How to Calculate Kilowatt-Hours (kWh Calculation)

Inspire Clean Energy

8 min read

category: Sustainable Living

Each and every one of our electricity bills revolve around one thing: our kWh (kilowatt-hour) usage. The problem is most of us are not exactly sure what that means. And since this measurement dictates how much we end up paying, it’s a good idea to understand what it is and how to calculate it for your home.

What is a kW and a kWh?

A “watt” is the unit used to measure quantities of power and is named after the Scottish inventor and engineer James Watt (1736-1819). A kilowatt, or kW, is equal to a thousand watts. So the number of kW is the amount of power an electrical device uses in order to run, and a kilowatt-hour (kWh) is the amount of energy that an appliance uses every hour. For example, if your electric radiator is rated at 3 kW and is left on for an hour, it would use 3 kWh of electricity.

More importantly, a kWh is the unit that electricity suppliers use to bill you for the electricity you use. They do this by either reading your usage for you, or by having you send them the reading from your meter. Usually, you are given a unit charge for your electricity; this multiplied by the number of kWh you use gives you the cost of the electricity on your bill.

How do you calculate the number of kWh used per day?

If you want to know how many kWh you use daily, simply divide your total kWh number by the number of days covered by the bill. In reality, you are not going to use exactly the same amount of electricity every day. This changes depending on how long you spend at home, what you do while you are there, the time of year, and the temperature.

You can even work out the number of kWh used by each appliance per day based on how long each is on. If you use a 3-kWh heater as an example, it will use 15 kWh of electricity if you have it on for 5 hours.

How do you calculate the number of kWh from watts?

If you want to know how many kWh an appliance uses and already know how many watts it uses, the calculation is pretty straightforward.

First, you need to convert the number of watts into kW. To do that, you divide the number of watts by 1,000. So 100 W is 0.1 kW, 60 W is 0.06 kW, and 1500 W is 1.5 kW.

To get the number of kWh, you just multiply the number of kW by the number of hours the appliance is used.

For example, a device rated at 1,500 W that’s on for 2.5 hours:

1500 ÷ 1000 = 1.5. That’s 1.5 kW. 1.5 x 2.5 = 3.75. So, a 1,500 W appliance that’s on for 2.5 hours uses 3.75 kWh.

How do I calculate kW to kWh?

Calculating kWh from kW is even easier, as you already know the number of kW for the appliance. All you need to do is multiply the kW number by the time in hours. The 3-kW heater, if used for 3.5 hours, would use (3 x 3.5) 10.5 kWh of electricity.

How many kWh is normal for a home?

In 2019, according to the U.S. Energy Information Administration, the average American home used 877 kWh of electricity every month, or 10,649 kWh each year. This varies depending on which part of the country you live in. The five states with the lowest electricity consumption include Hawaii, Maine, California, Vermont, and Rhode Island, all with about 500-600 kWh every month. The top five are Louisiana, Tennessee, Mississippi, Alabama, and North Dakota. All of these use more than 1,200 kWh every month on average.

The variation is partly climate-related, but construction regulations and the age of houses also play a part. We have become much more aware of how well-built and insulated homes are far more energy-efficient than older properties. In addition, today there are numerous ways to make homes more energy-efficient, such as using clean energy and switching to a renewable energy company.

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What is the kWh usage for common household items?

The electricity consumption of home appliances varies a lot. The rules are that anything that heats or cools is likely to be a big power user and that newer models tend to be more efficient than older units.

  • A furnace with a fan that uses 10 kW per hour is likely to be used for extended periods. In 12 hours, that would be 120 kWh.
  • A 1,500 W portable heater will use 1.5 kW per hour. In 4 hours, it would use 6 kWh.
  • A 1.5-ton heat pump without heat strips is rated at around 3 kW, so if on for 8 hours, it would use 24 kWh.
  • An 8 kBtu air-conditioner uses 2.93 kWh of electricity every hour. If on for 12 hours, that is 35.16 kWh.
  • An oven is around 2.3 kWh every hour.
  • An old-style 15-cubic-foot refrigerator uses 150 kWh per month. By contrast, a 17-cubic-foot Energy Star refrigerator uses just 35 kWh per month.
  • A 50-60” LED/4k UHD TV is about 0.071 kWh every hour, so in an evening it might use around 0.426 kWh.
  • A 50” LCD TV is just 0.016 kWh, so in that same six-hour period it would use just 0.096 kWh.
  • A desktop computer, when in use, uses about 0.05 kWh per hour, and in standby, this drops to 0.004 kWh.
  • A laptop is 0.02-0.05 kWh per hour.
  • A 300 W halogen lamp is 0.3 kWh per hour. Compare that with a 38 W LED lamp (equivalent to a 150 W incandescent), which consumes just 0.038 kWh.
  • A hot wash, cold rinse wash uses about 2.3 kWh per load, and a dryer uses somewhere between 2.5-4 kWh per load.
  • A vacuum cleaner uses something like 0.75 kWh per hour.
  • An iron uses 1.08 kWh per hour.
  • A hairdryer consumes 1.5 kWh per hour.

How do I calculate how many kWh an appliance uses?

If you want to know how to calculate kWh usage, you can usually find the electricity rating of an appliance in the instructions, on the manufacturer’s website, or on a label attached to the product. This figure will be the power consumption of the appliance. If you multiply this figure in kW by the number of hours it is on, you get the kWh.

If you are unsure of how to do this or want to check the manufacturer’s rating, you can buy a simple plug-in usage monitor that will measure kWh along with voltage, amps, cost, and so on. They are around $20-$25 and are a useful method of checking the figures are accurate.

Why is my kWh usage so high?

If you think your electricity usage is above what it should be, there are several things you should consider. If your house is old, it is likely it was built when the value of insulation was either not understood or unavailable. The construction industry has come a long way in the last 20 years, and today’s houses are like tightly sealed boxes compared to older homes.

The climate is also a major factor. If you live in an area that experiences very cold winters, super hot summers, or both, your energy usage will reflect this.

Also, older appliances really are more energy-hungry than newer ones. Not long ago, TVs in standby mode often used almost as much energy as when they were in use, today they hardly use any.

How do I reduce my energy usage?

If you want to reduce your electricity usage and lower your energy bills, there are a few things you can do:

Look at the standards of insulation in your home and upgrade where possible.
Think about replacing older, power-hungry appliances and getting newer, more efficient appliances.
Make certain all your lamps are LEDs.
Set your thermostat to a comfortable temperature. In the summer, set it so you can be comfortable in a T-shirt. In the winter, set it so you’re comfortable in a sweater. By not setting it to extremes, you can reduce your energy use.
Turn appliances and electronics off when you don’t need them.

Kilowatt-hours give you a real handle on how much electricity you consume and how you can reduce that figure. You can see which appliances are high-use items and which are more economical. When looking to buy a product, the kWh rating is important to consider.

Not only does it give you an idea of how the way you use electricity in the home can be improved, but the cost per kWh gives you a direct comparison of one electricity supplier with another.

If you’re looking for a way to reduce your carbon footprint, Inspire's 100% clean energy plan is an easy way to do your part for the planet.

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