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Week Twenty-Nine - Hot Water

Small changes save 302 kilograms CO2e per person per year

· Weekly experiments


- You have too many toys in the bath. How will you fit?

My kindergartner

- It's easy! My ponies make room by keeping their hair bands in.

My Little Pony, rubber duckie, lego, duplo and other toys in the bath

In hot water

Hands up if you love long showers? Me too, apparently. We measured our showers and baths for the week and I lost my hippie smug when I discovered I used most of our hot water. Oops. But when I looked at our overall efficiency, I realised we'd made some smart choices.

How much CO2e does a household generate from hot water?

In a dry country like ours, saving water brings its own benefits. But saving hot water also saves energy and carbon dioxide and equivalent emissions (CO2e). How much?

Various sources say hot water uses 16% - 37% of household energy. I picked 25% of household electricity based on my data and Australian government data. This means the average Australian household generates around 460 kilograms CO2e per person per year from hot water.

My household generates less than that. We use cold water in the washing machine, we have a low-flow showerhead and a solar hot water system with electric boost. I measured our hot water usage and found the three of us use around 750 litres per week. I used industry data to estimate our system's power usage, which showed we were generating around 65 kilograms CO2e per person per year to heat that water. We managed to cut down even further.

How can you measure your hot water usage?

It's simple to estimate your hot water usage.

- Find out how many litres of hot water go into one bath (72 litres for us). Record how many baths you have in a week (4 baths for us).

- Find out the flow-rate for your shower. I used a stopwatch to fill a bucket for 10 seconds, measured the litres in the bucket and then multiplied by 6. This showed our shower's flow-rate was 6.6 litres per minute. Our low-flow showerhead is working brilliantly, given that normal showerheads use 20 or more litres per minute. Once you've got your flow-rate, time and record showers for the week to calculate your hot water. My family had sixteen showers averaging 4 minutes each.

- We already wash clothes in cold water, so I didn't measure laundry. If you wash hot, work out how much you use per load from direct measurement or manufacturer specifications and record loads per week.

- I omitted hot water for washing dishes by hand, as it's very low. If it's high for you, include it.

- I omitted the dishwasher, as it fills with cold water and heats it via an element inside (this electricity is covered in Week Twenty-Eight).

Picture of a low-flow showerhead

A low-flow showerhead slashes water and energy use without you even noticing.

How did we cut back?

Timed showers and low-flow showerhead

The adults in my family shower after exercise which means a daily wash. My partner already took short showers. I've installed a timer to keep me under 4 minutes, which is easy most of the time but gets tricky when washing hair and shaving legs! Some people prefer a longer shower every second or third day, so pick what suits you.

The biggest win comes from a low-flow showerhead. If you don't have one, get one immediately. It will halve your overall usage without you noticing. I've tried out some that give pins-and-needle showers but ours is gorgeously comfy, so shop around if you don't like the one you have.

Fewer baths

My kindergartner loves a bath. We run it a few inches full three times a week. Baths are a bit of a luxury for adults, so have them if you love them, but limit the frequency.


Why waste hot water on laundry? If clothes aren't getting clean, soak them first, try different detergents or drop your standards and have an easy life. We only use laundry hot water to zap bacteria when someone's got a tummy bug.

Turn down the thermostat

Our hot water thermostat is already set low, but many are set to the highest temperature. Governments recommend setting it at 60°C for a storage system or 50°C for instantaneous, which kills bacteria without wasting power. A 50°C setting also reduces the risk of burns.

Solar thermal hot water?

Solar thermal hot water is another way to save power. I spoke to Robert Edwards of Canberra Solar Hot Water Repairs, who's been working in solar hot water for almost thirty years. Robert had some great tips.

'A large storage tank acts like a battery and needs less boosting,' Robert said. 'Put a timer on your booster so you only use it when you need it. Use a pitched roof frame to run efficiently in winter. And make sure you get the installation right because it's as important as the system itself.'

There are flat-panel, heat pump and evacuated tube systems, all of which suit different climates. Government and industry both recommend electric boosting rather than gas for environmental reasons (it's also cheaper to run). We've had good results from our evacuated tube solar hot water system. We installed it a decade ago and use around 15% of the power of an electric-only system. But solar PV panels and heat pumps have improved a lot in that time, so if I were upgrading now, I'd consider those first.

How much CO2e did I save?

Without changing systems, the average Australian could save a massive 302 kilograms CO2e per person per year by setting a smart thermostat temperature, switching to low-flow showerheads, washing clothes in cold water, limiting baths and sticking to a daily 4-minute shower.

I've already made the big savings, so the only thing left to do was install a shower timer. That only saves me 13 kilograms CO2e per person per year, but I'll do it anyway. It'll keep me honest and save some water as well as CO2e.

Share showers to save water? Less sexy than it sounds.

Spreadsheet calculations, data sources and references in 'Notes' section, Week Twenty-Nine.