I have been looking to buy a dedicated 220V AC offline UPS or portable power station with an offline UPS functionality. What surprised me is that, whereas both are very similar devices, is that portable power stations are often rightfully so specified with both power ratings in W and an energy capacity rating in kWh (or Ah for low voltage/DC devices, which too lets you figure out the energy capacity given the supply voltage), but that dedicated UPS devices are often specified with power ratings in VA (and confusingly so called "capacity") whilst not providing an energy capacity. Why are the ratings for these (seemingly?) similar type of devices so different? Is it marketing, or is there a technical explanation for this discrepancy? Or, if you will, I can add several sub-questions:

  • Why is power for UPS devices expressed in VA instead of W, and why is it called capacity?
  • Why do many UPS devices not come with an energy (kWh, J, or if you want... kVAh) or electric charge (Ah) figure?
  • Without this information, how am I supposed to understand how long it will last?
  • \$\begingroup\$ Welcome! Marketing. Rating it in VA at least used to produce higher numbers than W, but with full-on sinusoidal inverters and active PFC in just about every equipment, this makes little sense these days. Runtime is usually stated in minutes at X % load, so you need to calculate backwards from there. \$\endgroup\$
    – winny
    Sep 19, 2022 at 9:48

2 Answers 2


Power semiconductors in the inverter are rated based on current, not output power. If you use a load with a bad cos phi, that draws a lot of reactive current compared to its actual power, what matters is that the current rating of the power devices is not exceeded. So the inverter is rated in VA or Volts Amperes, not watts.

Some inverters are also rated in watts, for example 230V 1500VA/900W. This means the inverter can handle 1500/230=6.5A, but the DC-DC converter that elevates battery voltage to the inverter input voltage can only handle 900W. This is basically a 900W inverter with a higher current capacity in case the load has a bad cos phi.

Rating the battery capacity in kVAh would make no sense because VA isn't a unit of power, it is only apparent power and does not say anything useful about the active power flowing from generator to load. For the same reason, VAh isn't a unit of energy. If it outputs 1kVA into an inductor, it will output little actual power (W) but the losses in the inverter will still be about the same as when it outputs 1kW, maybe a bit more.

I don't know why UPS'es aren't rated in kWh, but I'd guess this is for marketing. Most consumer computer UPS are only intended to power the PC for a few minutes during which the PC will automatically shut down cleanly, so they use tiny batteries compared to their power. Datacenter UPS'es just cover the generator starting time, so it's a similar story.

If they give the battery voltage and capacity in Ah, you can estimate stored energy, but that doesnt' count the losses in the UPS. They usually run on deep cycle lead acid batteries, and these don't last many cycles, so the energy capacity would only be valid when new anyway. It's really about saving data in the event of a blackout or keeping the PC on through a short (few minutes) brownout.

If you intend to use it as a power station, with frequent discharge cycles, then a UPS based on a small high current lead battery is the wrong choice, a lithium based power station will last a lot more cycles.

  • \$\begingroup\$ Thanks for the elaborate answer, but the third paragraph is confusing. Aren't VA and VAh units of power and energy respectively, albeit non-conventional ones? \$\endgroup\$ Sep 20, 2022 at 9:05
  • 1
    \$\begingroup\$ VA is apparent power, so it does not say anything useful about the flow of energy from generator to load, that's what active power does. \$\endgroup\$
    – bobflux
    Sep 20, 2022 at 9:09
  1. VA is used instead of W because not all loads are purely resistive so it better describes how much load and what type of load can be powered with an UPS, due to power factor between VA and W. And it's called capacity likely because sales/marketing people nor users of UPS devices are not electrical engineers and any term seemingly capable of telling how much load in VA or watts you can put on the UPS.

  2. The difference is likely due to UPS being primarily equipment that needs to power a load of X VA for Y minutes, while portable power stations are just glorified power banks with built-in mains inverter, and power banks tend to be marketed based on their battery capacity, not how long they can power a certain equipment. Please note that the battery capacity is not an useful measure as it does not say anything about how much power/current is available and how long a certain load can be powered. Just as an example, UPSes typically use lead-acid batteries, and their capacity in Ah is measured at a rate of 20h discharge. If you discharge with larger current, you have less Ah available, if you discharge with less current, you have more Ah available.

  3. You just need to read the manuals. UPS devices have data sheets and manuals which describe how long a certain load can be powered.


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