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Cups of tea

Is this a way to make solar panels sound good? "Once all the 7 000 photovoltaic panels are in place, it is expected that the solar panels will create 180000 units of renewable electricity each year - enough energy to make nine million cups of tea." This announcement thus equates 1 kWh to 50 cups of tea.

As a unit of volume, 1 US cup (half a US pint) is officially 0.241; but a cup of tea or coffee is usually about 0.181. To raise 50 cups of water, at 0.181 per cup, from 15 °C to 100 °C requires 1 kWh.

So "nine million cups of tea per year" is another way of saying "20 kW."

Double-decker buses, Albert Halls and Wembley stadiums "If everyone in the UK that could, installed cavity wall insulation, we could cut carbon dioxide emissions by a huge 7 million tons. That's enough carbon dioxide to fill nearly 40 million double-decker buses or fill the new Wembley stadium 900 times!"

From which we learn the helpful fact that one Wembley is 44 000 double decker buses. Actually, Wembley's bowl has a volume of 1140000 m3.

"If every household installed just one energy saving light bulb, there would be enough carbon dioxide saved to fill the Royal Albert Hall 1,980 times!" (An Albert Hall is 100000m3.)

Expressing amounts of CO2 by volume rather than mass is a great way to make them sound big. Should "1 kg of CO2 per day" sound too small, just say "200000 litres of CO2 per year"!

mass of CO2 ^


2 kg C02 ^

1 m3

1 kg CO2 ^

500 litres

44 g CO2 ^

22 litres

2 g CO2 ^

1 litre

Table I.3. Volume-to-mass conversion.

Table I.3. Volume-to-mass conversion.

More volumes

A container is 2.4 m wide by 2.6 m high by (6.1 or 12.2) metres long (for the TEU and FEU respectively).

One TEU is the size of a small 20-foot container - an interior volume of about 33 m3. Most containers you see today are 40-foot containers with a size of 2 TEU. A 40-foot container weighs 4 tons and can carry 26 tons of stuff; its volume is 67.5 m3.

A swimming pool has a volume of about 3000 m3. One double decker bus has a volume of 100 m3. One hot air balloon is 2500 m3.

The great pyramid at Giza has a volume of 2500000 cubic metres. Areas

The area of the earth's surface is 500 x 106 km2; the land area is 150 x 106km2.

My typical British 3-bedroom house has a floor area of 88 m2. In the USA, the average size of a single-family house is 2330 square feet (216m2).


If we add the suffix "e" to a power, this means that we're explicitly talking about electrical power. So, for example, a power station's output might be 1 GW(e), while it uses chemical power at a rate of 2.5 GW. Similarly the

Figure I.4. A twenty-foot container (1 TEU).


= 104 m2


= 4050 m2

square mile

= 2.6 km2

square foot

= 0.093 m2

square yard

= 0.84 m2

Table I.5. Areas.

Land use area per person percentage

Land use area per person percentage


- domestic buildings



- domestic gardens



- other buildings



- roads



- railways



- paths



- greenspace



- water



- other land uses






Table I.6. Land areas, in England, devoted to different uses. Source: Generalized Land Use Database Statistics for England 2005. [3b7zdf ]

1000 BTU per


= 0.3 kW =

7 kWh/d

1 horse power (1 hp or 1 cv or


= 0.75 kW =

18 kWh/d

1 kW =

24 kWh/d

1 therm =

29.31 kWh

1000 Btu

0.2931 kWh

1MJ =

0.2778 kWh


277.8 kWh

1 toe (ton of oil equivalent) =

11630 kWh

1 kcal

1.163x10 kWh

1 kWh = 0.03412 3412


86x10-6 859.7

therms Btu


toe kcal

Box I.7. How other energy and power units relate to the kilowatt-hour and the kilowatt-hour per day.

suffix "th" may be added to indicate that a quantity of energy is thermal energy. The same suffixes can be added to amounts of energy. "My house uses 2 kWh(e) of electricity per day."

If we add a suffix "p" to a power, this indicates that it's a "peak" power, or capacity. For example, 10 m2 of panels might have a power of 1 kWp.

Petrol comes out of a petrol pump at about half a litre per second. So that's 5 kWh per second, or 18 MW.

The power of a Formula One racing car is 560 kW.

UK electricity consumption is 17 kWh per day per person, or 42.5 GW per UK.

"One ton" of air-conditioning = 3.5 kW.

World power consumption

World power consumption is 15 TW. World electricity consumption is 2TW.

Useful conversion factors

To change TWh per year to GW, divide by 9.

1 kWh/d per person is the same as 2.5 GW per UK, or 22 TWh/y per UK To change mpg (miles per UK gallon) to km per litre, divide by 3. At room temperature, lkT = ¿¡eV At room temperature, 1 kT per molecule = 2.5 kJ/mol.

Meter reading

How to convert your gas-meter reading into kilowatt-hours:

• If the meter reads 100s of cubic feet, take the number of units used, and multiply by 32.32 to get the number of kWh.

• If the meter reads cubic metres, take the number of units used, and multiply by 11.42 to get the number of kWh.


inland water








oil pipeline


gas pipeline


int'l water container


int'l water bulk


int'l water tanker


Calorific values of fuels Table I.8. Energy intensity of transport modes in the USA. Source: Crude oil: 37MJ/1; 10.3 kWh/l. Weber and Matthews (2008).

Natural gas: 38MJ/m3. (Methane has a density of 1.819 kg/m3.) 1 ton of coal: 29.3 GJ; 8000 kWh. Fusion energy of ordinary water: 1800 kWh per litre. See also table 26.14, p199, and table D.3, p284.

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