What Is the Difference Between kW and kWh?

When discussing solar, many people get confused by the different units of power and energy that are used to measure: the size of their solar system, the amount of energy it will produce annually, and how it will offset the amount of energy they are using.

To help simplify things, this article discusses the difference between two similar units that are often confused: kilowatts, abbreviated as “kW,” and kilowatt-hours, abbreviated as “kWh."

 

Energy vs. Power

In order to understand what separates kilowatts from kilowatt-hours, we need to first outline the difference between energy and power.

Energy and power may be similar words, but in physics, they have important differences. To understand the difference between kW and kWh, it’s necessary to understand these differences.

 

Energy is a measurement of the amount of work that can be done by a force. You might think of it as the amount of effort it would take to lift a heavy weight or run a marathon. 

However, there’s a difference between lifting a heavy weight and running a marathon. One is done quickly with a lot of effort, while the other is done over a longer period of time with a lower amount of effort at any given time. 

 

Power is a measurement of how much energy is being used at a given time, or the rate of energy use.

Let’s imagine that you’re a very strong person able to instantly lift a weight so heavy that it took the same amount of energy to lift as it took you to run a marathon. While the energy expended would be the same whether you ran a marathon or lifted the weight, if we measured the power of both actions, lifting the weight would have greater power. This is because lifting the weight took all the energy and compressed it into a very short amount of time, meaning that the rate at which the energy was used was much higher.



kW vs. kWh

Now that we know the difference between energy and power, it’s fairly easy to understand the difference between kilowatts and kilowatt-hours.

Kilowatts are a measure of power–they measure how much energy is being used at a certain time. For instance, if you ran ten 100 W lightbulbs in your home at the same time, you would be using one kilowatt of power.

Kilowatt-hours, on the other hand, are a measure of energy. Imagine you ran your ten 100 W lightbulbs for one hour. Congrats, you’ve used one kilowatt-hour of electricity! 

 

1,000 W x 1 hour = 1,000 wH or 1 kWh

 

Now imagine that you ran only five 100 W bulbs, but you left them on for two hours. Because it takes the same amount of energy to provide 500 W for two hours as it takes to provide 1000 W for one hour, you’ve used another kilowatt-hour of electricity!

 

500 W x 2 hours = 1,000 wH or 1 kWh



How Utilities Charge for Energy

Energy companies charge you for energy using a measurement of energy: a kilowatt-hour. Each  month, you’ll see that your electric meter tracked how many kWh of energy you used. The power company then charges you a certain rate for each of these kWh.

According to the US Energy Information Administration, the average cost for electric energy in the US was 13.72 cents/kWh in 2021. When the cost of a DIY solar install through Project Solar is divided by the number of kWh it produces throughout its lifetime, each kWh can cost as little as 4 cents.

 

Why are solar systems sized in kW when they produce kWh?

Just as household devices are sized by how much power they use at any given time (like a 100 W lightbulb), solar panels are sized by how much power they produce in full sunlight. A 100 W panel will produce about 100 W of power when the sun shines directly on it, a 200 W panel about 200 W of power, etc.

Solar systems are sized for how much DC (direct current) energy they are rated to produce in full sunlight. This is called the “DC system size.” Our microinverters convert this DC energy to usable AC (alternating current) energy.

How much energy a certain system will produce depends on its location, angle and orientation towards the sun, and the shade around the panels. 

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