Mastering the Basics: Understanding kW vs. kWh in Solar Energy

When setting up a solar system, you’ll see kilowatts (kW) and kilowatt-hours (kWh) ratings. The two measurements look similar. However, one measures the size of your system in terms of electricity storage, while the other measures power output over time. Both of these are relevant factors to your purchasing decision.

The amount of electricity your system can store and the maximum power output your system can output are related. Still, kW and kWh are different measures that significantly impact your solar home planning. 

Once you understand the difference between the two, it’s easier to determine what you need for your home and what you can expect from the system you purchase.

kW (kilowatt)

A kilowatt is the unit of measurement for power output in your electrical system. One kilowatt is 1,000 watts; watts, in turn, are the product of volts multiplied by amperes. Because the number of watts the average generator or battery backup system can output is so high, manufacturers often measure power output in kilowatts rather than watts. 

A solar generator or battery backup system’s AC output rating describes the maximum amount of electricity it can provide instantaneously. 

Watts or kW can also measure how much electricity a solar panel can capture under ideal conditions. Solar panels typically have a rated power capacity in watts. EcoFlow solar panels have power ratings ranging from 100W to 400W. 

It’s crucial to remember that this is the maximum amount of solar energy a solar panel can capture in direct sunlight, at precisely the right angle, among other determining factors. It is rare for a solar panel to deliver 100% of its rated power — at least not consistently.

In a solar energy system, the amount of electricity it can generate primarily depends on the number of solar panels, their rated power and efficiency, and environmental conditions.

As the photovoltaic cells in solar panels capture sunlight, it converts the sun’s rays into direct current (DC) electricity. The more solar panels you have — and the higher their rated power and efficiency — the more energy you can capture. 

However, remember that rated power is not a guarantee or a constant. Environmental factors such as shading over your panels, the angle relative to the sun, and the hours of peak sunlight will all impact the overall amount of electricity your panels provide.

Also, the number of solar panels you can add to your array is not infinite with solar generators. Every solar generator has a maximum capacity for converting and storing the DC power your panels capture. Typically this is expressed as the solar charging capacity. 

For example, EcoFlow’s EcoFlow DELTA 2 portable power station has a maximum solar charging capacity of 500W. That means you can connect up to two 220W bifacial portable solar panels, equalling 440W of rated power. If you connect additional solar panels to your array, the power they collect will potentially go to waste. Depending on the type of solar power system you use, connecting more panels than your setup is built to handle can damage your solar batteries and other balance of system components. It can even be dangerous.  

To select a solar power system that provides enough power to meet your home’s electricity needs, consider the following crucial factors:

• Number of watts or kW your solar panels can capture (Rated Power x Number of Panels)
• kW of DC power your system can safely convert and store (Solar Charging Capacity)
• Maximum electricity output in watts or kW (AC Power Output)

Manufacturers will express all of the above in watts or kilowatts.

kWh (kilowatt-hour)

Watts and kilowatts measure the amount of energy solar panels can capture and the maximum AC output of a solar power system. Watt-hours or kilowatt-hours (kWh) measure energy production over time. 

The formula is simple:

Kilowatt Output x Hours of Operation = Kilowatt Hours (kWh)

With solar energy systems, portable power stations, and solar generators, kWh is most often used to measure electricity storage capacity. 

Whether you’re using a DIY solar power system with a solar battery or a portable power station/solar generator with the battery built-in, the capacity with be measured in wH or kWh. 

For example, the EcoFlow DELTA Pro has 3.6kWh of storage capacity out of the box and is expandable to 25kWh.

One benefit of a top-of-the-line portable power station like the EcoFlow DELTA Pro is its expandable capacity. It can integrate with your household wiring to provide a Whole Home Backup Power Solution. 

With up to 3200W of solar charging capacity, the EcoFlow DELTA Pro can also be the foundation of a Whole Home Generator that can keep your house running off-grid indefinitely.

Differences between kW and kWh

The most obvious difference between kilowatts and kilowatt-hours is that kW measures electricity, while kWh measures electricity expanded over time.

As outlined above, with solar generators, kW measures maximum electricity output and generation capacity, and kWh measures the maximum amount of electricity you can store.  

When choosing which solar power solution is right for your home, you should determine the total wattage of the appliances, HVAC systems, etc., that you want to operate simultaneously. Make sure you account for surge power to get devices up and running.  

Here’s how to do it.

1. Identify the wattage requirements of your appliances. Survey the starting and running wattage requirements of the appliances and devices you plan to plug into the generator. You can usually find the wattage requirements labeled on the appliance, but we’ve also compiled the starting and running watts of typical household appliances in the table below. 
2. Convert volts/amps to watts. If your appliance’s power requirements are in volts or amps, you can calculate an appliance’s running watts with this equation: 

Volts (V) x Amps (A) = ​​Watts (W)  

3. Count the running watts of your appliances. Add up the running watts of the appliances you plan to use — does the total exceed the running watts listed on your generator? If so, consider buying a generator with more output capacity.
4. Factor in starting watt requirements. Identify the appliance with the highest starting wattage. Add that appliance’s starting wattage to the running wattage total.
5. Calculate the sum. That final number is the total starting watts you need from your generator.  As discussed above, to avoid overloading your generator, do not exceed its starting watts rating.
   

Starting and Running Watts of Typical Household Appliances 

Identify how much electricity you need (kW) and for how long (kWh).  Only then can you make an informed purchase decision of a solar power system that’s right for you.

Conclusion

Kilowatts (kW) and kilowatt hours (kWh) sound similar. The metrics are related but different. 

Kilowatts (kW) measure electricity output, generation, and operating requirements by wattage. 

Kilowatt hours (kWh) measure electricity consumption and storage over time. 

Before buying a solar generator or home backup battery system, familiarize yourself with both kW and kWh. Having a working knowledge of each unit of measure should not only inform your purchase decision but also serve you well in monitoring and optimizing your system’s performance once it’s up and running.

Check out EcoFlow today for off-grid and home backup solutions for you and your family. 

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