When talking about signal strengths, values are usually expressed in dBm, which is, according to Wikipedia 'an abbreviation for the power ratio in decibels (dB) of the measured power referenced to one milliwatt (mW)'. In other words, dBm is a value for the power or signal strength on a logarithmic scale.
In practice, signal strengths between -60 and -90 dBm are encountered at the receive antenna of a mobile device. So how much is that actually? On the high end of the scale, -60 dBm equals 0.000000001 Watts, while – 90 dBm are 0.000000000001 Watts.
I find two things quite remarkable in these numbers: First, both values are very small, definitely not enough to power anything and its amazing that receivers can pick up such small signals. And secondly, the wide range a receiver has to adapt to as the weakest signal compared to the strongest signal in this example is 1000 times weaker.