www.techmind.org logo
by W.A. Steer  PhD
Back to contentsAbout...



Energy Efficiency Calculations

This page is one of several on my site on the topic of domestic energy efficiency, and offers answers to popular energy dilemas. Unlike less scientifically-based sites, I include the reasoning, experiments and calculations to verify my claims. I intend to add to this page as and when I have the inspiration, and in response to questions submitted by the email using the address at the foot of this page.

Another page (shortly to be written) will cover some general principles including that gas is usually cheaper and more environmentally sound than electricity for domestic heating applications.

Questions and summary answers

Q. Is it better to make a cup of tea using an electric kettle or to boil the water on the hob?
Electric kettle - convenient, and a safe bet under most circumstances
Gas hob - mostly good, don't be afraid to use if more convenient (e.g. you want the water in the saucepan anyway)
Electric hob - avoid; there are cheaper and more environmentally-sound ways to make a cuppa!

Significance - you'd create ten times as much CO2 driving one mile in a typical car as you would boiling half a litre of water in a kettle!

The jug-kettle uses only one third as much electricity as a saucepan on an electric hob, and so will also produce only one third the amount of CO2, and cost one-third as much.
Comparisons to a gas hob are less clear cut. Although the gas hob still uses around three times the amount of energy at the point of use, the balence of CO2 emissions is very sensitive to the electricity generating mix. Using the 2003 figure for UK electricity places the gas hob at only 15% more CO2 than the kettle, a fairly negligible difference. In monetary terms, boiling water on a gas hob will cost much the as the electric kettle, because (in the UK) gas is so much cheaper than electricity per kWh.
In the kettle the fully-immersed element couples heat into the water with very high efficiency and boils the water speedily so there's little time for the kettle as a whole to lose heat to the room. Hob-methods couple the heat far less effectively to the water in the first place, and typically take much longer, resulting in greater heat losses.
Kettle experiment and calculations
Note: no "eco-kettle" can use much less energy than any cheap plastic jug kettle to boil the same amount of water. You can only really save money and energy by taking care to not boil more water than you need.
Q. "If every household [in the USA] replaced just three 60-watt incandescent light bulbs with CFLs [low-energy compact-fluorescents], we would reduce as much pollution as if we took 3.5 million cars off the roads!", claims the US Environmental Defense organisation. Is that realistic?
If you assume the three bulbs swapped across 100 million households are used for around six hours per day, then the saving is comparable to fuel used by 3.5 million typical US cars.
Significance - each household could make similar savings by cutting their own car-use by just 1.2 miles per day.
Calculations: CFL lamp savings comparison to cars

Typical energy use patterns in the UK

Ofgem (the energy regulator) is widely quoted stating that the average UK household consumes 3300 kWh electricity and 20500 kWh of gas annually. [Although I haven't yet found a definite source, apparently these figures date from 1999.]
Figures for 2003 put the average electricity figure at the rather higher value of 4600 kWh.

Source for recent UK gas and electricty consumption figures (including regional breakdown): http://www.dti.gov.uk/files/file20328.pdf

British cars are usually reckoned to have a typical annual mileage of 10000 miles.

4600 kWh electricity2500 kg CO2680 kg carbon
20500 kWh gas3900 kg CO21060 kg carbon
10000 car miles2900 kg CO2790 kg carbon
Note: to convert kgCO2 to kg carbon, multiply by (3/11) [molecular weight CO2 is 12+(16×2)=44, atomic weight of carbon is 12. 12/44 is equivalent to 3/11.]

[Annual figures for USA car, 12500 miles, 21.5mpg, 5000 kg CO2]

Examples from my house

Modest 3-bed terrace (built in 1980) in the south of England, double-glazed, 3 occupants.
Gas central heating/hot water, gas cooker, electric shower. Electric washing machine, no tumble dryer. Appliances probably UK-typical and about 5 years old.

Household electricity consumption (July 05 - July 06): 2400 kWh
Household gas consumption (July 05 - July 06): 11700 kWh
Car miles: very few. I don't own a car, cycle/walk daily to work, use trains for longer journeys

Tall larder fridge (250 litres), "energy efficiency class B" rated at 226 kWh/year
Tall upright freezer (181 litres), "energy efficiency class C" rated 418 kWh/year
Kettle usage estimated at 160kWh/year
Combined TV/satellite box standby power c.20watts, 175kWh/year


From a purely technical perspective (and I firmly believe technical problems need technical solutions, not just political rhetoric), the easiest ways to make the biggest savings in energy costs (i.e. money) and CO2 are:

Source for aluminium energy 16kWh/kg: http://www.world-aluminium.org/production/smelting/index.html

CO2 emissions figure

Standardised CO2 emission figures for common fuels are published by the British government, and these are the official factors for British businesses to use when making any environmental claims. For electricity the amount of CO2 emitted per kilowatt-hour (kWh) of electricity depends heavily on the method of generation; nuclear should be zero (or a bit more than zero if you take into account consequential emissions - fuel mining/purifying/concentration/transport - and power station construction and demolition), coal will produce a lot of CO2, and gas somewhat less. Consequently the official figure for electricity is a weighted average taking into account the mix of types of generating stations which provide electricity to the UK.

Key figures I'll be using here are:
Electricity0.43 kg CO2 per kWh
[0.54 kg CO2 per kWh *]
Natural Gas0.19 kg CO2 per kWh
Coal0.32 kg CO2 per kWh
Petrol (gasoline)0.24 kg CO2 per kWh
2.30 kg CO2 per litre
0.30 kg CO2 per mile, in a typical car
Diesel fuel0.25 kg CO2 per kWh
2.63 kg CO2 per litre
* The standardised figure of 0.43 kgCO2/kWh should be used to allow year-on-year comparisons until 2010. However, as nuclear stations become decommisioned and the fuel mix changes the actual CO2 estimation varies slightly from year to year. The most recent official estimate, for the year 2003 mix, is 0.54 kgCO2/kWh for electricity.

The full set of figures, qualifications and footnotes can be found at: http://www.defra.gov.uk/environment/business/envrp/gas/envrpgas-annexes.pdf.

Calculations and reasoning

Electric kettle vs boiling water on the hob

As an experiment I tried different ways of boiling 0.5 litre of water (measured using a small measuring jug).

N.B. 1kWh=3600kJ (because by definition 1 Watt is 1 Joule per second, and there are 3600 seconds in an hour).

Plastic electric jug-kettle (2200W rating)

Time to boil: 1m40s (100 seconds)
Energy used: 100s×2200J/s = 220000J = 220kJ
220kJ / 3600kJ/kWh = 0.061 kWh. Using 0.54 kgCO2/kWh gives 33 grammes CO2

Small-sized electric hob at full power (1200W rating)

In medium/small aluminium saucepan weighing 0.374kg
Time to boil: 9m00s (540 seconds) for vigorous boil [8m00s for begin to boil]
Energy used: 540s×1200J/s = 648000J = 648kJ
648kJ / 3600kJ/kWh = 0.180 kWh. Using 0.54 kgCO2/kWh gives 97 grammes CO2

Medium-sized gas hob burner at full power (2000W rating)

In medium/small non-stick aluminium saucepan weighing 0.337kg
Time to boil: 6m00s (360 seconds) for a decent rolling boil [5m20s for lazy bubbling boil]
Energy used: 360s×2000J/s = 720000 = 720kJ
720kJ / 3600kJ/kWh = 0.200 kWh. Using 0.19 kgCO2/kWh gives 38 grammes CO2

The electric hob is easily the worst option, costing three times as much energy and CO2 as the kettle. Although the gas hob still results in almost three times as much energy (heat) being delivered to the home, this comes at very little extra CO2 cost. If that warmth is useful to you, then why not? On the other hand, because the hob takes longer you're more likely to forget about it and/or leave the water boiling for longer than necessary, which uses more energy and will also put more humidity into the house. Further considerations might include whether using a saucepan is going to result in extra washing-up, and whether you really want the extra humidity from the burner and drying the saucepan afterwards...

Swapping a few light bulbs per house equivalent to millions of cars off the road?

If each house swapped three 60W bulbs for CFLs using one quarter the energy (15W), and they were used say six hours per day, the saving would be about 6 hours×3×0.045 kW = 0.8kWh per house per day.
Using 0.6 kg CO2/kWh electricity [USA figure], that's 0.49kg CO2 per day. Multiplying up for 100 million households in the USA, over 365 days in the year I make that 17751150000kg CO2 per year.

According to http://www.epa.gov/otaq/consumer/f00013.htm the average US car does 12500 miles per year, at [just!] 21.5 mpg.

12500 miles / 21.5mpg = 580 gallons = 2200 litres = 5000 kg CO2 per car per year.

Dividing 17751150000kg CO2 saved by all the houses installing CFLs by 5000 kg CO2 per car per year indeed gives about 3500000 car-equivalents.

BUT this completely misses the point that each household could make exactly the same savings by driving their gas-guzzling cars 1.2 miles less per day.

Essential reading

For further quantitative analysis of energy issues (with a UK bias), I thoroughly recommend you download the summary or book from http://www.withouthotair.com/.

You might also like to read the blog http://withouthotair.blogspot.com/

Given infinite time, I would have liked to have written something like these, but no need - David MacKay has already done it!

Created: October 2006
Last modified: 23 June 2008

Source: http://www.techmind.org/energy/calcs.html

©2006-8 William Andrew Steer