For many commercial buildings, HVAC is one of the largest electrical loads a standby generator will ever be asked to support.
That makes generator sizing more complicated than simply totaling the normal operating wattage of every piece of equipment.
Air conditioners, heat pumps, rooftop units, pumps and fans contain motors. Motors have starting characteristics that can place a significant temporary demand on a generator.
If maintaining heating or cooling during an outage is important to your business, those starting requirements need to be part of the generator design.
Start With the HVAC Equipment
Before selecting a generator, inventory the HVAC equipment that must remain operational.
For each piece of equipment, record:
- Manufacturer
- Model number
- Voltage
- Phase
- MCA
- MOCP
- Compressor information
- Motor information
- Available starting-current information
Don't forget indoor equipment, pumps, fans, controls and accessories.
Why Running kW Isn't Enough
Imagine a commercial building where the HVAC equipment operates normally within the generator's continuous capacity.
Everything appears fine.
Then two compressors start at approximately the same time.
For a brief period, electrical demand increases dramatically.
If the generator cannot adequately respond, the facility may experience unacceptable voltage drop, equipment may fail to start or protective devices may operate.
That's why generator sizing for HVAC requires considering transient motor-starting performance, not simply continuous kW.
Rooftop Units
Packaged rooftop units are common in commercial buildings.
A facility may have several RTUs serving separate zones.
Instead of assuming every unit will operate identically, review each model's electrical data.
Pay particular attention to how many compressors and motors could start simultaneously.
Multiple HVAC Systems
Consider a building with four rooftop systems.
During normal utility operation, several units can cycle independently without creating an issue because the electrical utility can generally accommodate their starting demand.
During generator operation, those same starts need to occur within the generator's capabilities.
A properly designed system may sequence equipment so large compressors don't all attempt to start simultaneously.
This can sometimes reduce peak starting demand without sacrificing the building's overall cooling strategy.
Three-Phase HVAC Equipment
Larger commercial HVAC equipment frequently uses three-phase power.
Generator selection must account for:
- System voltage
- Phase
- Load balance
- Motor characteristics
- Starting method
A 100kW generator with the wrong voltage configuration isn't an appropriate replacement for a correctly configured 100kW generator.
Electrical compatibility matters just as much as capacity.
What About VFD Equipment?
Variable-frequency drives are common in modern commercial HVAC systems.
They can provide controlled motor acceleration and operational efficiency, but their electrical characteristics still need to be considered in generator-system design.
Don't apply old motor-starting assumptions blindly to VFD-driven equipment.
Use the manufacturer's specifications and system requirements.
Refrigeration and HVAC Together
Restaurants, grocery operations, convenience stores and other businesses may have both substantial HVAC and refrigeration loads.
During an outage, refrigeration may be mission-critical.
That means generator sizing needs to consider refrigeration compressors alongside building HVAC.
The timing of compressor starts can become important.
Should Every HVAC Unit Receive Backup Power?
Not necessarily.
Some facilities may decide to back up only the HVAC systems serving critical areas.
For example:
- Server rooms
- Occupied office areas
- Medical/storage areas
- Critical production areas
- Temperature-sensitive inventory
Reducing the number of simultaneous backup loads can significantly change generator requirements.
80kW, 100kW, 150kW or 200kW?
There is no universal mapping between generator size and the number of HVAC tons it can operate.
An 80kW generator might be appropriate for one facility while another requires 150kW or more.
The answer depends on:
- HVAC equipment
- Starting characteristics
- Other building loads
- Voltage
- Phase
- Fuel
- Load sequencing
- Required redundancy
Avoid sizing commercial generators using HVAC tonnage alone.
Build the Backup Strategy Before Selecting the Generator
The best commercial generator design starts by answering:
What absolutely must operate when utility power is unavailable?
Once that's established, determine:
- Continuous load
- Motor-starting requirements
- Load sequencing
- Voltage and phase
- Fuel requirements
- Future expansion
Then select the generator.
ACUnitsForLess: HVAC and Backup Power
Commercial generator sizing becomes particularly important when HVAC represents a large portion of the facility's electrical demand.
ACUnitsForLess supplies both HVAC equipment and Briggs & Stratton standby-generator solutions, allowing customers to evaluate backup power with the actual heating and cooling equipment in mind.
If you're considering backup power for commercial HVAC, gather the model information and electrical specifications from your equipment before choosing generator capacity.
That information can prevent an expensive sizing mistake.
Explore Commercial Briggs & Stratton Standby Generators at ACUnitsForLess.
