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.