Understanding the Difference Between kW and kWh

When working with energy systems, two units constantly come up: kilowatts (kW) and kilowatt-hours (kWh). Though they sound similar, they represent very different things. Understanding this difference is critical for accurate system sizing, utility communication, and energy usage analysis.

kW = Power

Kilowatts (kW) represent power—the rate at which electricity is used or produced at any given moment. It answers the question: "How much electricity is flowing right now?"

• 1 kW = 1,000 watts

• It’s like the speedometer in your car, showing real-time demand or output

• Equipment and system capacities are typically rated in kilowatts

Examples:

• A 2,000-watt hair dryer = 2 kW

• A generator rated at 7.2 kW can supply up to 7.2 kilowatts of power

kWh = Energy

Kilowatt-hours (kWh) represent energy—the total amount of electricity used or produced over time. It answers the question: "How much electricity was used or generated?"

• 1 kWh = using 1 kW for 1 hour

• It’s like the odometer in your car, showing cumulative usage

• Your monthly utility bill is based on kWh

Examples:

• Running a 2 kW space heater for 3 hours = 6 kWh

• A home that uses 900 kWh in a month is averaging ~1.25 kW per hour

How to Convert kW to kWh Using a Usage Factor

To estimate energy consumption from a known power value (kW), you can use the following formula:

Annual kWh = kW x Usage Factor x Hours

• kW: The connected or diversified load (from NEC calcs or equipment ratings)

• Usage Factor: A percentage representing how often the load is actually in use (e.g., 0.15 = 15%)

• Hours: Total time period in hours (e.g., 8,760 hours per year)

Example: Let’s say a building has a calculated load of 50 kW, and a typical usage factor of 15% throughout the year:

Annual kWh = 50 kW x 0.15 x 8,760 = ≈ 65,700 kWh/year

This gives a reasonable estimate of annual energy consumption based on load and usage patterns.

Why It Matters

• System Sizing: Equipment is rated in kW, but total consumption is measured in kWh

• Utility Requests: Utilities often ask for estimated annual kWh to assess load impacts

• Load Calculations: NEC load calcs are in VA or kW, not kWh—conversion is required to estimate energy use

Quick Analogy

To help clarify the difference between kW and kWh, think of your car:

• kW (Power) is like your speedometer. It tells you how fast you're going right now—how much power is being used or produced at any given moment.

• kWh (Energy) is like your odometer. It tells you how far you've driven over time—how much total energy has been consumed.

So, just as speed and distance are related but not the same, power (kW) and energy (kWh) are connected but measure different aspects of electrical usage.

Conclusion

Kilowatts (kW) tell you how much power you're using or producing right now. Kilowatt-hours (kWh) tell you how much energy you've used over time. Both are essential—but they serve different roles.

At Empower Engineering, we help contractors, engineers, and facility managers understand and apply these concepts in real-world projects. Whether you're estimating usage or sizing systems, we're here to help.

Disclaimer:
The information, tools, and resources provided on this website are for general informational purposes only. While Empower Engineering strives to ensure accuracy and relevance, no guarantee is made regarding the completeness or applicability of the content to specific projects. Users are responsible for verifying all calculations, code interpretations, and design decisions with licensed professionals and local authorities. Empower Engineering assumes no liability for any loss, damage, or code violation resulting from the use of this website’s content.

Empower Engineering | July 15, 2025