Friday, November 28, 2014

Energy efficient behaviour pays


Investing in energy efficient buildings is a powerful way to save energy, but ultimately the occupants' behaviour is what determines power use. Without spending any additional money, you can reduce your consumption right now by changing your behaviour.


I work as an energy efficiency consultant. Generally, this involves finding investments that can be made (eg. LED lighting, better insulation, variable-speed-drives) that will result in decreased energy consumption. This works well, and there are generally good opportunities for people and organisations to make targeted investments in energy efficiency with good economic returns. However, it is easy to be blind to the most significant effect on energy consumption: human behaviour. Let me illustrate this by way of an example.

My family and I lived in Brisbane for almost three years, arriving in late 2010. We owned our home, and made some energy efficiency investments in it (solar PV, solar hot water, better lighting, blinds). Once these measures were completed, we were using about 5 - 6 kWh/day. When it came time for us to leave Brisbane, the real-estate market was poor, so we decided to rent out the house rather then sell it. We did this for a year and, because of the way the feed-in tariff (FiT) operates in Queensland, we kept control of the electricity supply and were reimbursed by our tenants (had we transferred the supply to them, they would have lost the solar PV FiT). Thus, we were able to see their consumption on a quarterly basis. What was immediately apparent was that their consumption was much higher than ours -- generally it was 2 - 3 times higher. Where we were using 5 - 6 kWh/day, they were using 14 to 18 kWh/day -- in the same house. Clearly, their appliances were different, which could account for part of this, but the majority of this difference I attribute to behavioural differences [1].

What this shows is that simple behavioural change can be hugely significant. Simple behaviours like these have a huge affect on household power consumption:

  • whether the oven is used in batches to cook a lot at once, or if it often turned on for only one small dish
  • whether appliances are turned off at the wall when not in use
  • whether there are many energy-hungry appliances (eg. large TVs)
  • when and how air-conditioning is used
  • whether lights are turned off
  • whether hot food is routinely put in the fridge without pre-cooling
  • how hot water is used
  • whether computers are left running when not used


This shows that energy efficient measures are important, but that the energy use of a building is determined in the end by its occupants. A positive way of seeing this is that you can save energy, without spending any money, merely by changing your behaviour.

[1] We were regularly washing nappies, and we also cook a lot. At that time, we had a relatively inefficient fridge -- thus we could have used a lot less also. Also, note that this house has one small through-window reverse cycle air conditioner, but is mostly not air-conditioned. Thus air-conditioning cannot account for this difference either.

This article was written by Angus Wallace, and first appeared at

Thursday, November 20, 2014

How fast do cars actually go?


I calculated the average speed that people achieve in cars, considering their actual average speed on the road, and the time they spend earning the money to maintain their cars. I found that, for an above-average income ($60000/year), a person driving 10000 km/year achieves an average speed of only 22.9 km/h. If you earn less than this, you must spend more time earning money to pay for your car, and your average speed is lower still.

How fast to cars go?

What I want to do in this article, is work out how fast people actually travel when they're driving cars. I want to consider actual average road speeds, and also consider the time they spend earning the money to pay for their car and its maintenance.

Traffic Speeds

Average speed of urban traffic in Australia, by time. Source:, page 104

This paper, has the 2005 statistics for vehicle speeds in Australia. From it I got this figure, showing that the average speed of cars in urban areas during the day time is about 42 km/h. Of course, congestion has got worse since then, and I would expect average speeds today to be lower, but I couldn't find those statistics.

Combine with time spent earning the money to pay for the car

Across Australia, once on the road, your average speed in the city is about 42 km/h (more like 37 km/h during peak periods). But what about the time you spend earning money to pay for your car? To work this out, I extended this spreadsheet (previously introduced in this article). It now also calculates the average speed of a motorist, based on the 42 km/h on-road speed and the time they spend earning money to pay for the car.

The average speed they attain depends on how much they drive -- as someone drives more, the marginal (time) cost of each extra kilometer decreases so their average speed increases. Here are some examples:
  • For a person on an above-average wage, driving 10000 km per year, they average just below 23 km/h
  • For a person on the same wage, driving 5000 km/year, they average just below 18 km/h
  • They won't reach an average of 30 km/h until they are driving 80000 km/year
  • For a person on $40000/year driving 10000 km/year, they average 18.5 km/h


  1. When you consider the time spent to pay for a car's maintenance, and the actual speed you achieve on the road, cars are not actually a quick way to get around. 
  2. The cost, to the taxpayer, of car infrastructure is huge -- I question whether it is good value for money.
This article was written by Angus Wallace and first appeared at

Thursday, November 13, 2014

What does it cost to own a car?

There have been a few good articles recently about the social cost of cars. Here is a good one -- it nicely summaries what many people are unaware of: vehicle registration does not pay for car infrastructure. Cars require a taxpayer subsidy.

People who ride bikes subsidise people who drive cars.

Here, I want to explore something different.

  1. What does car ownership cost its owner?
  2. How much time must the average car owner work to pay for their car?
These questions aren't often asked, so I ran a few numbers in a simple spreadsheet, which can be viewed here.

The cost of car ownership:

A private citizen on a slightly above-average income will spend 2.75 hours per week working to pay for their car. This includes a very small mileage (less than 40 kms/week). As the weekly distance driven increases, so too does the cost, though the cost per distance decreases.

A private citizen on a slightly above-average income will spend 2.75 hours per week working to pay for their car.

I ran the "model" with a heap of [distance travelled] options to explore how much work was required. I estimated several things:
  1. work hours per week
  2. Cost in Australian dollars per kilometer driven
  3. time spent working, per distance. This is measured in hours per 100km per week. This is the time that is worked by the car owner, each week, for each 100/kms they drive.

What I found really interesting is that it is really expensive.

For a person who drives 10000 km per year (just under 200 km per week) they must work nearly 4 hours per week, which equates to more than 2 hours work per 100km per week, for a total cost of $0.44/km driven. Suddenly public transport is looking very cheap indeed. Remember, this is the private cost borne by the car owner, and does not include all of the public costs borne by the Australian taxpayer.

A total cost of $0.44/km driven -- a limo is cheaper!

As you drive less, owning a car makes even less sense. For someone who drives 3000 km per year, the cost is more than $1 per kilometer. You could catch a limo cheaper than that!


Owning a private car does not make economic sense, even with the strong public subsidies that currently exist. Of course, the bottom line is not the only consideration, so I understand that people will want to keep a car, just for security and convenience. However, if you are a household with more than one car, I would strongly encourage you to consider getting rid of one car and exploring other forms of transport. Your wallet, and your country will thank you!

This article was written by Angus Wallace and first appeared at

Monday, November 3, 2014

National gambling day

Today is Melbourne Cup day: a national celebration of gambling and excess.

I tend to steer clear of gambling, but I wanted to offer a tip of how to maximise the expected return of any bets you make.

My tip: don't bet on the horse you think most likely to win

This probably seems like silly advice, but what you need to consider is that the odds determine your winnings. If a horse is very likely to win, and you bet on it, and it wins, you will not win very much (because its odds of winning are good (say 2-to-1 or even 3-to-2). When horses are very likely to win it is even possible to receive less than you bet (when your horse wins)!

Instead, what you need to do is identify the horses whose odds are overestimated. For example, if a horse is listed as 40-to-1, but you think it's really a 30-to-1, then you should consider betting on it. What this means is that your potential winnings are higher than the risk of the horse not winning.

Of course, doing this is a lot harder than just trying to pick the winner! That's why I don't bother gambling: in my opinion, the race organisers have it all worked out -- the best way to maximise your money at the end of the day, is not to place any bets ;-)


This article was written by Angus Wallace and first appeared at

Sunday, November 2, 2014

Water at my house - part 2: hot water

Hot water

Please refer to this article that I wrote previously, which gives an introduction to the hot water system I've chosen.

At this point, only the main solar hot water system is connected. This supplies the whole house. It is located above the laundry and near to the bathroom, but is across the house from the kitchen (which is on the Eastern side of the house). Refer to Figure 1.

Figure 1: Schematic of the water connections at my house. Note the two hot water systems.
Having a hot water system a long way from where you want to use the hot water is a waste. This is because of two reasons:
  1. the hot water cools on its way to where you want it. This can be reduced by lagging (insulating) the pipe
  2. when you're finished with hot water, you're left with a pipe full of hot water. This is wasted.
This is bad for two reasons:
  1. we're running rainwater, and don't want to run out in summer. Hence we catch the water that's coming out of the tap before it gets hot. This is doable, but is a hassle.
  2. I want to run on pure solar hot water (without electric boosting), and anticipate that this will sometimes be marginal in winter. Any wasted hot water will make this harder.
Because of this, it is very beneficial to have hot water closer to where we actually want it. This is particularly true in the kitchen where solar heated hot water can regularly be used, if it's convenient (eg. fill the kettle with solar-heated hot water to save electricity).

For this reason, I've put a second, smaller, solar hot water heater on the roof right above the kitchen.It holds 30 L and cost AU$300 on ebay. Internally, this is different from the main heater in that there is no heat exchanger (refer to Figure 2 (b) in this article for a detail of the main solar hot water heater). The difficulty is that the unit can tolerate no more than about 5 psi, so can't be used in a normal fashion (ie. the supply fills and pressurises the tank, and a tap at the point-of-use controls the exit of water).

As far as I can see, there are two options to use this tank (which is not yet connected):
  1. set up a small header-tank, and use this hot water system as gravity fed. This is not ideal because it necessitates a float valve and a tank up on the roof that is higher than the hot water system
  2. use a tap that "pushes" water into the tank, causing its hot water to overflow down a pipe, and that water is what comes out of the faucet. This is not ideal because there will be quite a bit of latency between when one turns off the tap and when the water stops coming out. Also I can imagine that in summer the tap could drip if the tank boiled.
I haven't yet decided which of these arrangements to go for. At the moment I'm focusing on building a chicken house!

This article was written by Angus Wallace, and first appeared at

Water at my house - part 1: rainwater

This post will evolve, but the idea is that is summarises what we've done to manage the water at our place. It will contain links to other relevant posts I've written.


Our house was built in the 1950s, and is typical of houses from that era. It is mostly brick, with a tile roof on the main part of the house and a lean-to at the back with corrogated iron. There are four downpipes on the house, one in each corner. Those on the Northern side of the house drained to the back yard. The downpipe at the South-East corner drained to the street (that has possibly the largest flow of the four), and the pipe at the South-West drained to the neighbour's  front yard!
At the back, our garage had a gutter down the East and West side.

Some considerations for our rain water system:
  • We wanted to catch all the water that fell on our roofs, and not waste any
  • We didn't want big tanks in the front yard
  • We wanted to use the rainwater in the house
  • Where possible, we wanted to avoid pumping the water (gravity feed) (in keeping with low energy, as described in these posts)

Rainwater plan

Below is a schematic for our rainwater system
Figure 1: Schematic (not to scale) of the water systems as installed
We decided to install 5 rainwater tanks. We installed
  • 2 x 15 kL tanks in the NE corner of the block as bulk storage.
  • 1 x 5 kL tank near the pergola
  • 2 x 1 kL tanks at the front of the house
I have performed a detailed analysis (using historical meteorological data) of whether this will be sufficient rainwater storage in this article.

This leaves one downpipe without a tank attached in the NE corner of the house. For now, I've raised the pop (within the gutter), so that the water will preferentially drain to the West (the other downpipe that's being caught by the 5 kL tank) and will only go down the NE downpipe if the rain is exceptionally heavy and the gutter is in danger of overflowing (refer to Figure 2(a) for detail). I plan eventually to run a new gutter around the house-attached pergola on the northern side.
Figure 2: (a) cross-section of a gutter showing the raised pop that will only drain when the gutter is in danger of overflowing. (b) detail of the manner of connection of the main solar hot water system

The five rainwater tanks equilibrate via blue-line poly pipe. The 5 kL and 15 kL tanks are connected by 25 mm blue-line. This is low-pressure, and I've also included a gravity fed tap in the middle of the vegetable patch to allow gravity fed watering. The two 1 kL tanks are connected to the 15 kL tanks via 40 mm blue-line. Because of the front tanks' small capacity and distance to the rear tanks, I wanted to avoid a situation where heavy rain caused them to overflow despite there being storage space at the rear tanks. The larger diameter of the 40 mm blueline allows water to more-quickly move from the front to the back of the house.

The 25 mm blue-line from the 5 kL tank is connected to a Grundfos variable speed pump which supplies the house. To use this, we also installed a valve that allows us to turn off the water supply near the street. To save energy, we run our pump at a lower pressure than the main supply (~25 psi, versus 55 psi for the supply). I think that this makes our plumbing work better (our shower in particular works better at lower pressure).

System advantages

  1. Because the tanks are in equilibrium, and are scattered, we have low-pressure rainwater available around the yard. This is a very simple system, and lets us access water under its own pressure without using a pump. Simple, non-powered systems are cheap and resilient, as described in these posts.
  2. Little/no rainwater is wasted. It would have been possible to create a wet sump system, whereby the stormwater from the front of the house was routed to the tanks at the back underground. This would have allowed us to avoid having tanks in the front yard. The problem with this system is that any water left in the pipe between rains tends to go bad. To avoid this bad water entering the rainwater tanks, this needs to be drained. Apart from being a waste, it is very difficult on our block which is almost flat. Although there is water inside the blue-line pipe, this water is often moving: as we use water in the house, it causes all the tanks to re-equilibrate, which cycles water through the pipe and prevents it going bad. It would not have been possible to adjust our gutters to direct water out the back without extensive modifications.
  3. Because we have tanks (and not just piped water) all around the yard, the pressure we achieve by gravity feed is maximised [1]. This means that we can mostly avoid pumping water in the garden (saving electricity).

System disadvantages

  1. Digging the trenches and installing the pipe was a lot of work and the components were fairly expensive. A wet sump system would be cheaper overall (there would be fewer tanks, for example) though installing the piping would be more labour-intensive.
  2. Some might perceive the 2 x 1 kL tanks in the front yard as being an eyesore. I will write more about aesthetics in another post


Through a wide-gauge pipe, it is quite amazing how much water will flow even at very low pressure. Last summer, I would water the garden through a 25 mm hose with as little as 20 cm of water head (~ 0.2 psi), and found it very effective. It doesn't spray, but a lot of water still comes out if one holds the end of the hose near to the ground. Also, the water rate can be regulated by how high one holds the hose.

If I was building a house from scratch, setting up a similar system would be simpler because I would have all the gutters drain to one or two points.

A sizable expense has been the connectors for the blue-line. If I did this again, I would seek a solution that minimised the number of connectors and provide a significant saving.

[1] When water is moving through a pipe, there is a pressure loss that is related to the speed of the water and the diameter of the pipe. By having a column of water (ie. a tank) right next to  where we want to use it, there is less pipe and thus less pressure loss.

This article was written by Angus Wallace and first appeared at
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