Sizing UPS Systems

In Solar Power by Kayla Jlassi

What are UPS Systems?

Uninterruptible Power Supply (UPS) Systems are continuous power systems designed to provide emergency power to the load when the main power supply fails. UPS Systems are used for their reliability. They used stored energy from batteries to instantaneously provide power when needed. Used in many industries, they provide are used to power critical equipment.

The Standards

IEEE 946 – Recommended Practice for the Design of DC Power Systems for Stationary Applications

The Standards

IEEE 485 – Recommended Practice for Sizing Lead-Acid Batteries for Stationary Applications

The Sizing Method

Begin by calculating the required load backup in Ah. The formulas are as follows:

• Load (A) x backup time (hours) = Ah
• Load (W/V) x backup time (hours) = Ah

Following are a couple of examples of how to calculate the load in Ah:

Example 1: 6A continuous load at 24VDC for 8 hours
6A x 8hours = 48Ah @ 24VDC

Example 2: 15W continuous load at 12VDC for 24 hours
15W/12V x 24hours = 30Ah @ 12VDC

Some other considerations for load calculations:

• Loads at non-system voltage to include inefficiencies (inverters, DC/DC converters)
• Intermittent loads should be averaged
• Future expansion considerations
• Factoring in uncertainty on run-time or power-draw (radios, pumps, lighting)
• Consider inrush current for AC loads, making sure inverter is properly sized to handle

Summing it all up!

UPS Systems are reliable power systems provide backup power to the load during power failure. Correctly sizing them means calculating the load, battery capacity, and desired charge current. Fortunately, SunWize does all of this legwork for you through our System Sizing Form. Prefer to speak to someone directly? Contact us via chat or by phone at (866) 827-6527.