My time traveller has a maybe 20-minutes-into-the-future smartphone and laptop. They can store large amounts of data, on the Petabytes level or higher. The time traveller can effectively store the entirety of the current day's internet and library of congress or other information on his laptop.
The problem is, they have a limited battery life. On very conservative use, it would be around 2-3 weeks before it runs out. Any DC current can charge the time traveller's equipment, though it supply a minimum of 5W of power on average to charge.
The so long as the time traveller's equipment is working, he can consult his database to help him construct devices necessary to charge his equipment. eg. Give him a motor and he would be able to rig it so that he can charge his equipment.
However, once the battery dies, he would no longer be able to do anything except the anything say, a college graduate would be able to do.
How far can this time traveller go back in time before his electrical equipment becomes unchargeable?
EDIT: I did not expect a solar powered solution, though it seems like a pretty good idea. However, some of the answers gave rise to some clarification I should make about the time traveller's equipment.
His laptop and smartphone require a DC current to charge, simply because a AC current would discharge when the polarity gets reversed on the other side of the waveform. (average current is 0) Essentially, this would mean that a rectifier would be neccessary if a AC current is used.
The idea is that the laptop and smartphone are off-the-shelf from the time traveller's time, so such external power methods would not be incorporated by default. The batteries are high-capacity such that although the laptop and smartphone consume significant amount of power, they can last for a relatively long period of time. Their actual specs are that on general continuous use, they last about 24 hours. But if you turn them off and on just for minor uses (such as just referencing some data), they can be expected to last 2-3 weeks.
In a sense, this question could be converted to: when is the earliest time that a DC power source with at least 5 watts of power (or more likely, enough a rectifier) can be obtained within 2-3 weeks?