#### Problem:

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

#### Solution:

Use this four-step process to estimate UPS runtime.
1. Identify amp hours (AH).
2. Determine DC amperage for existing load.
3. Create a base ratio from available AH.  Create a second ratio from the DC Amperage estimate. Cross multiply both ratios.
4. Apply power-factor multiple for final conservative runtime estimate.
N.B., This process is automated with my UPS runtime calculator and spreadsheet tool:  http://www.stevenjordan.net/2016/06/ups-runtime-tool.html

#### 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

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!