What Is a Kilowatt-Hour?

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What Is a Kilowatt-Hour?

You might hear the term “kilowatt-hour” occasionally in daily life, although it’s more likely in specific situations. 

For instance, you might see the term on light bulbs, cooking appliances and other household items. But you’ll certainly see it on your solar power equipment — specifically on its meters.

If you’re a solar user, it’s important to know what a kilowatt-hour (kWh) is. That knowledge helps you understand how much energy your panels generate and how much energy you consume, which hopefully will help you lead a more energy-responsible life. 

[Related: Tips To Save on Energy Costs: Solar Energy Savings and More]

What Is a Kilowatt?

Put simply, a kilowatt (KW) is the main unit we use to measure electrical energy. The root word, “Kilo” means “thousand” — so KW is 1,000 times greater than a single watt.

A Little Background

We can credit the watt (W) unit to Scottish scientist James Watt, who invented the measurement in 1786.

Watt was an instrument maker and inventor whose improvements on Thomas Savery’s 1712 steam engine led to the Watt steam engine in 1776. It was a major milestone in the Industrial Revolution. Watt also patented an early steam locomotive and a double-acting engine. 

When Watt was repairing the Newcomen steam engine in 1764, the amount of steam it wasted surprised him. To lessen the loss, he devised a separate condenser — his first and most significant invention. Watt realized that latent heat loss was the main problem with the Newcomen engine. (Latent heat is heat that alters a substance’s state, such as liquid to air and solid to liquid.) Furthermore, he affected the condensation in the engine’s chamber (which is distinct from the engine-connected cylinder). 

Later, Watt signed a patent: “A New Invented Method of Lessening the Consumption of Steam and Fuel in Fire Engines.”

Thanks to Watt’s substantial scientific contributions, the watt became a unit of power in the International System of Units (SI), equaling one joule of work performed per second.

However, we use KW instead of W when measuring electrical energy because of the sheer volume of electricity that appliances need. One watt is a tiny amount of energy in comparison, so it makes perfect sense to use KW when measuring lots of energy. 

[Related: 50 Solar Power Facts]

Kilowatt vs. Kilowatt-Hour: What’s the Difference?

A kWh is a non-SI unit of energy. That is, it’s not in the International System of Units (i.e., the metric system) and includes a quantity of energy’s direction and magnitude. 

For clarity, SI units include the following:

  • Length — meter (m)
  • Time — second (s)
  • Amount of substance — mole (mol)
  • Electric current — ampere (A)
  • Temperature — kelvin (K)
  • Luminous intensity — candela (cd)
  • Mass — kilogram (kg)

A non-SI unit would measure these factors:

  • Velocity
  • Force
  • Momentum 
  • Magnetic field

So, what is a kilowatt-hour in concrete terms?

Because we know 1 KW is 1,000 times bigger than 1 W, we can now compare 1 KW to 1 kWh. One kWh equals 1 KW of power in one hour’s time. In SI units, 1 kWh equals 3.6 megajoules.

So in other words, we can measure the KW of energy that a device produces or consumes within one hour’s time using the kWh. 

Solar Power and Kilowatt-Hours

When billing electrical energy, we most commonly use kilowatt-hour. So when reading your solar electric bill from your utility, you can expect to see the abbreviation “kWh.” Plus, utilities sometimes use metric prefixes for watt-hour (Wh) multiples and submultiples.

But how do KW and kWh apply to various appliances and items in (and on) your household, such as a solar panel system? 

As far as a solar company is concerned, the nameplate value representing a solar system’s size is in KW. For example, a 6-KW solar panel system could have 20 solar modules, each at 300 KW. You’d divide 300 KW by the number of modules (20) and end up with 6 KW. 

In theory, your 6-KW solar system should produce 6 KW of solar power at any given moment (with enough sun exposure, of course). 

Note. Check out our solar potential calculator to determine how much solar energy you could produce from panels based on your electricity bill amount and level of house shading. 

Solar Panel Systems: KW vs. kWh

However, the kWh number that a solar company assigns a solar panel system is a bit different than a KW rating. That’s because kWh measures the amount of energy a device uses or produces during one hour. 

For example, a 100-W light bulb that’s on for 10 hours consumes 1 kWh of energy. When measuring electronics’ energy consumption or generation, we take the wattage of the item or system and then multiply it by the amount of time (in this case, the number of hours) that it was on.

So, if a solar panel system is valued at 6 KW, it should produce around 6,000 kWh of solar power in one year. The amount of energy depends on the system’s efficiency rating, age, performance level, location, orientation and sun exposure. 

For example, if you live in one of the sunniest U.S. states (like Florida, Colorado or Texas), you could reasonably bump up that prediction around 1.5 times. You’d expect 7,500 kWh of solar power production. 

Note: If you want to go solar but are unsure which questions to ask, remember that a system’s size differs from its expected energy production. 

Consider asking your utility provider what your kWh usage is for one year. Then, you can take that number to your solar provider. Energy consultants there will use the kWh number to determine the number of panels you need per your energy consumption and geographic location. 

[Related: Five Best and Worst U.S. States for Solar in 2021]

Go Solar With Freedom Solar Power

If you’re ready to join the green energy revolution, contact solar power installers Freedom Solar. 

We provide SunPower solar panels, the highest-rated panels on the market, as well as backup batteries for homes and other backup power solutions to ensure you meet your energy needs and goals.

Freedom Solar also has it’s own solar repair and maintenance service department if needed. Our experienced team can maintain, repair and upgrade any system for maximum energy production and efficiency

Call (800) 504-2337, or complete our online inquiry form. One of our energy consultants will be in touch!


Featured image via Pixabay

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Willie Nelson for Freedom Solar

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