Nominal capacity is the maximum output or storage a device can deliver under a specific set of standard conditions, as stated by its manufacturer. Think of it as the “on paper” number: the baseline performance you’d see if temperature, load, and other variables all lined up perfectly. It appears most often on battery spec sheets and HVAC equipment ratings, but the core idea is the same everywhere it’s used.
How Nominal Capacity Works for Batteries
For batteries, nominal capacity is measured in ampere-hours (Ah) or milliampere-hours (mAh). It represents the total electric charge a brand-new, fully charged battery can deliver when discharged under controlled conditions: typically a temperature of 25°C (77°F) and a standard discharge rate of 1C (meaning the battery is fully drained over one hour). The manufacturer lists this number on the data sheet as a promise of what the battery will provide at the start of its life.
Three variables determine the number:
- Discharge current: A battery discharged slowly (at a lower current) delivers more total charge than one discharged quickly. This is why manufacturers must specify the discharge rate alongside the capacity figure.
- End voltage: The cutoff voltage at which the battery is considered “empty” affects how much charge is extracted.
- Temperature: Standard testing is done at 25°C. Real-world temperatures shift actual capacity up or down from the nominal value.
Because the number depends on these test conditions, nominal capacity is application-specific. The same physical cell could carry a different nominal rating if tested under a different standard.
Nominal Capacity vs. Actual Capacity
Nominal capacity is a fixed reference point. Actual capacity changes constantly depending on how the battery is used, how old it is, and the environment it operates in. A lithium-ion battery sitting in a hot warehouse at 70°C, for example, degrades roughly three times faster than one kept at moderate temperatures. At extreme heat (100°C), researchers have measured capacity dropping by nearly 39% in just two charge-and-discharge cycles.
This gradual loss over time is called capacity fade. As a battery ages, its maximum available capacity shrinks below the nominal figure. That’s why state of charge (SOC), which tells you how much energy is left in a battery, can be calculated two different ways. The simpler method divides remaining capacity by the original nominal capacity. A more accurate method divides remaining capacity by the battery’s current maximum available capacity, which accounts for aging and real-time conditions. The first method is easier to compute; the second gives you a truer picture of how much runtime you actually have left.
Nominal Capacity in Electric Vehicles
EV battery packs add another layer to this concept. The total nominal capacity of an EV battery is the full energy the pack can theoretically store, measured in kilowatt-hours (kWh). But automakers don’t let you access all of it. They reserve a small buffer at the top and bottom of the charge range to protect battery longevity, prevent damage from full depletion, and maintain consistent performance.
The portion you can actually use is called “usable capacity,” and it typically falls between 95% and 99% of the total physical capacity, depending on the manufacturer. So if an EV is advertised with a 75 kWh battery, its usable capacity might be closer to 71 to 74 kWh. This is one reason real-world driving range sometimes comes in slightly below official estimates, even on a brand-new vehicle.
Nominal Capacity in HVAC Systems
The term shows up in heating and cooling equipment too, especially chillers and air conditioners. Here, nominal capacity refers to the cooling output at “typical” conditions, measured in tons. One ton of cooling capacity equals 12,000 BTUs per hour, a unit based on the energy needed to melt one ton of ice in 24 hours.
For an air-cooled chiller, the standard test conditions (set by the industry group AHRI) assume 44°F leaving water temperature, a 10°F temperature difference across the system, and 95°F outdoor ambient temperature. A “100-ton chiller” is rated to remove that much heat per hour under those assumptions. In practice, there is no universal rule that locks manufacturers into identical test conditions. Each company defines nominal tonnage slightly differently, which means two chillers labeled at the same nominal tonnage may perform differently once installed. The actual cooling output shifts with outdoor temperature, humidity, water flow rates, and building load, so the real tonnage on any given day is almost never exactly what’s printed on the nameplate.
Why the Nominal Number Matters
Nominal capacity exists to give engineers, buyers, and consumers a common reference point for comparing products. Without it, you’d have no standardized way to tell whether a 3,000 mAh phone battery from one brand is comparable to a 3,000 mAh cell from another, or whether a 5-ton rooftop AC unit is sized correctly for your building. International standards, like the IEC 61960 series for lithium batteries, formalize the testing conditions so that rated capacity figures are at least roughly comparable across manufacturers.
The key takeaway is that nominal capacity is always a best-case, controlled-environment figure. Real performance will vary based on temperature, usage patterns, age, and load. When you’re comparing products, the nominal number is a useful starting point, but understanding the gap between that number and real-world performance is what helps you make a better decision.