kVA vs kW: Apparent Power vs Real Power Explained
Understand the difference between apparent power (kVA) and real power (kW). Learn how power factor affects your generator sizing and electrical loads.
kVA vs kW: The Core Difference
In AC (alternating current) power systems, kVA (kilovolt-amperes) and kW (kilowatts) are both units of power, but they measure different aspects of it. The confusion often arises when sizing generators or transformers, where ratings are typically given in kVA rather than kW.
- kVA (Apparent Power): This is the total power that the system is carrying. It is the product of volts and amps without factoring in efficiency.
- kW (Real Power): This is the actual power converted into useful, working output (like heat, light, or mechanical motion).
The Golden Rule: kVA (apparent power) is always greater than or equal to kW (real power) because kVA represents the total power the system carries while kW represents power actually converted to useful work.
Power Factor: The Missing Link
The difference between kVA and kW is governed by the Power Factor (PF). Power factor is a ratio (from 0 to 1) that represents the efficiency of the electrical system.
The relationship is simple: kW = kVA × Power Factor.
In purely resistive loads (like an electric heater or incandescent bulb), the power factor is 1.0, meaning 1 kVA = 1 kW. However, for inductive loads (like motors, compressors, and transformers), the power factor is lower (typically around 0.8), meaning some power is lost as reactive power.
Worked Example: Sizing a Generator
Generator sizing is the most common scenario where confusing kVA and kW leads to problems. Most backup generators are rated in kVA with an assumed power factor of 0.8.
Suppose you have calculated that your home appliances and well pump require a total of 8,000 W (8 kW) of real power to run. You see a "10 kVA" generator on sale and assume it will provide 10 kW of power. Will it work?
Using the formula kW = kVA × PF, and assuming a standard 0.8 power factor for the generator:
10 kVA × 0.8 = 8 kW.
The 10 kVA generator provides exactly 8 kW of usable real power. It will run your loads, but it will be at 100% maximum capacity with no headroom. If you hadn't known the difference between kVA and kW, you might have assumed you had 2 kW of spare capacity.
Summary
Always remember that kVA is the total electrical requirement of a circuit or device, while kW is the amount of work you get out of it. When sizing equipment, ensure you are matching the kW requirements of your loads to the kW output of your source, not just the kVA rating.