When I wrote in December about our summer shading system I thought we had shaded everything we could reasonably manage. But as it got hotter we found more exposed areas that could be protected and added:
additional dense (90% block-out) shade cloth to patch a gap in the rear (southwest) courtyard shading,
on very hot days, a towel inside the (northeast) bathroom window, which has no eaves,
on very hot days, an old painting canvas inside the (southwest) laundry glass sliding doors, which get blasted with afternoon sun,
an old woollen wall hanging on the southwest wall outside Eva's bedroom, and
three towels left on the clothesline all summer along the northwest living area wall.
These also make a good cubby.
Initial Time: Shade cloth: fifteen minutes to bend hooks and hang the cloth; wall hanging:five minutes to put a hook in; towels: 30 seconds each to hang; paint canvas: 30 seconds to balance in place on the grey water bin.
Initial Cost:$20 for shade cloth, zero for towels, paint canvas or wall hanging (which was being thrown out by a family member)
Ongoing time or cost commitment: One or two minutes every week or two to readjust the rear shadecloth as it slips a bit in windy weather; 30 seconds each on hot days to put up the inside towel and paint canvas; probably about ten minutes at the end of summer to pack away for cooler weather.
Impact:A southwest facing vertical surface in Perth in summer receives approximately 3kWh of energy per square metre per day. If the surface is unshaded transparent glass, almost all this energy gets inside the house as heat. The additional shading to our rear courtyard gives 90% shading to about 3 square metres of glass, reducing the amount of heat entering our living area in summer by approximately 8.1 kWh per day. We have a 300% efficient reverse cycle air conditioner in the living area which would use 2.7 kWh of energy to remove 8.1 kWh of heat.So, by shading instead of turning on the airconditioner most days, we are saving 2.7 units of electricity per hot day just with this one area of shade cloth. Put another way, we start to get ahead of the $20 spent on shade cloth on the 27th day of not turning on the aircon. (Thanks Tyson for knowing the numbers!)
In less precise terms, I estimate that this summer it has generally been the third day in any heat wave before we turn on the airconditioner, and last summer I think it was more often the second day. I consider a heat wave to be when the temperature stays at the mid thirties or higher for more than one day. Our airconditioner rarely runs on days with temperatures up to the low thirties (celsius!).
Calculations for the towels and wool hanging shading wall areas, and the paint canvas inside vertical blinds, are more difficult, but we notice that these rooms heat up slower. (In terms of impact, one woolen wall hanging also has a new purpose in life and has been saved from landfill, at least for a while.
Our recent power bill for 22 Dec to 24 Feb shows an average electricity usage of 8 units a day (530 units total), a very very small reduction on the same period last year (10 units less over two months). Most of the past year our average usage has been around 6 units a day, so summer is still presenting the biggest challenges for reducing electricity use at our house. It was the fourth hottest summer in Perth ever recorded (since 1897) and I was going to excuse our tiny reduction by pointing to this fact - until I discovered two of the other three hottest summers were 2009-2010 and 2010-2011. (Its hard to deny climate change when three of the four hottest summers here in the last 115 years were the last three consecutive years... the other was 1997-98). So my alternate excuse for our electricity usage not reducing compared with last summer is that this year I am pregnant and feel hot quicker!
To put our usage in some perspective, however, Synergy (our local electricity supplier) estimates the average household electricity use in WA's southwest grid is 6250 kWh per year (over 17 units per day). Our household's 2011 usage was 2452 kWh (6.7 units per day). We are living a perfectly comfortable existence on that amount of energy.
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