I did say it was an older circuit , construction of solar cells has come a long way in the past few years and conversion efficiency has been improved. Yet they still remain an expensive item to purchase.
£1 (one english pound) gets you a single solar cell with the characteristics..
[lifted straight from the Maplin website]
Ouput voltage: 0.45V
Output current: 100mA (in bright light)
Dimensions: 47 X 27 X 7mm
Weight: 10g
And with the circuit that would supply 3mA to a 9v battery, in bright light.
So in 1 day of bright light, no clouds, that would supply 30mA to the battery and as the battery would save only around 80% of the charging current it really makes for a slow charger. If you were to buy 16 of the solar panels you could charge a 7.2 PP3? in 1 hour
Ouch I've got 14 kaneka 65 watt thin film panels on the house and 8-65 watt poly crystaline panels along with an 80 watt panel on my shed roof. I'm glad we don't get hurricanes around here or I'd be stuck running gensets just to turn a light on if the winds not blowing.