We all know you should unplug your mobile the moment it’s finished charging, but if you plug it in when you go to bed and then undock it next morning as you head out to work, the chances are you’ll have left it trickling all through the night. So why not cut the current after two hours with a mechanical timer, like the kind you’d use for security to switch on and off your lights.

Why not indeed?

I’ve got a little dehumidifier chugging away in the outhouse right now, which set me thinking about power consumption. Would it be better to put it on a mechanical timer and have it switch on and off for a couple of hours at a time, or leave it running all day?

That particular point is still in debate - at least until the initial damp has gone - but it did set me thinking about doing the same thing with the mobile, iPod and other gadgets that need a regular charge. It’s proving very difficult to source any accurate power consumption figures for mobile chargers - particularly in the trickle mode they slip into once the battery is charged - but the numbers for mechanical timers are less obscure.

After various tests, Brian Cryer puts it at around 2 Watts, or two hundredths of a unit of electricity per hour (two-hundredths of a kilowatt hour). My supplier charges 9.15 pence for a kilowatt hour of electricity (including VAT at 5%), so over the course of a year a mechanical timer, left plugged in and switched on 24 hours a day for 365 days, would cost £1.60 to run. You could cut that considerably if you only switched it on when you went to bed, and remembered to switch it off - at the plug - when you picked up your phone again next morning, say seven hours later. If you did that, the annual cost would be a negligible 47p for the year.

That’s pretty good. It’s less than an eighth of a penny a day, but obviously it doesn’t include the cost of actually charging the phone.

Some phone chargers draw around 4 Watts or 5 Watts of power, so a two hour charge of your phone would around 0.07p at the rates stated above. Even if you left it plugged in for the whole year, and it never went into trickle mode it would only add about £3.20 to your overall electricity consumption.

But of course they always draw that much power. Once the batteries have a full charge, the charger itself switches into a trickle mode and dramatically cuts the consumption.

Whether this consumption would be higher than the 2 Watts drawn by a mechanical timer - thus making the cost of running the mechanical timer lower - is unclear, but given that a phone in full-strength charging mode takes only double that it seems unlikely.

So are mechanical timers a effective means of cutting your energy costs? Yes, if used the right way. Their value when it comes to switching off a fully-charged mobile phone is in doubt, but a 23 Watt dehumidifier…? Almost certainly worth the effort.

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