19 October 2013

Back to Basics: Boiling the Kettle

How much energy does it take to boil the kettle? Is it more than I need to use?

Some time ago I realised that I almost always overfill the kettle, and I have been working at reducing how much water I put in. It took a while to get the hang of it, but I can now say we are committed to putting in only as much water as we intend to use. As my mum and dad are visiting from Victoria for a couple of weeks, our kettle is working extra hard and it seemed timely to publish this commitment.

This is a simple post about a simple action in tune with our original intention to take up one additional act towards sustainable living per month. 

Initial Time: zero - but it did take many months to change old habits. 

Like my habit of filling and boiling the kettle half an hour before guests arrived, so they wouldn't have to wait so long for their cuppas. This then required reboiling the kettle when they arrived and often I had more hot water than I needed because I didn't know in advance how many cuppas I would be making. I can't think of a guest to our house who would mind waiting a few extra minutes for their hot drinks.

Or the practice of someone in this house (who shall remain un-named) of boiling the kettle then an hour later remembering the cuppas have not been made so reboiling the kettle... and perhaps  remembering this or the next time to actually do something with the water...

Initial Cost: zero - a cost saving (see below)

Ongoing time or cost commitment: zero

Impact: When I fill our kettle I am generally boiling around 1.25L more water than I need. It takes 430.5kJ to heat 1.25kg of water from 18°C (ambient temperature) to 100°C (boiling). This is equivalent of approximately 120Wh. 

To put that in perspective, every time I boil a full kettle to make one or two cuppas, the excess energy I am using is equivalent to leaving two sixty Watt light globes on for an hour, or to having our energy-saver 20 Watt kitchen light on for six hours.

If I boil the kettle full four times a day, in a year I would have used 174.5 kWh of electricity doing nothing at all - boiling water I don't need. Our Perth electricity company currently charges 26 cents per kWh (unit) and we have opted for a natural power premium of an extra 5 cents per unit. This means our commitment to only boil the water we need is saving us $54 per year (plus GST - total close to $60).

[for those interested in formulas: 

change in temperature (ΔT) x mass (M) x heat capacity of water (C), 
where ΔT=82°C M=1.25kg C=4.2kJ/kg°C = 430.5kJ
1 kWh = 3.6MJ
0.4305 / 3.6 = 0.1195kWh = 120Wh

0.1195kWh x 4 boils per day x 365 days = 174.47 kWh]

Thanks Tyson for knowing the formulas... I'll have a cuppa next time you're boiling...