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
- kW vs. kWh
- How Utilities Charge for Energy
- Why are solar systems sized in kW when they produce kWh?
Energy vs. Power
In order to understand what makes kilowatts different from kilowatt-hours, we need to know the difference between energy and power.
Energy and power may be similar words in everyday language, but in physics, they have important differences.
Energy is a measurement of the amount of work that can be done by a force. You might think of it as the total amount of effort it would take to do something, like lift a heavy rock or run a marathon.
Power is a measurement of how much energy is being used at a given time, or the rate of energy use.
Let’s imagine you’re a very strong person able to quickly lift a rock so heavy that it took you the same amount of energy as running a marathon. While the energy expended would be the same whether you ran a marathon or lifted the rock, lifting the rock would have greater power. This is because lifting the rock 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 at any given moment.
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. A 100 W panel will produce about 100 W of power in ideal conditions, 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.” Inverters 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.