Imagine a cold winters day – you have just gotten back from work and are looking to enjoying a nice hot shower.
Oh no! The kids were thinking the same and there is no hot water left.
Having had this exact issue today – I have set about seeing how I can optimise my hot water usage which should save both my sanity and my power bill.
How to save hot water
There are three basic steps that I use:
- 1 – Check how much water each shower uses
- 2 – Test some options
- 3 – Check out the savings!
Step 1 – Check how much water each shower uses
To do this you need two things:
a) A bucket with clear litre markings
b) A stopwatch
The process is simple – fill up the bucket to a set mark from each of your showerheads, and time how long it takes.
The New Zealand Water Efficiency Labelling Scheme (WELS) allocates a star rating to how water efficient your shower is. The WELS suggests:
- 0 Stars: More than 16 litres per minute
- 1 Star: 12 to 16 litres per minute
- 2 Stars: 9 to 12 litres per minute
- 3 Stars: 7.5 to 9 litres per minute
- 4 Stars: 6.0 to 7.5 litres per minute
- 5 Stars: 4.5 to 6 litres per minute
- 6 Stars: 3 to 4.5 litres per minute
Typically a flow rate of 7.5 – 9 litres per minute is recommended as a good balance between comfort and efficiency for family showers. Showerheads with WELS (Water Efficiency Labelling Scheme) ratings of at least 3 stars meet this flow rate target.
To measure my house flow rates I used a basic Bunnings Warehouse bucket along with a Vivid to mark the “7 Litre” mark in black.
Here’s what I found:
| Location | Time to fill up 7 Litres? | Flow rate | WELS rating |
|---|---|---|---|
| Kids shower (normal) | 64 seconds | 6.6 litres / minute | **** |
| Kids shower (rainhead) | 58 seconds | 7.2 litres / minute | **** |
| Family shower | 70 seconds | 6.0 litres / minute | **** |
| Mum & Dad's shower | 37 seconds | 11.4 litres / minute | ** |
Ok, so I think I may have found the problem – my shower! Whoops.
With a flow rate around double of our family bathroom, my wife and I are chewing through more than our fair share of hot water.
Step 2 – Test some options to reduce hot water usage
There are three main options I can think of to reduce my hot water usage:
- Take shorter showers (uhhh…. next!)
- Replace my shower head with a lower water consumption version ($100 – 200)
- Install a water restrictor ($10 – 20)
I popped down to Mitre 10 and brought a pack of these to try:
[I’m doing this on the weekend… want and see]
3 – Check out the savings
Ok now is the fun part (for me at least!). To figure out how much water and energy this change may save, let’s assume:
- Daily shower time: 10 minutes for each Mrs and Mr That Power Guy
- Old 2-star showerhead: 11 L/min
- New 4-star showerhead: 6 L/min
Water used per day:
- 2-star: 11 L/min × 10 min × 2 people = 220 litres/day
- 4-star: 6 L/min × 10 min × 2 people = 120 litres/day (i.e. 100 litres saved per day)
I expect this is actually on the conservative side, as if I go for a run or kayak after work I can easily have two showers a day. Same with Mrs That Power Guy (that power gal??).
Estimating annual savings in water consumption:
100 × 365 = 36,500 litres of water saved per year!
To calculate annual power savings it’s a bit more complicated.
First of all, I need to approximate how much of my shower water is heated (from the hot water cylinder) and how much is just from the mains supply.
A quick Google tells me that showers are typically 60-70% hot water mixed with around 30%-40% cold water. This is backed up by an American study I found that says “…73 percent of all water use for showerheads and lavatory faucets is hot”.
As we live in Auckland, and it’s kinda warm most of the time, I am going to be conservative and assume that 60% of my shower water is hot and the other 40% cold.
My hot water cylinder is set to 60 degrees celsius, and the cold tap water is about 15 degrees celsius, so I am raising the temperature of each litre of hot water by 45 degrees.
To calculate this in units of energy, I need to use the following formula:
Energy (kWh) = (Volume of water in litres x 1000 grams x temperature rise x Specific heat of water) / (3600 × 1000)
For my shower, this is :
(36,500 litres x 45°C x 1000 grams x 4.186 joules/gram/°C) / (3,600,000) = 1,910 units of electricity.
My average cost of per unit last year was around 25 cents – therefore the extra cost of heating this water I can estimate as : 1,910 x $0.25 = $477 per year
I’m pretty impressed at this. Almost $500 per year is a large saving on my power bill – equating to around two months free power.
I’ll update this post after a few months and see whether the change has made a real difference….