10:10—notes on the maths

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By Annabel McAleer

Here’s how to cut your carbon footprint 10% in 2010.

Below are some calculations you can use to calculate the carbon savings you and your family could make around your home. All figures represent CO₂-equivalent greenhouse gas savings in tonnes.

HOME

Install double glazing = 0.19

Consumer energy use per household: 10,549 kWh/year
Space heating: 34% of household energy
34% × 10,549 kWh/year = 3,587 kWh/yr for space heating
Assume 25% heat loss reduction (proxy for otherwise fully insulated houses)
25% × 3,587 kWh = 900 kWh/yr
900kWh saved/year × 0.21 CO₂e/kWh (aka the "electricity emissions factor", or EEF) = 189kg CO₂e
NB This is extremely variable due to different climates, house sizes, house thermal efficiencies, heating regimes

Replace your old electric hot water cyclinder with a solar hot water heating system = 0.47

Based on 50%–75% reduction of energy use for heating water, with a solar hot water system for a 4–5 person household (AS/NZS 4234:2008)
Based on average savings of between 1,800 kWh and 2,700 kWh a year
1,800–2,700kWh = 2,250 kWh median
2,250 kWh median × 0.21 EEF = 473 kg CO₂e

Buy a wood- or pellet-burner = 0.75

Consumer energy use per household: 10,549 kWh/year
Space heating: 34% of household energy
34% × 10,549 kWh = 3,587 kWh for space heating
3,587 kWh × 0.21 EEF = 753kg CO₂e
NB extremely variable due to different climates, house sizes, house thermal efficiencies, heating regimes

Install 2 kilowatt solar PV panels = 0.49

Assume capacity factor of 12–15%
2kWh × 12–15% × 8,760hrs/yr = 2,100–2,600kWh (EECA supplied info)
median = 2,350kWh
2,350 kWh x 0.21 EEF = 494 kg CO₂e

ELECTRONICS

Don’t leave appliances on standby, and turn off lights and heated towel rail when not in use = 0.16 (per household)

Don’t leave appliances on standby, and turn off lights and heated towel rail when not in use = 0.06 (per person)
For all of NZ, 1.4 billion kWh are used to power appliances in standby, heated towel rails, faulty refridgeration, minor loads, lights left on and other misc, generating 238,000 tonnes CO₂e (carboNZero comissioned report)
divide by number of NZ households = 1,471,746
238,000 / 1,471,746 = 162 kg CO₂e wasted per household
divide by number of NZ residents = 4,027,947
238,000 / 4,027,947 = 59kg CO₂e wasted per person

Replace your old fridge with a new Energy Star refrigerator = 0.11

Assume 300kWh/yr for new EnergyStar fridge/freezer
Assume 800kWh/yr for old fridge/freezer (comparison on fig 48 in HEEP YR10 report)
500 kWh saved
500 kWh × 0.21 EEF = 105 kg CO₂e

Wash your clothes on cold washes rather than warm = 0.07

Median electricity consumption for a top loader 312.5 kWh, based on 250 kWh average for medium top loader (warm wash 27c / cold wash 3c per load) and 375 kWh average for large top loader (warm wash 40c / cold wash 4c per load), from Consumer NZ Appliance Running Costs report (updated 2 Jul 2009), converted back to kWh using quoted electricity cost of 20 c/kWh; assumes 4 loads a week
312.5 kWh × 0.21 EEF = 66 kg CO₂e

Use the sun to dry your clothes, rather than a dryer = 0.17

Median electricity consumption for a dryer 800 kWh, based on 700 kWh average for 3.5kg dryer and 900 kWh average for 5kg dryer, from Consumer NZ Appliance Running Costs report (updated 2 Jul 2009), converted back to kWh using quoted electricity cost of 22c/kWh; assumes 4 loads a week
800 kWh × 0.21 EEF = 168 kg CO₂e

Install an efficient shower head = 0.17 (per household)
Install an efficient shower head = 0.06 (per person)

Assume before shower flow rate of 12l/min, afterwards 9l/min
Assume 3 showers per day of 9 minutes each (NB extremely variable)
Assume electric water heating
830 kWh × 0.21 EEF = 174 kg CO₂e (EECA supplied)
divided by 3 showers = 58 kg CO₂e per person
Doesn’t include CO₂e emissions of treating water supply and wastewate

Only use your heated towel rail four hours a day (instead of all the time) = 0.11

Average wattage of a heated towel rail was stated as 70W in BRANZ year 10 HEEP report
70W × 20hrs × 365 days/yr = 511 kWh/yr saved
511 kWh × 0.21 EEF = 107.3 kg CO₂e

Get rid of the second fridge (or just switch it off) = 0.13

Average electricity consumption for a fridge was stated as 621±30 kWh/yr in BRANZ report EC1208/01 Analysis of Refrigeration Appliances from HEEP
621 kWh × 0.21 kg CO₂e/kWh electricity emissions factor 2008 (EEF) = 130.4 kg CO₂e
FYI: average consumption of new fridge = 400 kWh/yr; average consumption of pre-1986 fridge/freezer in good repair = 1,300 kWh/yr (as per Fig 48 year 10 HEEP report)

Replace one 100W bulbs with a 20W eco-bulb = 0.03

Assume bulbs on for five hours a day
100W bulb × 5 hours × 365 = 182,500 = 182.5kWh
20W bulb × 5 × 365 = 36,500 = 36.5kWh
182.5 − 36.5 = 146 kWh saved per bulb
146 × 0.21 EEF = 31 kg CO₂e per bulb

BEEF

one NZ beef cow contributes 1,700kg CO₂e/year (info from NZPGGRC and MAF) = 141.66 kg CO₂e/month
most steers slaughtered at 27–34 months of age (info from Beef NZ) = median age 30.5 months
141.66 kg CO₂e/month × 30.5 months = 4,320.83 kg CO₂e per beef cow over life
Kobe beef production in Japan = 36.4 kg CO₂e/kg beef, calculated from 4,550 kg CO₂e per beef cow over life (includes total contributions of one beef calf throughout its life cycle to global warming, acidification, eutrophication and energy consumption). Source: Ogino et al. 2007
The Ogino beef cow emissions figures over lifetime (4,550 kg) closely matches the NZ figures (4,321 kg), so I have estimated 35kg CO₂e/kilo beef for NZ beef
Per capita consumption of beef and veal = 34.2kg / year (Meat & Wool NZ)

Go beef-free 2 days a week = 0.34

34.2kg beef per year × 35kg CO₂e per kilogram beef = 1,197kg CO₂e
1,197 ÷ 7 = 171 kg CO₂e per day’s beef consumption
171kg × 2 days = 342 kg CO₂e

Halve beef consumption = 0.6

34.2kg beef per year average consumption × 50% = 17.1kg
17.1kg beef × 35kg CO₂e per kilo beef = 599kg CO₂e

Compost all food and garden waste = 0.1

52 × 6kg bags rubbish
CarboNZero calculator = 106kg CO₂e

CAR

Car-pool to work with 1 other = 0.58

Medium car (1.5 – 2 litre) commuting 10km twice/day, five days/week, 48 weeks/year = 4,800km
4,800km = 1156.8 kg CO₂e (CarboNZero calculator)
1,156.8 kg CO2e shared between two people = 578 kg CO₂e saved

Car-pool to work with 2 others = 0.77

Medium car (1.5 – 2 litre) commuting 10km twice/day, five days/week, 48 weeks/year = 4,800km
4,800km = 1,156.8 kg CO₂e (CarboNZero calculator)
1,156.8 kg CO₂e shared between two people = 578 kg CO₂e saved
1,156.8 kg CO2e shared between three people = 385.6 kg CO₂e per person
1,156.8 – 385.6 = 771 kg CO₂e saved

Work from home one day a week rather than commute by car = 0.23

Medium car (1.5 – 2 litre) commuting 10km twice/day, five days/week, 48 weeks/year = 4,800km
commuting 10km twice/day, four days/week, 48 weeks/year = 3,840 km
4,800 – 3,840 = 960 km
960 km = 231 kg CO₂e (CarboNZero calculator)

Take the bus instead of drive = 0.75

Medium car (1.5 – 2 litre) commuting 10km twice/day, five days/week, 48 weeks/year = 4,800km
4,800km = 1,156.8 kg CO2e (CarboNZero calculator)
City bus commuting 4,800 km = 411.2 kg CO₂e (CarboNZero calculator)
1,156.8 – 411.2 = 746 kg CO₂e

Work from home one day a week rather than commute by bus = 0.08

City bus commuting 10km twice/day, five days/week, 48 weeks/year = 4,800km
City bus commuting 10km twice/day, four days/week, 48 weeks/year = 3,840km
4,800 – 3,840 = 960 km
960 km = 82 kg CO₂e (CarboNZero calculator)

Cut your annual mileage in half = 1.69

Average mileage 14,000km/year
14,000 km in medium car (1.5 – 2 litre) = 3374 kg CO₂e (CarboNZero calculator)
3,374 × 50% = 1,687 kg CO₂e

Walk or cycle 10% of average annual travel distance = 0.34

Average mileage 14,000km/year
14,000 km in medium car (1.5 – 2 litre) = 3,374 kg CO₂e (CarboNZero calculator)
3,374 × 10% = 337 kg CO₂e

If your car is old and inefficient, buy a new small car then scrap the old one = 1.7

Average mileage 14,000km/year
Assume using old, inefficient car. New Zealand car uses on average 11.2 litres of fuel per 100 kilometres (km) travelled. You can expect a new small car to deliver fuel economy of less than 6 litres per 100 km (via EnergyWise)
0.112 litres/km × 14,000 = 1,568 litres
0.06 litres/km × 14,000 = 840 litres
1,568 – 840 = save 728 litres/year
728 litres × 2.33 kg CO₂e/litre = 1,696kg CO₂e/year saved

Remove roof rack, correctly inflate tyres (and check every month), have vehicle tuned = 0.42

A new medium-sized family sedan can deliver fuel economy of around 8 to 9 litres per 100 km (via EnergyWise)
0.085 litres/km × 1,4000 = 1,190 litres
If a car’s tyres are under-inflated, there is more ‘rolling resistance’. This means more energy is needed for the wheel to roll – just as a soft soccer ball will not roll as easily as a properly inflated one = uses 5% more fuel (via FuelSaver)
A vehicle that is out of tune will not burn fuel as efficiently as it is designed to = uses 5% more fuel (via FuelSaver)
A roof rack, ski box or bike rack attached to your car causes more wind resistance. More of the vehicle’s power is needed to overcome this resistance in order to travel at a given speed = uses 5% more fuel (via FuelSaver)
15% × 1190 litres = 178.5 litres
178.2 litres × 2.33 kg CO₂e/litre = 416 kg CO₂e

Slow down, brake earlier and more gently = 0.69

Fast driving, heavy acceleration and braking uses more energy (and therefore more fuel) than traveling at a constant speed = uses 25% more fuel (via FuelSaver)
0.085 litres/km × 14,000 = 1,190 litres
25% × 1,190 litres = 297.5 litres
178.2 litres × 2.33 kg CO₂e/litre = 693 kg CO₂e

Open window instead of using air con = 0.14

A car's air conditioning system needs power to work, in the same way a domestic fridge does. This power comes from the engine and increases fuel consumption = uses 10% more fuel (via FuelSaver)
Open window/sunroof (at speeds under 80 km/h; the higher the speed, the more wind resistance and therefore the more fuel the car uses) = uses 5% more fuel (via FuelSaver)
10% - 5% = opening windows saves 5% fuel
0.085 litres/km × 14,000 = 1,190 litres
5% × 1,190 litres = 59.5 litres
59.5 litres × 2.33 kg CO₂e/litre = 139 kg CO₂e