What’s in a Kilowatt Hour?

The current price spikes for electricity in Europe has driven a new interest in saving energy, and part of doing that is to understand just how much energy different things use. I realized while I knew that modern LED lights are magically efficient, just how much electricity is used by other machines? No idea! So, I set out to find some examples the utility you get from a one kilowatt hour of electricity.

Updated in November 2022 with additional data.

Lighting is Cheap

Lighting has historically been very expensive. Candles and oil lamps required ridiculous amounts of labor in order to produce very little light. Electrical lighting revolutionized life in the late 1800s. Today, thanks to LED technology, we can generate both work lighting and decorative lighting extremely cheaply.

For example, a 470 lumen LED light source (equivalent to an old 40W incandescent bulb) draws about 6W. Which means it can run for about 170 hours on a single kilowatt hour.

Compare that to the older “energy efficient” technology of halogen lights, where you need 33W to provide the 40W-equivalent. In this case, it runs for about 30 hours. Unfortunately, not all of those old halogen mini-lamps are easily replaced with LED equivalents.

Example power specification for some decorative outdoor LED lights (from Clas Ohlson)

Even more impressive is the efficiency of decorative lights. For example, a simple cheap 240-LED outdoor string light draws around 7W. Thus, a very nicely lit Christmas tree can run for about 140 hours. Given the huge number of LEDs the amount of light given off by each one is really low – but the point is decoration.  

Household Machines

The EU regulations for energy labeling are really helpful. It made very easy to understand the nominal energy usage of typical household machines. I went through some sites and tried to get a sense for the typical use of machines currently being sold in Sweden.

Example label, for a Siemens SN45ZW49CS dish washer from Elon.se.

It appears that one kilowatt hour can do the following:

Boiling a liter of water in a water kettle (and presumably also in a microwave or on induction stove) from about 20°C to 100°C uses something like 0.2 kWh. Thus, it would boil five liters of water.

Run a standard household (60cm) dish washer once (based on what is being sold today).

Keep a modern refrigerator running for four days (assuming EU energy declarations are realistic). Quite amazing, as that means a power use that is only about twice that of a 470 lumen light! A freezer is worse, but it can still keep a freezer running for two days.

I checked the specifications of our current washing machine, and as expected the energy use depended heavily on just what program was used. One kilowatt hour can get you anywhere from half to one-and-a-half load. Quite impressive considering the work being done (just watch this classic Hans Rosling TED Talk about the transformative power of this particular appliance).  

Computing and Entertainment

The power intensity of computing varies enormously between use cases and systems. On one hand, we get remarkable utility out of mobile devices for a comparably small investment in energy. On the other hand, the world’s top supercomputers consume tens of megawatts. One kilowatt hour is enough to run the Frontier system for about one fifth of a second.

 Current smartphones have batteries that seem to range from 10 to 20 Wh (required some digging, as most batteries are declared in mAh – which means the number has to be multiplied by the voltage being used). Thus, a kilowatt hour would suffice to fully charge a mobile phone from 50 to 100 times.

Laptop batteries are bigger. As an example, the laptop I am writing this on has a 65 Wh battery. My old Alienware 15 R4 can use a 99 Wh battery (which happens to be the limit set for most airline flights). Which means a kilowatt hour can charge a laptop 10 to 15 times.

The energy consumption of graphics processing units (GPUs) vary greatly with use case and model. However, they do have the nice property that they tend to get maxed out when in use (after all, if you do not use all the performance of a certain GPU, why would you buy it in the first place). A typical mid-range card like the Nvidia GTX 3060 Ti appears to use around 200W at full speed. You would get about five hours of gaming graphics. With a more powerful card, this can easily be halved.

Modern displays are far more efficient than old-style cathode-ray tubes (which gave off truly noticeable amounts of heat). For example, the Dell 2520d displays I use my home office are specified to use 22W, making it possible to run a 25-inch display for about 50 hours. Checking the EU energy labels on displays currently for sale, it seems that lower-power displays are available. Unfortunately, lower power seems to be achieved by using a lower resolution (which should come as no surprise).

Large television screens use more power, unsurprisingly. It seems a current 75-inch 4K television uses around 120W. Activate HDR, and it goes up to 160W. Thus, it is possible to run a 75-inch TV for between 5 and 9 hours.

Moving Stuff

The focus on range as a factor for electric cars has made it very easy to access data on their energy use. Currently, it seems a good rough guideline is that an average electric car uses about 2kWh to move ten kilometers. That means you move a two-ton electric car about 5 km. Which will take a few minutes. A few minutes of driving or weeks of light? Same energy. Moving things is hard work, at least if you want it to be fast. It also means that the concept of using car batteries as backup power makes perfect sense, since the energy amounts needed for mobility dwarf most household usages.

Another data point for where 1kWh can get you is the Gossamer Albatross – this pedal-powered aircraft pulled about 300W of sustained power from its pilot-cyclist during its flight across the English channel in 1979. Since the flight took almost three hours, the energy provided for propulsion was close to 1kWh. Indicating the rather high energy density of a human as compared to an electrical battery (but this was not electric).

For trains, several sources I found indicate that a slow freight train uses about 20Wh per ton and kilometer. Thus, a single kWh would move 50 tons of train freight 1 kilometer. This is about 5 times more efficient than the electric car. An example of regional passenger train that I found (120 ton, carrying up to 170 passengers) would use 6kWh per kilometer. I.e., a regional passenger train could move 170 passengers about 160m.  

To avoid making electric car fans annoyed, it should be noted that a gasoline-powered car would require at least twice the energy of the electric car to move the same distance. Assuming a very efficient car it would require about 0.5 liters of gas to move 10km, and 1l of gas contains about 9kWh. Due to the known inefficiencies of combustion engines, this works out to 3x or more than what an electric car requires.

Public Lighting

Despite the efficiency of LED lights, cities across Europe are still cutting back on street lighting and other public lighting. This can be seen as a symbolic measure designed to remind people of the need to save energy. But it is also a fact that even a small saving is a saving and that given a large enough base, even a few percent can have a real effect.

Image from 2021.

It is still good to keep things in proportion. Here in Uppsala, the city has scaled back this year’s light festival to save energy. Officials note that for the full original program the expected energy use was at most 1700 kWh. For a month’s worth of public art that really make the darkest time of the year nicer. If you want to spend energy somewhere, this is a good place I think.


I have not been really able to find any useful numbers to quantify the amount of heating that a kilowatt hour provides. It obviously depends on aspects like insulation and method of delivery, and while there are many rules of thumb floating around for expected energy and/or electricity consumption, none of them work for the purpose of this blog.

Update, November 2022. I have found some good sources on the heating aspect that are worth repeating.

Fortum estimates that one kilowatt hour gives you about two minutes of hot showering. This is using an electrical heater, and assuming a flow of 16 liters of water per minute, heated from 10 degrees C (as it arrives in the pipes) to 37 degrees C.

A staff sauna at our local hospital was closed down, as the hospital claims that running a sauna in regular use for 10 minutes uses one kilowatt hour. Sure, it is a pretty high level of heat, but still… also interesting that this actually uses less energy than heating shower water. Water is most annoying in how hard it is to heat it.

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