What is the difference between Kilowatts and Kilowatt-Hours

In the solar energy world, you will often see the terms kilowatt (kW) and kilowatt-hour (kWh). These terms are crucial to understanding energy usage, solar panels, energy savings, and return on investment (ROI). In this article, we will explain this basic terminology and how it is applied when discussing solar power.

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Basic Solar Terms to Know

Power: the rate at which energy is produced or consumed.

Watts (w): measure rate of power. A kilowatt (kW) is 1000 watts. A megawatt (MW) is 1000 kilowatts.

Watt-hour (Wh): measures the number of watts produced or consumed over 1-hour.  A kilowatt hour (kWh) is 1000 watt-hours.

How are watts (W) and Kilowatts (kW) used in the Solar industry?  

In the solar industry, we use Watts (W) and Kilowatts (kW) to describe a panel or solar systems power capacity and maximum output.  

For example, a:

• 450w panel can produce up to 450 watts of power under ideal conditions

• 200kW solar system could produce up to 200kW of power under ideal conditions

What about Watt-Hours and Kilowatt-Hours?

When talking about solar energy, kWh is used to describe how much energy a solar panel or solar system can produce over time.  

In a general example, it would take

• 10 hours for a 100-watt light bulb to use 1 kWh

• 30 minutes for an oven to consume 1 kWh  

For example, if a solar panel continuously produces 1kW of power for a period of 1 hour, the panel will have produced 1 kWh of energy.

• 10 hours of direct sunlight a 450w panel will have produced 4.5 kWh

• 10 hours of direct sunlight a 200kW system will have produced 2,000kWh

What is the difference between kilowatts (KW) and kilo-watt hours (kWh)?

A watt (w) is the measurement of power while a watt-hour (Wh) is the length of time something can use a watt.  When talking about solar panels, kilowatts refers to the amount of power the panel can produce and kilowatt-hours refers to the amount of energy the panel produces over time.  

• 1 Wh = 1 W of power expended for 1 hour of time

• 1 kWh = 1 kW of power expended for 1 hour of time

For example, under ideal conditions a panel rated at 400 watts (0.4kW) could produce:

• 400 watts of power

• 1.2 kWh of energy over 3 hours (0.4 kW x 3 hours)

When talking about an entire solar system, the same applies:

• kW = size and capacity of system  

• kWh = how much energy the system can produce over time.  

For example, under ideal conditions a 200kW solar system could produce:

• 200 kW of power

• 600 kWh of energy over 3 hours (200 kW x 3 hours)

Understanding kWh is crucial in understanding energy savings, ROI, and how solar can help offset the cost of a utility bill.  To get a better understanding of how to read your utility bill, check out our article here.  

Why don’t solar systems always produce their full KW or kWh?

Photovoltaic (PV) solar panels will never produce at full capacity 24 hours a day, due to a range of factors from amount of sunlight to panel degradation.  

Panel KW and kWh outputs are determined under ideal conditions ‒ temperature-controlled labs with perfect lighting and no weather, debris, or shading to obstruct the panels.

In the real world, factors that can affect solar panel output include:

• Hours of Sunlight: The more sunlight a panel receives, the more energy it generates.  

• Panel Orientation: The angle and direction the panel faces affects its output.  

• Shading: Dirt, debris, or other objects that block sunlight can reduce a panel's output.  

• Season: Solar panels produce less energy in the winter when there's less sunlight.  

• Climate: The climate zone where the panel is located affects its output. For example, panels placed in CA tend to be more productive than panels placed in WA.  

• Panel Age: Output of solar panels drops slightly each year with general wear and tear and as the components degrade. For example, a 400W panel with an 80% performance guarantee after 25 years, would only be producing 320W by the end of its lifecycle.    

At TENCO SOLAR, we take all of these considerations into account when designing your custom solar system to ensure that your business receives the largest energy savings, ROI, and offset on your utility bill.

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