What size UPS should I use?

How to Calculating UPS size?
“the safest approach is to keep the sum of the load nameplate ratings below 60% of the UPS VA rating”

Example: Consider the case of a 1000VA UPS. The user wants to power a 900VA file server with the UPS.
The file server has a Power Factor to be considered.
The file server has a Watt rating of 900W and a VA rating of 900VA. Although the VA rating of the load is 900VA, which is within the VA rating of the UPS, the UPS will not power this load.
That is because the 900W rating of the load exceeds the Watt rating of the UPS, which is 60% of 1000VA or around 600W.

Ensure correct loading of your UPS.

Equipment nameplate ratings are often in VA, which makes it difficult to know the Watt ratings. If using equipment nameplate ratings for sizing, a user might configure a system, which appears to be correctly sized based on VA ratings but actually exceeds the UPS Watt rating.
By sizing the VA rating of a load to be no greater than 60% of the VA rating of the UPS, it is impossible to exceed the Watt rating of the UPS. Therefore, unless you have a high certainty of the Watt ratings of the loads, the safest approach is to keep the sum of the load nameplate ratings below 60% of the UPS VA rating.
Note that this conservative sizing approach will typically give rise to an oversized UPS and larger run time than expected. If optimization and accurate runtime of the system is required, call our power experts who have the ability to accurately size your UPS.

For the engineering mind!

To calculate the power usage (Watts) from battery capacity (VA) use the following formula:

Factor 0,7 x VA = Watts
To convert VA to Watts
E.g. 0.7 x 500VA Power Supply = 350 Watts max load
If UPS specs quote 500VA can run for 3-4hrs
i.e. UPS can deliver 350 watts for 3-4hrs or 175 Watts for 4-6hrs

To see Amps usage in Watts

Watts = Amps x Volts
E.g. If a power supply draws 5 Amps
= 5A x 220VAC
= 1100 Watts

Watts to VA – To convert 1100 watts to VA (VA power supply rating)
1100 / 0.7 = 1571VA (1.57KVA)
i.e. (UPS) Power supply needs to be 1.6KVA – 2KVA

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