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How to Calculate UPS Load and Runtime

When it comes to selecting an uninterruptible power (UPS) system, there are several factors to consider. Beyond determining the desired topology and whether you require a single-phase or three-phase unit, it is essential to properly calculate the size of the UPS you need. To do so, you must take into account the intended total load (the combined voltage and amperage of all connected devices), the capacity (the unit’s maximum power output), and the amount of backup time desired from the UPS batteries (how long the UPS can support the equipment during a power outage).   

Although most major UPS manufacturers provide helpful product selectors and runtime calculators, read on to learn how to size your UPS load and factor runtime the old-fashioned way.

Factors to Consider When Calculating UPS Load and Runtime

Because UPS products have different specifications, it is critical to carefully consider a variety of aspects when selecting the optimal model to meet your needs. The three most important factors to take into account are the UPS’s rating, the load it will support and the amount of runtime desired.

UPS Rating

UPS systems are typically rated in either kilowatts (kW), volts amps (VA), or kilo-volt-amperes (kVA). While VA or kVA power rating represents the power limitation accepted by the UPS, the Watts rating is the power output of the UPS and determines the unit’s ‘real power.’

In a direct current (DC) circuit, watts = volts x amps (in other words, 1 kW = 1 kVA). However, when the uninterrupted power supply system uses AC (alternating current) ── as most data centers and other buildings do ── it reduces the available power (watts) in apparent power (volt-amperes). The ratio of these two numbers is called the power factor. The power factor of an AC power system is defined as the ratio of the real power absorbed by the load to the apparent power flowing in the circuit and is calculated as: watts = volts x amps x power factor. Power factors differ depending on the UPS. For example, a 100 kVA UPS system with a power factor of 0.8 can only support 80 kW of real power.

Power Load

The UPS load is the combined amount of power that attached electrical devices will consume. To calculate the load, you add the total watts of each piece of equipment that will be connected to the UPS. For example, if you want the UPS to support a 120W PC, a 30W VPN router, a 960W server, a 280W network switch and a 480W storage device, the total load required is 1870 W.

Battery Capacity

While the load represents the total equipment the UPS will be tasked with protecting, you also must determine how much time you want that load to stay up and running during a power outage. This will establish the UPS runtime. Every UPS has an internal battery that will supply a limited amount of power if the utility supply isn’t available. However, many UPS models can incorporate additional battery modules to increase runtime by minutes or hours. The battery capacity is measured in AH or Amp-hrs.

How to Calculate UPS Load and Run Time

Many UPS manufacturers offer online UPS calculators and sizing tools that make it easier to match a backup power supply to your needs. However, if you don’t have immediate access to a UPS selector, you can calculate the load by gathering measurements and completing a series of simple steps. It is important to note that while adding more batteries to a UPS can increase the battery runtime to support the load, it does not increase the capacity of the UPS. You must first ensure that the UPS is adequately sized for your load, and then add batteries to accommodate your runtime needs.

To size the UPS:   

1. List all the equipment and devices you want the UPS to protect.

2. List the amps and volts for each device. These ratings can typically be found on the label on the back

of the equipment.

3. Multiply amps by volts to determine VoltAmps (VA). Some devices may list their power requirements in watts. To convert watts to VA, divide the watts by power factor.

4. Multiply the VA by the number of pieces of equipment to get the VA subtotals.

5. Add the VA subtotals together to get the total power requirement.

6. Multiply the total by 1.2 to get the grand total. This step accounts for future expansion.

When choosing a UPS, be sure that the VA requirement of supported equipment does not exceed the VA rating of the UPS.

To determine the backup time:

1. Multiply the battery rating (in Ah) × the battery rating (in V) × the number of batteries × the battery efficiency.

2. Then divide that number by the load in Watts (W).

While runtime may seem like a simple thing to quantify, understanding the facts behind the numbers helps determine the optimal battery backup solution for your particular business or application.

Consider the following solution scenarios:

1. UPS with 10-15 minutes of runtime and no generator ── This solution allows time to safely shut down connected equipment and save work-in-progress.

2. UPS with 10-15 minutes of runtime and a generator ── This solution will keep connected systems up and running until the generator powers on.

3. UPS with two or more hours of battery runtime ── In some cases, generators may not be practical and organizations that wish to remain up and running during an extended outage must rely solely on UPS batteries.

Rely on the Experts: UPS Services with Unified Power

Regardless of the size of your UPS solution, Unified Power is uniquely qualified to help ensure it remains in optimal condition and ready to perform when you need it most. With a nationwide staff of professional service engineers, we provide a wide range of UPS and generator maintenance plans, replacement battery services, extended warranties, UPS rentals, site surveys, and other critical power services. Our team can also help you cross-reference the ideal power protection solution to meet your needs. Contact us today for more information.

Contact Unified Power for UPS Power Solutions. We provide immediate help for UPS failure and mission critical power services to help you keep your UPS in the best condition possible, preventing failures and unexpected downtime.
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