how much energy does it take to charge a cellphone?

Its hard to say without knowing what phone you have, but the short story is, you wont save much at all. I believe the idea behind the solar chargers, is the convenience of charging when you are away from the grid, hiking or sailing for example.

a Razr V3 cell phone uses 7.6V @ 800milliAmps for about 2 hours to become fully charged. The electricity drawn by the power supply is approximately .006 kW/h x 2 hours. In my hometown, we pay 15 Cents per kwh. So 1 charge cycle for my cell phone is costing me literally a fraction of a penny. Even if I needed to charge my cell phone everyday, (which I dont) My yearly cost to charge would still be very low.

If you leave your cell phone charger plugged in all the time, it will always be draining energy from the grid (Source: Treehugger.com, which suggests that 95% of cell phone charger energy is wasted, because the cell phone isn't plugged into the charger).

If you never plug in, you could be saving .006 kW/hr x every hour of the day.

bob is right, a lot of people leaved them plugged in overnight which wastes energy. What most people don’t know is that even when the phone isn’t plugged in, but the charger is, it is still wasting a lot of energy! Sometimes, the adapters plugged into the wall even get warm, which is a total waste of energy. I can’t remember exactly how much, but many things that remain plugged in all the time, like your tv, waste incredible amounts of energy. I remember from somewhere that a tv, while off, was still using 90% of the power it uses while on. If you leave things plugged in, which everyone does, you can use a surge protector or something, and just flip the switch on it to cut all power.

Of course, it is a cell phone, so your not saving much, but you are saving more than just what it takes to charge it. I built a solar phone charger last summer for fun/learning experience. It has a cigarette lighter connection so I can use for a lot of stuff :)