Short answer
Retail RTU tonnage comes from a load calculation, not floor area alone. Lighting, occupancy, outside air, glazing, roof insulation, and local design temperatures together decide how many tons a sales floor needs.
What does "tonnage" actually measure?
Tonnage is a rate of heat removal. One ton equals 12,000 BTU per hour, so a 15-ton rooftop unit is nominally rated to pull about 180,000 BTU out of a space every hour at standard rating conditions.
Real-world capacity shifts with outdoor temperature, indoor humidity, and airflow, which is why a unit that is "big enough" in a mild coastal store can struggle on the same floor plan in an inland valley or desert site.
Related terms, from ton and BTU to sensible and latent load, are defined in the commercial HVAC glossary.
Why are square-foot rules of thumb misleading?
Most facility teams have heard a figure in square feet per ton. Those shortcuts were never designs; they are averages of buildings that may share nothing with yours.
Two 12,000-square-foot stores can have very different loads because the things that generate heat inside a store are not proportional to its floor area.
A proper commercial load calculation, such as one following ACCA Manual N, adds up each source of heat and moisture at the local design conditions, then sizes equipment to that peak. Rules of thumb remain useful for one thing: spotting a number that looks wrong and deserves a second look.
Which loads drive a retail sales floor?
Five loads drive most retail sales-floor sizing: lighting and display power, people, outside air for ventilation, solar heat through the roof, and glazing at the storefront.
- Lighting and display power. Every watt of lighting becomes heat in the space. Track lighting, illuminated signage, and display screens add up, and LED retrofits can substantially reduce this load.
- People. Shoppers and staff add both sensible heat and moisture. Peak occupancy on a holiday weekend is a different design case from a weekday afternoon.
- Outside air. Ventilation air must be cooled and dehumidified from outdoor conditions. In hot, humid hours this can be one of the largest single loads on the unit.
- The roof. Single-story retail has a large roof area relative to its floor. Insulation level, roof color, and the condition of the roof membrane all affect how much solar heat reaches the space.
- Glazing and doors. Storefront glass facing west, and entry doors that stand open during trading hours, bring in heat and unconditioned air.
- Equipment. Refrigerated cases, ovens, coffee equipment, and back-of-house servers each add or remove heat depending on how they are vented.
How do climate zones change the answer across Southern California and Arizona?
Climate changes the tonnage. The same store layout calculates to different capacities in coastal Orange County, the Inland Empire, and the Phoenix area, because each sizing starts from local design temperatures and humidity.
Design conditions are set from local weather data. A coastal Orange County store, an Inland Empire store, and a Phoenix-area store with the same layout will calculate to different tonnages, and the desert store must also plan for long runtimes at very high outdoor temperatures, where condenser performance drops. Arizona's monsoon season adds a humid period to a climate that is otherwise dry, which affects latent (moisture) load as well as sensible load.
This is one reason a single equipment standard rarely fits a whole portfolio. The sizing logic should be consistent across sites; the result often should not be.
What goes wrong when a unit is oversized or undersized?
An undersized unit runs continuously on peak days, cannot hold setpoint in the afternoon, and generates comfort complaints that are easy to misread as a repair problem.
An oversized unit is subtler: it satisfies the thermostat in short bursts without running long enough to remove moisture, so the store feels cool and clammy, and the frequent starts wear the compressor and electrical components.
Staging helps. Units with multiple compressors or variable-capacity compressors can match part-load conditions, which are far more common than design peak hours.
When should an existing store be re-checked?
Re-check sizing whenever the use of the space has changed or a unit is coming up for replacement:
- A lighting retrofit or a new fixture package has been installed.
- The floor plan was reset, a stockroom was converted to sales floor, or a space was demised.
- Kitchen, grooming, or process exhaust was added or removed, changing the outside air the RTUs must condition. See make-up air and exhaust balance in retail stores.
- One zone runs constantly while the neighboring zone short-cycles, which often points to a mismatch between units and the areas they serve.
Replacement is the natural moment for this check, because the curb, crane, and electrical work are already being scheduled. The repair-or-replace decision factors article covers how sizing fits into that wider decision.
How does staging affect the sizing decision?
Peak load sets the maximum capacity a unit needs, but a store spends far more hours at part load than at peak.
A unit sized correctly for the hottest afternoon may spend most mornings, evenings, and cooler months at a fraction of that. Two-stage, multi-compressor, and variable-capacity designs let the unit step down to match those hours, which improves humidity control and reduces cycling. When comparing replacement options of the same nominal tonnage, ask how each one performs at part load, not only at its full rating, and how its staging is controlled.
What information speeds up a sizing review?
Bring the existing unit nameplates, the zone each unit serves, floor plans with current fixture layouts, the lighting type, store hours, and any comfort complaints by zone and time of day.
Twelve months of service history is useful too, since repeated high-pressure trips or frozen coils can reveal airflow or sizing issues that the nameplate will not.
Replacement is the time to review sizing; we can look at it with you as part of a rooftop unit installation and replacement, so the new equipment is matched to the store as it trades today. For background on the equipment itself, start with how a commercial rooftop unit works.
Most of that information should already sit in the site's records; building an HVAC asset register sets out how to capture it.
Methodology: Written by REE Mechanical's team from field experience on multi-site commercial portfolios; standards cited link to the issuing body.