Aside from the electricity we use in our homes and offices, and gasoline in our cars, we expend other energy without realizing it: The energy used to manufacture, package, transport and finally dispose of a product is known as gray energy. One bar of chocolate for example uses 0.25 kilowatt-hours of gray energy. That same amount of energy could be used to cook a pot of pasta about 20 times.

Half a liter (17 ounces) of bottled mineral water requires 0.7 kWh of gray energy. That's about a thousand times more energy than for the same quantity of tapwater. Products that have been transported over long distances require a lot of grey energy. Yet transport of locally produced-goods by car over short distances can be more polluting than mass transport over long distances.

The production of a laptop's hardware amounts to 1,000 kWh of grey energy. That's equivalent to 40 days of continuous vacuuming. If gray energy is not taken into account in comparisons of energy consumption, a misleading picture can result.

A single pair of cotton jeans is estimated to represent more than 40 kWh of gray energy. With the same amount of electricity, you could watch 400 hours of television. When it comes to measuring gray energy, the extraction of raw materials are taken into account, and the energy used in all production processes added up.

Producing an average single family house with approximately 120 square meters (1,300 square feet) of living space requires more than 150,000 kWh of gray energy. This is about the electricity consumption of a family of four for almost 40 years. Experts say that every euro that a household spends translates into around one kilowatt-hour of gray energy.

An average newspaper weighing about 200 grams (7 ounces) requires about 2 kWh of gray energy. For the sake of comparison, with 2 kWh you could brew 150 cups of coffee. The data required to calculate gray energy is often difficult to obtain — results can vary significantly depending on details around the manufacture and transport of products.

For the manufacture, transport, storing, selling and discarding of a smart phone, 220 kWh of gray energy is needed. With that same energy, you could charge your phone for 50 years. The difficulties around measuring gray energy are a major hindrance to providing that information to the consumer.

Producing one pair of shoes uses about 8 kWh of gray energy. This is the same amount of energy that an average refrigerator consumes in two weeks. Gray energy is a major contributor to global energy consumption, as well as "gray" CO2 emissions, which greatly increases the carbon footprint of many products.

A mid-range car has devoured around 30,000 kWh of gray energy before it even hits the road. Translated into gasoline, that means driving 36,000 kilometers (22,000 miles). Import and export play a decisive role in tallies of grey energy: If a car is manufactured in Germany and exported to another country, the emissions during production should be charged to that country and not to Germany.

A roll of chlorine-free bleached toilet paper includes 20 kWh of gray energy. A single roll of toilet paper thus represents as much energy as washing 20 loads of laundry. Yet gray energy is rarely in the minds of consumers — not least because there is extremely little data about it. Despite that, consumers should take gray energy into account if sustainability is important to them.