Dishwashing, hand vs machine

What are the costs of dishwashing and the relative costs of machine vs hand dishwashing?

Hand washing
A typical bowl has a capacity of approximately 12 litres when full. Suppose that one fills it to 10 litres and then, in the process of rinsing, replacing, topping up, etc, one uses twice that amount, ie 20 litres, which has a mass of 20 kg.

This water has to be bought and there is a charge for disposing of it. If both supply and sewerage are metered, these are in the order of £1.50 per cubic metre for both supply and sewerage, ie a total of £3 per cubic metre or £3/1000 = 0.3 p/litre. So the 20 litres of water cost 20 x 0.3p = 6p.

That water has to be heated, from an average temperature of 10°C to around 50°C. That is a rise of 40 K. (The kelvin, symbol K, is the unit of temperature difference, but you could, perfectly adequately, say that it was a rise of 40 °C.)

The energy needed to raise 20 kg of water by 40 K is
20 kg x 4200 J/kg K x 40 K = 3,360,000 joules = 3.36 megajoules (MJ). (4200 J/kg K is the specific heat capacity of water, the energy needed to raise the temperature of 1 kg of water by 1 K.)

The standard unit of energy supply is the kilowatt-hour (kWh), which is 3.6 MJ. So the energy needed to raise 20 kg of water to 50°C for washing up is 3.36/3.6 = 0.93 kWh.

If one heats the water by full-rate electricity, at 17 p/kWh, the heating of the water costs 0.93 x 17p = 15.8p.

If one heats the water by off-peak electricity at 10 p/kWh, the cost is 0.93 kWh x 10p/kWh = 9.3 p.

If one heats the water by gas, when gas costs 3 p per kWh, the cost of doing that depends on the efficiency of the water heating process. Typically gas water heating is 50% efficient, meaning that the heating actually costs 6 p per kWh. So for 0.93 kWh, the cost is 0.93 x 6p = 5.6p.

Washing up liquid
A modest squirt of washing up liquid is about 8 g. If the washing up requires 4 squirts, that is a total of 32 g, about 32 ml.

Tesco Lemon washing up liquid (Aldi price match they say) costs 73p/litre. So 32 ml costs (32/1000) x 73p = 2.3p.

So hand washing our significant load of dishes costs:
water supply and disposal – 6p
heating the water – 9p (using the middle calculated value above)
washing-up liquid – 5p (I’ve allowed for a number of extra squirts)
Total – 20p/wash

Machine dishwashing
Let’s compare this with using a dishwasher.

The Hotpoint HFE 1B19 has an energy rating of A+. It uses 12 litres of water per cycle and an energy consumption of 1.039 kWh.

The water costs are 12 litres x 0.3p/litre = 3.6p.

The energy costs for the water are 1.039 kWh x 17 p/kWh = 17.6p. This is significant more than that for the hand washing because, unless the dishwasher is run overnight, it heats the water on full-price electricity and neither on off-peak electricity, nor gas.

Tesco All-in-one lemon dishwasher tablets are £3 for 30 tablets, so 10p/tablet.

This Hotpoint dishwasher costs £270. If it does 1 wash per day for 5 years that’s 365 washes per year x 5 years = 1825 washes. The capital cost of the machine per wash is £270/1825 = 14 p/wash.

So the cost of machine dishwashing is:

water supply and disposal – 4p
heating the water – 18p
dishwasher tablet – 10p
capital cost of machine – 14p
Total – 46p

Our dishwasher may last more than 5 years with 1 wash per day. So perhaps the capital cost may be down to 10 p per wash. That brings the cost per dishwasher cycle down to 42 p. Our handwashing may actually use 4 bowls’-worth of water, doubling the cost to 40p. That makes cost per hand wash 40 p per wash, compared with the machine wash cost of 42p.

Even if most of the dishes are washed in the machine, there are usually some items that need hand washing. A comparison between hand washing and dishwashing needs to take into account that machine dishwashing is usually accompanied by some hand washing.

There isn’t really much to choose between hand and machine dishwashing. They are of similar cost but the indications are that machine dishwashing is possibly somewhat more expensive.
Dishwashing costs in the order of 40p per big load, perhaps 40p x 365 = £146 per year. Let’s call that £150 per year.
Energy costs are about 40% of that, around £60 per year.