#### Problem:

How to manually estimate UPS runtime based on server load and battery capacity.#### Solution:

Use this four-step process to estimate UPS runtime.- Identify amp hours (AH).
- Determine DC amperage for existing load.
- Create a base ratio from available AH. Create a second ratio from the DC Amperage estimate. Cross multiply both ratios.
- Apply power-factor multiple for final conservative runtime estimate.

#### Identify Capacity (i.e., Total Amp Hours):

Internal batteries usually connect in a series. That means that the voltage increases and the amperage remains the same. In most situations, we only need the AH from one internal battery (in a series of two batteries) -not both. For example:2 x 12V18AH batteries (i.e., series) = 18 AH.

N.B., batteries that connect in parallel doubles the AH while the voltage remains consistent. Parallel connections are common in external battery packs.

#### Determine Load (i.e., DC Amperage):

Load describes power consumption. Determine the active load based on amps consumed per hour (AH). Use the standard power formula to calculate amp use:W = V x A.

For example, assume that a HP server uses 300W at any given time. We're calculating battery capacity so use battery voltage (e.g., 24V) -not 120V. N.B., It's OK to use server-manufacturer estimates. It's better to take measurements from UPS or server management tools.

300W = 24V x A

#### Runtime Calculation:

Runtime uses a ratio of capacity over load.Capacity:

__18AH__

Load: 12.5A

Therefore, our initial runtime estimate is 1.44 Hours or 86.4 minutes.

#### Power Factor

Our initial runtime estimate does not account for power factor, diversity factors, efficiency loss, etc. We must, therefore, multiply our initial runtime estimate with a conservative power factor (e.g., .70):Original runtime estimate: 41.6 minutes

Power Factor: .7

86.4 minutes x .7 = 60.2 minutes.

or 1.44 Hours x .7 = 1.008 hours.

That's it! APC Network Administration UPS

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