Kilowatt hours explained: Your essential energy guide

kW and kWh are often used interchangeably. And while they're similar, they measure different things. To understand the distinction between these two terms, you need to start with the most basic unit of energy: the watt. 

A watt is the measure of power1. In other words, watts are the rate at which energy is produced or consumed at any given moment. A good way to understand this is by considering a 100 W lightbulb2 and a 60 W lightbulb. The 100 W lightbulb uses energy at a higher rate than the 60 W lightbulb, which is dimmer and has a lower wattage.

Watts can easily be converted to kWs: 1,000 watts is one kW. While watts and kWs are useful, they're only a snapshot of energy usage. And that's where kWhs come in. This is a measurement of energy usage over a period of time.

In other words, kWh is the measurement of the amount of power a device or appliance needs in order to run for an hour. One kilowatt-hour (1 kWh)3 is equivalent to a power of 1 kW being used for 1 hour. kWh takes into account how many watts are used and for how long. In the case of your electric bill, you're billed for the amount of electricity consumed in kWhs. 

To simplify all this, here are some easy-to-use formulas4 that illustrate the relationship between these different energy units:

Watts to kilowatts

watts/ 1,000 = kWs.

Example: A dishwasher of 1,200 watts could also be written as 1.2 kilowatts.

Watts to watt-hours 

watts x time in hours = watt-hours 

Example: The same dishwasher of 1,200 watts that runs for an hour will use 1,200 watt-hours.

Kilowatts to kilowatt-hours

kilowatts x time in hours = kilowatt-hours

Example: The same 1.2 kW dishwasher which runs for an hour will use 1.2 kWhs.

Understanding kWhs is vital for calculating electricity usage. As an example5, take a 1.5 kW hair dryer. Even though it's a 1.5 kW appliance, the amount of energy it uses depends on how long it's used for. And that's where kWhs come in. If used for an hour, the hairdryer will use 1.5 kWhs of energy. But let's say you only use it for 15 minutes; it will use a quarter of the amount of energy. The same thinking applies to power-generating technologies like solar panels. If, for example, the solar panel has a power rating of 500 watts, it will, under ideal conditions, generate 0.5 kWhs in an hour and 1 kWh in two hours.

When it comes to measuring energy consumption, your utility bill might be the best and most obvious place to start. There, you should find a breakdown of your energy consumption. Most bills include a Usage or Energy Consumption section6, which details the kWh usage for the specific billing period. Of course, there's more you can do to further measure and understand your usage. 

This could include installing an electricity usage monitor7. This will help you see how many watts a specific device uses. If you want to know how many kWhs that device is using, leave the monitor plugged in for a specific period of time, like an hour, for example. Another option is the US Department of Energy's8 appliance and electronic energy use calculator. This enables you to estimate your annual energy use and the cost of operating specific appliances. By entering the wattage of your appliance, you'll get an accurate estimate of this device's energy footprint. 

In addition, there are several formulas for calculating daily and annual appliance usage. 

Formula for calculating daily appliance usage:

(Wattage × Hours Used Per Day) ÷ 1000 = Daily Kilowatt-hour (kWh) consumption

In order to calculate the daily energy usage for an appliance, multiply its power consumption in kilowatts by the number of hours it's used per day.

Formula for calculating annual appliance usage:

Daily energy usage x number of days it's used per year = annual energy usage.

This involves multiplying the daily energy usage of an appliance by the number of days it is used in the year. The other thing to keep in mind when calculating annual usage is that all appliances have energy usage labels that provide information about their energy consumption. Typically, these energy guide labels also estimate the annual electric usage and cost for the appliance. 

The US Department of Energy9 also provides data on how much energy different appliances use and estimates of their annual costs. Here are some examples from the US Department of Energy of common household appliances and their energy consumption:

Refrigerator: 100 to 400 watts

Air conditioner: 500 to 3000 watts

Washing machine: 500 to 1,500 watts

Laptop computer: 30 to 180 watts

LED light bulb: 5 to 20 watts

Formula for calculating the annual cost to run the appliance:

Annual energy consumption × utility rate per kWh = annual cost to run appliance 

Energy efficiency is a good place to start when trying to reduce one's energy bills. A recent study from the International Energy Agency10 found that in countries with energy efficiency standards and labeling (EES&L) programs, the energy efficiency of new appliances can increase by as much as two or three times. Which over a 20-year period can amount to an energy savings of as much as 10-30%. By upgrading to energy-efficient appliances with Energy Star labels, you have an opportunity not only to reduce energy consumption but also benefit from considerable energy savings. 

In addition to switching to energy-efficient appliances, it's also a good idea to unplug unused devices. When not in use, appliances still consume power, contributing to your energy consumption. The same goes for lights. It's important to turn off lights when you're not in the room. Other energy efficiency measures include insulating your home and adjusting your thermostat settings in summer and winter to conserve energy. 

Consider switching to solar

Making energy efficiency swaps isn't the only way to save money and care for the planet. By switching to solar11, you can save on your monthly utility bill because solar power allows you to generate your own electricity from sunlight. This means that you'll be relying less on traditional energy sources provided by utility companies, leading to reduced consumption of grid-based electricity. In many cases, excess solar energy generated during sunny periods can even be fed back into the grid, potentially earning you credits or payments from utility companies in a process known as net metering.

Our expert Solar Advisors guide you through the complexities and details of solar—from understanding your energy bills, to installation and more. And each Sunrun solar system is custom designed to fit your home’s unique needs.

As the nation's #1 home solar & battery installer, we provide dedicated support from initial system design to powering on your system. Our personalized approach ensures that you make the right decisions for your home solar system, enabling you to harness the full potential of solar energy without the stress.