by W.A. Steer PhD
We are increasingly bombarded by all kinds of "environmental" and "energy-saving" messages;
some confusing, some genuine, some alarmist, some misguided, some with a hidden agenda. How
can we make sense of them? Importantly, how can we concentrate our limited time and effort
on making the biggest difference? In most cases, reducing your energy consumption should
personally save you money, too.
On this page I'm hoping to explain energy (and carbon-dioxide) basics, offer some
calculations and show how headline-grabbing comparisons are arrived-at, and give you an
intuitive feel for energy-issues.
Introduction: Energy demand and usage
Reducing our "environmental footprint", or national CO2 emissions is not just
about a move away from fossil fuels and towards so-called renewable (or nuclear) energy
sources. Managing and reducing demand also has a vital role to play. Particularly with
the rising energy costs, saving energy makes good financial sense too.
But we're so used to just flicking a switch, lighting the oven, turning on the central
heating, or starting a car that it's often difficult to visualise how much energy we're
using, and where. This page is here to make energy "real", based on sound fundamental
Domestic electricty consumption is measured and billed in kilowatt-hours (kWh).
The rate at which any appliance uses energy (i.e. its power) is measured in watts (W), or kilowatts (kW).
1kW = 1000W.
To calculate the total amount of energy used by an applicance over a given time, multiply
its power by the number of hours of use.
For example, a 3kW electric heater operated for 1 hour
uses 3kWh of electricity. The same amount of energy, 3kWh, would be used by running five 60W light
bulbs (5×60W = 300W) for 10 hours. An electric shower, typically rated at
9kW, would consume 3kWh of energy in a mere 20 minutes!
Note that the label on an appliance always states its maximum power. Basic appliances
such as lamps, kettles, and toasters will draw the full power the whole time they are in use, while
things like heaters and fridges may be thermostatically controlled and switch off and on intermittently.
Many electronic products such as televisions and computers will vary the amount of power they take
depending on how bright the picture or how hard they're working.
Domestic gas consumption is measured in cubic metres (or cubic feet) and billed in kilowatt-hours (kWh).
For British gas, 1 cubic metre is approximately equivalent to 11kWh (The exact
conversion should be explained in the small print on the back of your bill).
Petrol (for cars) is sold by the litre. The "calorific value" (energy content) of petrol is around
60kWh per gallon (13kWh per litre).
UK retail prices for energy
British consumer energy prices (as at August 2006) including taxes:
For the same amount of energy, electricity is four times the price
of gas! This price differential is largely justified by the poor efficiency (around 30%) of
generating electricity from the burning of gas or coal.
- Gas is charged at 3 or 4 pence per kilowatt-hour
- Electricity costs between 10 and 14 pence per kilowatt-hour
One litre of petrol retails for around 99 pence, making petrol work out at around 7.5 pence per kWh.
In the UK and Europe, the biggest fraction of our total energy consumption is used for heating
[No-one would argue this, but would be good to quote some reputable
figures, even so.]
The amount of energy required to boil the water for one cup of tea (250ml) is about 90kJ. [3600kJ = 1kWh]
This amount of energy could alternatively:
- Run an electrically-heated shower for about 10 seconds
- Run a conventional 60W light bulb (or any 60W appliance) for 25 minutes
- Run a low-energy 12W lamp (light allegedly equivalent to a 60W conventional bulb) for 2 hours
- Power a personal MP3 player all day every day for around two weeks
- Power a mobile phone for around one month of typical use
- Refine the aluminium to make one tenth of a drinks can
- Accelerate a 1500kg mass (eg car) from stationary to 25mph (40km/h), once
- This is also roughly the total power stored in a brand new alkaline (eg Duracell) 'D' battery
Understanding energy usage and reducing demand have an important role to play in meeting our
environmental obligations. Energy consumption and efficiency is very well understood by
scientists and engineers. In a laboratory setting it is quite straightforward to measure,
calculate, or simulate the power requirements of almost any machine or activity. In contrast,
considering an existing house, office premises, or lifestyle, it is often not obvious where
the energy goes, and how best to make savings. The primary purpose of this web-page is to
offer practical guidance in understanding energy use.
Energy and Power
There is a key distinction: power is the rate of use of energy. ["use" should really read "conversion" since strictly energy is neither created nor destroyed!] Power is effectively the
"flow rate" of energy.
Energy is measured in Joules (J), kiloJoules (kJ), megaJoules (MJ) etc and kilowatt-hours (kWh)
1kWh=3600kJ (3600 because that's how many seconds there are in an hour)
Power is measured in Joules-per-second, more commonly known as watts (W).
Energy cannot be created or destroyed
You just convert it from one form to another.
Classes of energy
Potential (stored) energy - energy by virtue of height (against gravity), chemical energy
(batteries, fuels etc), energy stored in a stretched or compressed spring...
Kinetic energy - energy of movement
Energy in different forms can easily be compared.
Energy conversions are almost always inefficient; if any process is less than 100% efficient,
it's pretty certain something is getting hot (or at least warm) somewhere! Light bulbs get hot,
fluorescent tubes get warm, electric motors get warm, car engines get very hot, most electronic
equipment (televisions, DVD players, set-top-boxes, computers, mobile phones...) gets warm in
use. Rub yours hands together - in any mechanical system friction makes things warm.
A couple of myths
These are very common misconceptions which are still spread by many people who should know better!
- It takes as much (if not more) energy to re-heat a cold house than it would have done to keep it warm
by leaving the heating on all day long. MYTH
- Myth 2
- Myth 3 ?
We'll come back to why these ideas are wrong later.
A couple of core facts
- Space-heating and transport account for around 80% of an individual's energy consumption. (These have greatest scope for savings)
- Myth 2
- Myth 3 ?
The efficiency of any process or applicance is the ratio of the useful (or wanted)
energy or work obtained to the amount of energy put in in the first place. If, for example, a
light bulb is "only 30% efficient", the other 70% of the power we put in doesn't disappear, rather
it is manifest as heat. Whether it's friction in a mechanical system, warmth from
a computer or television, or a car engine, "lost" energy almost universally appears as heat.
- Energy cannot be created or destroyed; it can only be converted from one form to another.
It's generally very easy (and efficient) to convert any form of energy into heat, but much
more difficult (and inefficient) to do the reverse.
"Super efficient" halogen (or whatever) heaters advertised in the Sunday papers or mail-order
catologues are a mockery: every electric heater ever made is 100% efficient at converting
electricity to heat!
The rate of heat loss (energy loss) from a premises due to conduction through the walls and
windows etc is proportional to the difference between the inside and outside temperature.
Far better to be tucked up under a thick duvet at 5am than have the central heating working
hard to keep the whole house at 21 Celcius while the outdoor temperature is at its coldest!
Drying clothes and towels
Covering radiators with clothes or towels (eg to dry them) greatly reduces the effectiveness of the
radiator for warming the room, and is likely to upset the balence of heating in the house. If, as a consequence,
you turn up the thermostat you are likely to excessively-heat the rest of the house and waste a significant
amount of energy. If you must dry things on the radiator, use a rack which hangs the clothes in front of the
To dry more than a small amount of washing (and to avoid condensation and damp problems), you need a change of air as well as
heat. For the months when your house is heated, it is normally going to be more energy-efficient, and healthier
to dry your washing outside or in a (unheated/minimally heated) conservatory where there is a good airflow.
http://news.bbc.co.uk/1/hi/uk_politics/4719654.stm - patio heaters etc
http://news.bbc.co.uk/2/hi/science/nature/4754109.stm - more prominent meters for user-feedback
http://news.bbc.co.uk/1/hi/sci/tech/4789780.stm - BBC Energy Week / energy saving
Production of the alumina precursor from the bauxite ore creates the same weight in CO2 as the
weight of ore itself.
Then 16kWh of electricity needed to reduce alumina to 1kg of aluminium metal.
(16kWh would be equivalent to leaving a 3kW immersion heater on for just over 5 hours, or about
£2 (2 GBP) worth of electricity at residential prices!)
I also quote:
Recycled aluminium requires only 5 per cent of the energy required to make "new" aluminium. Blending recycled metal with new metal allows considerable energy savings, as well as the efficient use of process heat. There is no difference between primary and recycled aluminium in terms of quality or properties.
Aluminium smelting is energy intensive, which is why the world's smelters are located in areas which have access to abundant power resources (hydro-electric, natural gas, coal or nuclear). Many locations are remote and the electricity is generated specifically for the aluminium plant.
Overview of Energy sources
These are some necessarily brief notes to set the scene; please follow up references
for more detail if required. At this point I'm trying to flag key concerns, rather than
go into detail of arguments.
Combustion and Carbon-Dioxide
Most fuels we might burn (wood, coal, gas, petrol, kerosene...) are hydrocarbons; that is
they are composed principally of carbon and hydrogen. When they are burned the combustion
products are mainly carbon-dioxide (CO2) and di-hydrogen monoxide (H2O - ie water vapour).
The oxidation of carbon to CO2 and hydrogen to H2O is exothermic; it releases energy -
usually as heat. Wood-burning releases CO2 which was previously captured from the
atmosphere by the tree as it grew (probably in the past 50 years), so is potentially part
of a cycle which happens within a human lifetime. Our present burning of coal and oil
reserves, which formed many millions of years ago, rapidly releases CO2 which would
otherwise have been locked away. Whether such fossil fuel deposits will last another 30 years,
50 years, or 100 years is really neither here nor there - we're using them up thousands of
times faster than they took to form, so they will run out; greenhouse effects aside, this
fuel source is not sustainable in the long-term.
See also CARBON CYCLE
Hydrogen is not the solution to any energy crisis; hydrogen is merely a (relatively)
convenient energy store.
H2O + energy -> hydrogen gas + oxygen gas
hydrogen + oxygen -> H2O + energy
To store an appreciable amount of energy requires quite a lot of hydrogen, so it has to
be compressed (using more energy) and transported or stored in heavy pressure-vessels or
cylinders. It's also dangerous as it burns very rapidly or even explodes when mixed with
Electric cars may make for cleaner cities, but though there may be some efficiency savings,
they mainly just shift the combustion and pollution to the power station.
Since there's no combustion involved, the nuclear reactor generates power without releasing
CO2. Mining and enriching the uranium requires a significant input of (typically fossil-sourced)
energy, so it not completely carbon-neutral. As uranium resources become more scarce, the
energy required in uranium-enrichment becomes progressively more prohibitive. Some types of
reactors (breeder reactors) may create enough future-fuel to be more sustainable?
Nuclear power still has plenty of unresolved issues of what to do with the waste, problems of
nuclear proliferation, and security/safety.
"Renewable" sources - wind, wave, solar etc.
These are important, but with the best will in the world could not realistically meet our
present vast demand for energy on their own. In general the energy density of these natural
sources is quite low, meaning that the capture device (turbines, solar panels etc) need to
be spread out across a large area of land. With solar there are issues of storage, since we usually
require most energy for heating during the winter, and not in the summer when the sun is
strongest! Photovoltaic (PV) cells convert sunlight directly into electricity but are only
around 20% efficient, they're also very expensive to make (although even less efficient but
much cheaper technologies may be on the horizon). In many cases the much lower-tech solar
water heaters are more practical.
Created: March 2006
Last modified: 21 May 2006
©2006 William Andrew Steer