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I would like to ask about the volt-ampere (VA) rating of UPS. Is there any effect of this value in terms of the time period that the UPS can supply the power?

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No.

The VA rating expresses the maximum output power (apparent power S) that the UPS can provide:

$$ \langle S \rangle = \frac{V_{\mathrm {max}} I_{\mathrm {max}}}{2} = V_{\mathrm{rms}} \cdot I_{\mathrm{rms}} $$

The apparent power is used only to choose output cable size, so it's not related to the UPS duration at all. What you must seek for is the size of the batteries inside the UPS, that is measured in \$Ah\$, ampere time hours, dimensionally this number is a charge, coulomb.

ie:

PSU rated \$10Ah\$, your load needs 1A, theoretically the PSU can provide it for: $$ \frac{10 Ah}{1A} = 10h $$ ten hours. This assuming your batteries can fully empty and your PSU has unitary efficiency, that of course is not true. The manufacturer should provide some sort of "duration vs consumption" tables, and through them you can choose the PSU suitable for your needs.

Just choose your VA rating based on your load maximum consumption maybe adding a 10~20% of safety coefficient, then between the PSUs meeting this requirement choose one whose batteries are big enough.

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    \$\begingroup\$ Couple small remarks: \$A\cdot h\$ is commonly written as Ah. Your 10h calculation is only valid when the UPS output voltage is the same as the battery voltage, that is often not true. I'd personally keep a higher safety margin as it is often difficult to find the actual max. consumed power. \$\endgroup\$
    – jippie
    Dec 26 '12 at 9:56
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    \$\begingroup\$ In practice this answer isn't actually 100% correct. The standard practice is that UPS devices with higher VA rating are paired with batteries of higher capacity. Sometimes UPS won't specify the battery itself. If such is the case, UPS with higher VA will (usually) have higher battery capacity. \$\endgroup\$
    – AndrejaKo
    Dec 26 '12 at 11:31
  • \$\begingroup\$ @jippie thank you I fixed that. AndrejaKo what you say may or may not be true, I believe that stating the higher the wattage, the higher the duration is not 100% correct, also if as you say it usually goes like that. Thanks for your contribution too! \$\endgroup\$ Dec 27 '12 at 9:27
  • \$\begingroup\$ Also, the energy the battery can supply is power dependent, if you have a lower discharge power, the total energy you will get out is larger. \$\endgroup\$
    – Gunnish
    Jan 11 '14 at 17:17

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