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?