There's this idea being promoted that washing at 30°C rather than 40°C will help reduce power consumption. I am dubious about whether things will be cleaned well enough, especially given the advice "It is however recommended that towels, underwear, sportswear, baby clothes, all bedding, and heavily stained items still be washed at higher temperatures to ensure they get completely clean."

Currently I'm working my way through the draft of Sustainable Energy - Without the Hot Air, a book on sustainable energy by Professor David MacKay of the Department of Physics in Cambridge. It is an excellent facts-and-numbers-driven analysis. His debunking of the mobile-phone charger myth inspired me to do some calculation on "washing at 30".

The manual for our washing machine states that it uses 59 litres of water for a standard wash, and 0.5kWh of electricity. It is plumbed into both hot and cold mains, and for wash temperatures up to ~65°C it uses the house hot water rather than doing any heating itself. As a household, we do a nappy and a non-nappy wash most days of the week, nappies at 60°C and everything else at 40°C.

Non-nappy washes
For the sake of easy calculation, I'll call it 6 washes a week currently at 40°C, and 60 litres of water per wash. The specific heat capacity of water is 4.2 kJ/kg/K and water is handily 1kg/l.

60 kg x 4.2 kJ/kg/°K x 10°K = 2.52MJ per wash, so 15.12MJ per week.

Our water is heated by gas and we are billed for gas in kWh. 1kWh = 1000 J/s x 3600s = 3.6MJ.

15.12/3.6 = 4.2kWh per week. We are currently charged 2.574p/kWh inc VAT, so the saving would be a grand total of 11p per week, or £5.72 per year.

Either there is something wrong with my calculation or this is a fairly minimal effect on energy consumption.


Nappy washes
We wash the nappies at 60°C but strictly speaking, only the soiled nappies need to go at 60, the rest could go at 40 with the non-nappy washes. Without worrying too much about implementation, we could cut from 6 x 60°C washes per week to 3 x 60°C washes, and 2 x 40°C washes (we could probably eliminate one wash a week by mixing the wet-only nappies with other laundry).

Pleasingly, 3 x 60 x 4.2 x 20 is the same as 6 x 60 x 4.2 x 10, so we know that part of the answer already. What about saving 1 washload a week? If we assume the water is heated from mains cold at 10°C to 40°C then we have 1 x 60 x 4.2 x 30, which is half the previous answer. Plus we save the 0.5kWh of electricity which costs just under 11p/kWh.

So in total, we could save 22p per week by separating out the soiled nappies, and we could only do this by continuing to wash at 40 most of the time, so it's not additional to the 11p per week above.


Showers
Showers are also usually taken at about 40°C. Some quick experimentation with a measuring jug and the shower tells me that our shower flows at about 8 litres per minute. So if we shower for 7.5 minutes that's the same as one non-nappy washload. My guesstimate from our morning routine is that I spend 5-10 minutes in the shower and Tony spends 10-15 minutes. Plus Jason and Jonny take showers every day in the other bathroom, but I don't observe for how long. Our showers are both fed from the hot water tank and do no additional heating of their own.

If we assume an average of 10 minutes per adult per shower per day, that's 280 minutes of showers a week, equivalent to 37 washes at 40. The energy used by heating water for a wash at 60 is 5/3 that for a wash at 40 (heating from 10 to 60 rather than 10 to 40), so our current laundry is equivalent to 6 x 8/3 = 16 washes at 40 (16x5.5p=88p/week), less than half of the cost of showering. Without the nappy washes, it would be less than one-sixth (6x5.5p=33p/week).

Baby costs
Jonny asked just now "so how much does Charles cost then?" to which the answer is 6 nappy washes and 1 non-nappy wash per week.
Water heating is (6x5/3 + 1) x 5.5p = 60.5p/week.
Running the washing machine is 7 x 0.5kWh x 11p/kWh = 38.5p/week.
A total of 99p/week on baby laundry energy costs. Detergent costs are left as an exercise for the reader.

Thank you for all the interesting comments! I'm sorry it took me a while to respond to all of them, but Gmail helpfully started tagging all my comment notifications as spam shortly after the post.

The most clear conclusion, though only 3 of you were brave enough to tick it outright, was "Rachel, Tony & Charles are strange". That is, we are different from the majority of UK parents in the following ways:

1. We don't own a car, so we have no default travel option.

2. Although we do use hired or borrowed cars when that seems the best option, only one of us can drive (and it's the mother rather than the father).

3. We are comfortable using public transport, and have years of experience travelling together thereon to calibrate how much we can reasonably manage.

4. Most of our UK journeys are long-distance (and thus requiring more effort from the sole driver), between cities/towns well linked by trains, and with reasonable buggy-friendly urban bus provision. Our experience is that train+bus doesn't add much more time to each journey than the many necessary comfort stops when driving with the baby.

5. We live in a particularly compact, walking- and cycling-friendly city which shapes our expectations of how to get around.

6. All of the above constrain our purchases of luggage, baby equipment, in fact anything we might want to take on holiday, to the lightweight and portable.

7. We breastfeed and co-sleep, both of which cut down our required baby baggage considerably.

Because driving isn't the norm for us, when we do drive, we are probably more stressed than on trains/buses. Certainly I am more tired. These will both communicate themselves to Charles and affect his mood. I am also ignoring Charles for long periods in order to drive, and he's still clingier to me than to Tony. It's likely also that he's not terribly used to the car or the car seat, and the car seat is more restrictive than the buggy. He is also at a particularly wriggly phase and doesn't seem to sit still anywhere for more than five minutes unless restrained or asleep. I suspect the biggest improvement we could make to long-distance car journeys would be to have a second driver, both to give me a break and to give Charles more mummy-time. Perhaps when I'm feeling less traumatised about the last few long car trips, we can experiment if an appropriate journey comes up.

In any case, I am now much less puzzled about having to repeatedly defend not-driving-with-Charles, which was mostly the point of the exercise. Thanks for helping me examine my assumptions.

I was told today "you're very brave to come all the way from Cambridge with a baby on public transport", and answered honestly "it would have been much harder any other way". I keep coming up against this perception that cars are the easiest way to transport children, in complete contradiction to our experience. Either we are strange, or the vast majority of parents are self-deluding on this matter. Or, more charitably, encouraged into delusion by advertising, social expectation, and reluctance/unpreparedness to try the alternative.

Poll #1133041 People are strange, when you're a stranger
Open to: All, detailed results viewable to: All

So, what's going on?

I have an email from 11 months ago where I wrote "[the pc] takes so long to boot up and down that I am *not* doing so every time I'm pulled away from it". That was when I was still running Windows. It would take 5-10 minutes from power-on to being able to do useful work. I think after that I did start getting it to hibernate rather than shut down, and that improved things, but it was still a great faff.

One of the things I noticed after switching to Ubuntu was the startup was much quicker. I timed it this morning and it takes 60s to get to the login screen, and then about 35s more after typing username and password to stop opening windows and restoring my previous state. It takes 30s to shut down.

All this means that I am much happier to shut down every night, and during the day if I know I'm going out, and as a result my computer has gone from being on pretty much all the time to about 70 hours a week or less, saving 98+ hours a week. I base that 70 hour estimate on a rough schedule of 2pm-10pm Mon-Fri, 8am-11pm Sat & Sun. It goes on later if Charles is demanding or we have other things to do, and of course once it is on, I am repeatedly and unpredictably drawn away from it at no notice.

I haven't got data for my new hardware yet, but using a cheap power meter from Maplin, I collected data on the old hardware before deciding to be rid of it. Among other things, the meter measured consumption in kWh and number of hours on. Following the timetable given above, the pc unit was drawing 130W and the monitor 80W. Being off for 100 hours a week saved 21kWh per week. Assume leave and weekends away mean I'm away from home 6 weeks of the year, that's 966kWh per year, assume a cost of ~10p per kWh (reasonable based on my bills during 2007) and that's nearly 100 pounds saved annually just by an improved start-up process.