Line SRT1500XLI (online double-conversion, pure sine wave): 1500VA / 1,500 W, a ratio of 1.0.
All three are "1500VA" models, but the online unit can carry 73% more real load than the Back-UPS Pro. The same pattern runs across other brands' catalogues, so check the watt rating on each model's datasheet. Our APC vs Eaton vs Vertiv guide covers the other things to compare.
Why doesn't a "1500W PC" draw 1,500 watts?
The wattage on a PC power supply is its maximum DC output to the components, not a constant draw. A PC pulls only what its processor, graphics card and drives need at that moment, plus losses in the supply itself. A 1,500 W supply in a PC that needs 600 W draws roughly 650–700 W from the wall, not 1,500 W. We explain why bigger supplies don't make PCs faster in our 1000W power supply myth post.
The catch is that a heavily loaded workstation can get close to its supply's rating, and draws more than that from the wall because no supply is 100% efficient. Graphics cards also spike briefly. For example, Igor's Lab measured sub-millisecond spikes of about 900 W on an RTX 5090. A line-interactive UPS running on battery has less tolerance for those spikes than an online unit, so leave headroom on high-end GPU systems. Our RTX PRO 6000 power guide shows how to estimate a GPU workstation's draw.
Size a UPS from the watts your PC actually pulls from the wall, not from the number printed on its power supply — then stay under 80% of the UPS's watt rating.
What else can trip a UPS that looks big enough?
Inrush at transfer. APC says power supplies with active power factor correction (PFC), common in modern PCs, can draw a high inrush current when the UPS switches to battery, for up to 8 ms. This can overload some UPS models or drop the load. APC's advice for its Back-UPS and Smart-UPS SC ranges is to multiply the PFC supply's rated output by 1.25 when sizing: a 600 W supply should be planned as 750 W of input.
Waveform. Some entry UPS models output an approximated or stepped sine wave on battery, such as the Smart-UPS SC series, according to APC. For workstations and servers with active-PFC supplies, a model with a pure sine wave output is the safer choice.
Everything else on the UPS. Monitors, a NAS, a network switch and speakers all count toward the total. Add them up, or measure the whole desk.
How do you size a UPS for your PC?
- Measure or estimate wall watts. A plug-in power meter, or the load display on an existing UPS, under your heaviest normal workload is best. Otherwise add up the processor's and graphics card's power ratings, plus about 100 W for the rest of the system, and allow for supply losses.
- Add everything else on the UPS. Monitors and peripherals you want kept on during an outage.
- Divide by 0.8. APC recommends not exceeding 80% of the UPS's rated capacity. That gives the minimum watt rating to shop for.
- Check the watt rating, not just VA. Then check the waveform, and that the runtime at your load is enough to save work and shut down.
In Singapore, a 13A socket at 230 V can supply about 2,990 W (CALC: 13 × 230), so a 1500VA UPS sits comfortably on one socket. Electricity is a separate cost: a PC drawing 400 W for 10 hours a day uses about 1,460 kWh a year, roughly S$455 at SP Group's Q4 2026 tariff of 31.16¢/kWh including GST (CALC). For runtime planning, see our UPS runtime guide, and for when an online design is worth it, our line-interactive vs online post.
Buying in Singapore
SourceIT lists UPS units across these watt ratings (listed 8 Nov 2026). For office PCs and modest gaming systems, the APC Back-UPS Pro 1500VA (BR1500GI, 865 W) is S$886.25 and the Back-UPS Pro 1600VA (960 W) is S$782.50. For workstations that need the full 1,500 W, the online APC Smart-UPS SRT 1500VA rackmount is S$2,436.25. Eaton and Vertiv alternatives, including the Eaton 9PX 1500VA at S$2,018.40, are in our UPS and power protection collection; check each datasheet's watt rating. Tell us your PC's specification or measured draw and we'll suggest a size. Prices change; ask for a current quotation. Use Request for Quotation on the product page, call (65) 6978 3502, email Sales@sourceit.com.sg or use our contact page.
Frequently asked questions
How many watts is a 1500VA UPS? It depends on the model. APC's 1500VA units range from 865 W (Back-UPS Pro BR1500GI) to 1,500 W (Smart-UPS On-Line SRT1500XLI). Always check the watt rating on the datasheet.
Can a 1500VA UPS run a PC with a 1500W power supply? Often yes, because most PCs draw far less than their supply's rating. But a workstation near full load can exceed a lower-watt 1500VA unit. Measure the wall draw and stay under 80% of the UPS's watt rating.
What does the 80% rule mean for a UPS? APC recommends not loading a UPS beyond 80% of its rated capacity. For a 1,000 W-rated UPS, that means about 800 W of connected load.
Why does my UPS shut off when the power fails? The load may be too high for its watt rating on battery, or a power supply with active PFC may draw a high inrush at transfer. APC suggests sizing PFC supplies at 1.25 times their rated output.
Do I need a pure sine wave UPS for a gaming PC or workstation? It is the safer choice. Some entry UPS models output an approximated sine wave on battery, and APC flags active-PFC power supplies as a sizing concern on those models.
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