kW vs. kWh

Understanding the difference between kW and kWh helps consumers choose the right solar panel

When shopping for solar panels for your home, you’ll come across the terms kilowatts (kW) and kilowatt-hours (kWh). While these seem like technical terms that only electricians need to know, they’re important to understand if you’re planning to go solar.

Key insights

  • Kilowatts are measurements of energy flow. A kilowatt is 1,000 watts.
  • A kilowatt-hour is how much energy can be collected or used steadily for an hour.
  • A 5-kW solar system, for instance, is capable of producing 5 kilowatts of power under optimal sunlight conditions.
  • Your monthly electric bill charges a rate based on how many kWh of energy you used during the previous month.

Kilowatt and kilowatt-hour: understanding the difference

“When a homeowner decides to go solar, we find that they usually do their research online and often get extremely confused over the difference between kWh and kW,” Brian Decker, CEO of SOAR Energy, explained.

It doesn’t need to be confusing, though. When you break it down, kW and kWh aren’t interchangeable, but they are closely related.

“You can think of kilowatts as a speedometer, measuring the instantaneous power being used, and kilowatt-hours as an odometer, representing the total energy consumed over time,” according to Phil Roth, technical sales lead at Lumin, an energy solution provider.

Watts are the measurement of energy flow, while kilowatt-hours express how much energy can be collected or used for an hour. Your monthly electric bill is calculated based on how many kWh of energy you used during the course of a month, for example.

What are kilowatts and kilowatt-hours used for?

Understanding kilowatts and kilowatt-hours are important when determining the energy requirements for powering your home, especially when considering a solar energy system.

Decker explained the relationship between kW and kWh in a solar system this way: If you have a 10-kW solar panel system, it will produce approximately 10 kWh of energy if it runs for one hour in optimal conditions. However, actual energy production fluctuates due to factors like cloud coverage, the angle of the sun, panel orientation and other factors.

The kW rating will give you an idea of how much power the system can produce at any given moment under ideal conditions, and the kWh will give you an idea of how much energy it can produce over a certain period, which will vary. This can help you determine whether a particular system can produce enough energy to meet your home's energy needs.

For example, as Roth explained, a 7-kW system might produce at its maximum capacity of 7 kW from 12:00 p.m. to 2:00 p.m., resulting in 14 kWh of bill savings during that period. However, outside those hours, when sunlight intensity is lower, the system may produce at a reduced level (e.g., 3 kW from 8:00 a.m. to 12:00 p.m. and 2:00 p.m. to 6:00 p.m.). This would contribute an additional 24 kWh of bill savings.

If your panels generate more electricity than your home consumes, the excess kilowatts can be stored in a solar battery for later use. This lets you harness solar energy even when the sun isn’t shining, such as during the evening or on cloudy days.

The battery system manages the discharge of stored kWh based on predetermined settings and energy demands. It can be programmed to prioritize essentials during a power outage or to optimize self-consumption by using stored kWh when electricity rates are higher. “By strategically deploying stored kWh, homeowners can reduce reliance on the grid, minimize peak-hour energy costs and maximize the utilization of their solar energy system,” Roth told us.

How to calculate kilowatt vs. kilowatt-hour

Put simply, a kilowatt is equal to 1,000 watts. You can divide watts by 1,000 to find the equal number of kilowatts.

For kilowatt-hours, you can use this equation: kW x time = kWh. So, if you’re using a 100-watt appliance for 10 hours, that’s 1 kWh. If you use a 1,000-watt appliance for one hour, that’s 1 kWh. The higher your appliance wattage, the quicker your energy usage equals a kilowatt-hour.

When you’re going solar, the main thing you need to know is how many kilowatts your home uses so you can purchase enough solar panels (you should be able to see exactly how many kilowatts your home uses per month on your electric bill). Decker advises analyzing 12 months of your energy usage to determine which solar panels are right for you.