VA, watts and power factor
VA (volt-amperes) is apparent power, volts times amps. Watts is real power, the work your equipment actually does. They are equal only at a power factor (PF) of 1; otherwise watts = VA × PF. UPS makers headline the VA figure because it is the bigger number, and two power factors decide whether it is useful:
- The UPS’s own. Its watt rating divided by its VA rating. Entry-level consumer units commonly sit around 0.6, so 1000 VA delivers only about 600 W; better line-interactive and online units are nearer 0.9 to 1.0. The spec sheet lists both numbers.
- The load’s. Modern computer power supplies have power-factor correction and sit around 0.9 or better. The Rack Planner assumes 0.9.
UPS must satisfy both: watts ≥ load × 1.25 and VA ≥ load ÷ 0.9 × 1.25 The 25% headroom keeps the UPS below about 80% load, which leaves room for start-up surges such as drives spinning up.
Size it in five steps
Step one is the only one that needs a measurement. A plug-in power meter between the wall and the UPS shows the real draw, ideally while the drives are busy. If you cannot measure, use planning values like these and keep the 25% margin:
| Device | Typical range | Used in the sizer |
|---|---|---|
| Router and modem | 8–15 W | 12 W |
| 8-port network switch | 5–10 W | 8 W |
| Wi-Fi access point | 8–15 W | 10 W |
| Mini PC or small server | 10–40 W | 35 W |
| 2-bay NAS | 15–35 W | 30 W |
| 4-bay NAS | 25–65 W | 65 W |
| 8-bay NAS | 50–120 W | 120 W |
| 1U rack server | 80–250 W | 250 W |
Runtime: what you are really buying
A UPS is a battery plus an inverter, so runtime follows from the battery’s energy in watt-hours divided by the load, reduced by losses:
That 0.51 is a planning figure. A small sealed lead-acid battery gives less than its nameplate energy at a high discharge rate and loses capacity as it ages, so this guide shows a range: a conservative share of 30% and the planner’s 51%. Treat the real number as lying between them and use the maker’s runtime chart as the authority.
| Battery | 30 W | 60 W | 120 W | 240 W |
|---|---|---|---|---|
| 84 Wh (one 12 V 7 Ah) | 50–86 | 25–43 | 13–21 | 6–11 |
| 108 Wh (one 12 V 9 Ah) | 65–110 | 32–55 | 16–28 | 8–14 |
| 216 Wh (two 12 V 9 Ah) | 130–220 | 65–110 | 32–55 | 16–28 |
A NAS needs minutes, not hours. It does not need to ride out an outage; it needs to survive one. If the NAS is set to start a clean shutdown a few minutes after the power fails, a 120 W load on a 108 Wh battery has a conservative estimate of about 16 minutes, three times what a 5-minute shutdown needs. The remainder is margin for an ageing battery. Many outages last seconds, and the UPS simply carries the load through them.
UPS sizer
Tick what will share the UPS, pick a UPS class, and the sizer checks watts, VA and runtime together. The page address updates with your choices, so you can copy it and share the exact result.
UPS sizer for a NAS or home server
Runs in your browserFeatures that matter for a NAS or home server
- Automatic shutdown support. Most NAS systems, including Synology DSM, QNAP, TrueNAS and Unraid, can watch a UPS over USB or the network and shut down cleanly. Network UPS Tools (NUT) is an open-source project with a networked protocol for exactly this, so machines other than the one with the USB cable can watch the same UPS.1
- Pure sine wave output. Some computer power supplies with active power-factor correction can misbehave or shut down on the stepped “simulated sine” output of cheaper units. Pure sine is the safer choice for a NAS or server.
- Line-interactive or online. Line-interactive units are the usual home choice: they correct small voltage swings and switch to battery in milliseconds. Online (double-conversion) units are always on the inverter and cost more.
- A replaceable battery. The sealed lead-acid battery in most UPS units wears out, commonly in about 3 to 5 years, and runtime shrinks as it does. A model with a user-replaceable battery turns that into a cheap swap instead of a new UPS.
- Battery outlets for the right things. Plug the NAS, router and switch into the battery-backed outlets, and keep printers, heaters and anything with a motor off the UPS entirely.
Questions people ask
What size UPS do I need for a NAS?
Measure the NAS and anything sharing the UPS in watts, multiply by 1.25, and check both the UPS’s watt and VA ratings against it. A 4-bay NAS with a router and switch is typically well under 150 W, which any entry-level UPS covers, so choose on battery runtime and shutdown support rather than a big VA number.
Is VA or watts more important when choosing a UPS?
Watts is the real limit, but both ratings must clear your load. VA is apparent power and watts is real power; they differ by the power factor, which is about 0.6 on many entry-level units and 0.9 to 1.0 on better ones.
How long will a UPS run my NAS?
It depends on battery energy and load. As a planning estimate, a 108 Wh battery at 65 W runs roughly 30 to 50 minutes (battery Wh × 30% to 51% ÷ load W × 60). Real runtime is shorter as the battery ages or the load rises, so check the manufacturer’s runtime chart.
Is a 1500 VA UPS overkill for a NAS?
For power capacity, usually yes. But a 1500 VA unit often carries a larger battery, and runtime scales with battery energy, so it can be worth it if you want longer to ride out outages or to keep network gear up.
Do I need a pure sine wave UPS for a NAS?
It is the safe choice. Some power supplies with active power-factor correction can shut down or behave badly on the stepped output of simulated-sine units. Check the UPS and power supply manuals if you are using a cheaper model.
Should my router and switch be on the UPS?
Usually yes. They draw little power, and keeping them up lets the NAS finish a shutdown, send alerts and be reached over the network while the outage lasts. Add their watts to the load.
How often should I replace a UPS battery?
Commonly every 3 to 5 years for the sealed lead-acid batteries in most units, sooner in a hot room. Follow the maker’s guidance and test runtime occasionally with the load you actually use.
Can I plug a laser printer or heater into the UPS?
It is better not to. Printers and heaters draw high power that would eat the UPS’s capacity and shorten runtime. Keep the UPS for equipment that must shut down cleanly.
Put the plan to work
When comparing estimates or adding headroom, check which value is the baseline in our percentage change and difference guide. Use the Percentage Calculator to keep the arithmetic visible, and explore related planning workflows before choosing equipment.
Sources and method
- Network UPS Tools (NUT): an open-source project providing a common protocol and tools to monitor power devices, including UPS units, across many vendors’ protocols.
- The 25% headroom, the 0.9 load power factor and the runtime formula with 85% inverter efficiency and 60% usable depth are those documented in the Rack Planner. The conservative 30% usable share is this guide’s own allowance for small batteries at high discharge or after ageing.
- Device wattages are planning ranges, not measurements of your equipment; a plug-in power meter gives the real figure.