Add each device the UPS needs to support, such as the BAS controller, network switch, and critical sensors.
🔋 Recommended UPS Capacity
Sizing Breakdown
Full Calculation Table
| Device | VA | Running Total |
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How to Use the UPS Sizing Calculator
Add the BAS controller, network switches, sensors, and any other equipment that needs to stay powered through a utility outage.
Use the nameplate VA rating if available. If only watts are listed, the device's power factor converts it to VA.
This is how long you need the connected devices to stay powered on battery, whether that's a few minutes for an orderly shutdown or longer for a generator transfer.
The calculator totals the connected VA load, applies the standard 80% loading guideline, and gives you the minimum UPS capacity to look for.
Sizing a UPS for Building Automation and HVAC Controls
A building automation system or critical HVAC control panel that loses power mid-cycle can leave equipment in an unknown state, lose scheduling data, or require a full restart and recommissioning before it's back to normal operation. A properly sized UPS bridges short utility outages and gives controls time to shut down cleanly or ride through a generator transfer, without the controller ever losing power.
Why UPS Sizing Uses an 80% Loading Rule
Keeping a UPS loaded at or below 80% of its rated capacity, rather than right at 100%, is standard practice across the industry. This headroom absorbs the brief inrush current that happens when electronics power on, leaves room for adding devices later without a UPS replacement, and keeps the battery running cooler, which generally extends its service life. This calculator divides your total connected VA load by 0.8 to find the minimum UPS rating that respects this guideline.
Converting Watts to VA
UPS capacity is rated in VA (volt-amperes), not watts, and the two aren't the same number unless the connected load has a power factor of exactly 1.0. Most electronic equipment, including BAS controllers and network switches, has a power factor somewhere between 0.8 and 0.95. If a device's nameplate only lists watts, dividing by its power factor gives the VA figure needed for UPS sizing. When in doubt, use the device's nameplate VA rating directly if one is printed, since it's already accounted for.
Runtime Doesn't Scale the Way You'd Expect
UPS battery runtime doesn't follow a simple straight-line relationship with load. The same UPS unit might run a lightly loaded circuit for an hour but only manage a few minutes at near-full capacity, because of how battery discharge and inverter efficiency behave differently at different power draws. Manufacturers publish runtime charts showing this relationship for each model, and the right way to confirm a UPS meets your runtime requirement is checking that chart at your calculated load, not just looking at the headline runtime number quoted at the unit's full rated capacity.
Choosing the Right UPS Topology
Standby and line-interactive UPS units are common for smaller BAS panels and cost less, but introduce a brief transfer time when switching to battery power that most modern electronics handle fine. Double-conversion online UPS units provide continuous, cleaner power with no transfer time, which matters more for sensitive control equipment or longer cable runs prone to voltage sag, at a higher cost. For most HVAC control panel applications, a line-interactive UPS sized per this calculator's guidance is adequate, but check the specific controller manufacturer's power quality recommendations before finalizing the choice.
Next Steps
Once you know the required VA capacity, check the transformer sizing calculator if the UPS needs its own dedicated control transformer, and the breaker size calculator to confirm the UPS's input circuit is protected correctly.
Frequently Asked Questions
Keeping a UPS loaded at or below 80% of its rated capacity leaves headroom for future load growth, accounts for inrush current from electronics powering on, and generally extends battery life since UPS batteries run cooler and degrade more slowly at lower sustained loads. This is why standard UPS sizing practice divides the connected load by 0.8 rather than sizing the UPS exactly to the connected load.
UPS battery runtime doesn't scale in a straight line with load. Most UPS manufacturers publish a runtime chart showing the same unit lasting far longer at 30% load than at 80% load, often by a factor much larger than the load difference itself, because of how battery chemistry and inverter efficiency behave at different discharge rates. Always check the manufacturer's runtime chart at your calculated load, not just the headline runtime figure quoted at full rated capacity.
Include every device that needs to stay powered through an outage, not just the main controller. Network switches, gateways, sensors with their own power supplies, and any field devices that report critical alarms all belong on the load list, since losing any of them during an outage can mean losing visibility or control exactly when it matters most.
Yes, in most cases. A generator typically takes anywhere from several seconds to a couple of minutes to start and transfer load, and a UPS bridges that gap so the BAS controller never sees a power interruption at all. Without a UPS, the controller would reboot every time the generator starts, even if the generator itself works perfectly.
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