David Zwartz, of Wellington, asks :-

If you shake the loose water off your hands before drying them under a hand-dryer in a public toilet, will this make a significant difference to the time taken to dry your hands? How much electricity would this save? Could it be extrapolated to an estimate for how much electricity would be saved if everyone in NZ did the same and whether we should have a national campaign to shake hands before drying?

Andy Edgar, a physicist at Victoria University of Wellington, responded.

A simple experiment with a bowl of water and some kitchen scales showed that the amount of water taken up by my hands was 6 grams per wash, and the amount shaken off was 1.5 grams.

Hand dryers come in two forms, one uses a stream of heated air to dry the hands by forced evaporation, the other uses a blast of room-temperature air at speeds up to 690km/hr to physically blast water off the hand surface, though with some evaporative effect.

Both types of dryer generally use an infrared sensor to detect when the exhaust air is dry, rather than relying on a timer, so any saving made by shaking the hands would be a real saving because the unit would be on for a shorter time. If we assume that the drying time is proportional to the mass of water on the hands, that saving amounts to 25%.

For both types of dryer, the manufacturersâ€™ data suggests roughly the same electrical energy consumption, about 0.0056 kilowatt-hours (kWh) per usage. Thus the savings would be 0.0014 kWh per usage. For comparison, a typical household power consumption per day is 20 kWh.

To calculate the national savings, I have assumed an average of 30 hand-dries per person per month, to get annual savings of approximately 2.3 million kWh for 4.5 million people. This may be an overestimate.

In comparison, the national energy consumption in 2013 was 41,876 million kWh. On this basis, the national savings would be less than 0.005%, rather small. The annual power bill for the machine owners would be reduced by about \$10 per dryer. The time saving for users would be about three seconds per usage.

So an interesting suggestion, and a surprisingly large reduction in the water carried by the hands after shaking. But the absolute saving in electrical energy is probably not large enough in comparison with other ways such as energy-saving light bulbs, intelligent hot water cylinders etc. to justify a national campaign.