68Modern phone chargers, when left plugged in with no phone attached, use
about half a watt
. The Maplin power meter in figure 11.2 is not accu-
rate enough to measure this sort of power. I am grateful to Sven Weier
and Richard McMahon of Cambridge University Engineering Department
who measured a standard Nokia charger in an accurate calorimeter; they
found that, when not connected to the mobile, it wastes 0.472 W. They
made additional interesting measurements: the charger, when connected to
a fully-charged mobile phone, wastes 0.845 W; and when the charger is do-
ing what it’s meant to do, charging a partly-charged Nokia mobile, it wastes
4.146 W as heat. Pedants sometimes ask “what about the reactive power of
the charger?” This is a technical niggle, not really worth our time. For the
record, I measured the reactive power (with a crummy meter) and found it
to be about 2 VA per charger. Given that the power loss in the national grid
is 8% of the delivered power, I reckon that the power loss associated with
the reactive power is at most 0.16 W. When actually making a phone-call,
the mobile uses 1 W.

Further reading: Kuehr (2003).

Figure 11.6. Advertisement from the “DIY planet repairs” campaign. The text reads “Unplug. If every London household unplugged their mobile-phone chargers when not in use, we could save 31,000 tonnes of CO2 and £7.75m per year.”