Connect 25 AA batteries in parallel and series

[w0z]

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It is neither recommended nor good electronics practice to connect AA batteries in parallel, if one cell goes low resistance it will drain the others (unless you use a diode on the output of each cell which in the case of AA's isn't practical as the forward voltage drop on the diode will be too significant). AA's also vary from cell to cell, nominally 1.5V alkaline but I've had new ones measure nearly 1.7V. NiMh cells measure 1.2V. (Don't even think about lithium, the cost would be huge)

LEDs work at constant device current for optimum output and the resistance in series with the led or leds is calculated to supply the maximum (or near maximum) current at the highest voltage the power source can provide, after taking into account the LED voltage drop. (or you could use an led driver chip).

You would also need to add in the cost of battery holders, unless he's going to try to solder all the cells together (surely not!).

As an example, a 1.2V nimh cell at say 1500mAh, not taking into account any losses, would provide 1.2x1.5=1.8Wh- you'd need 555 cells to provide 1 kWh, at a quid each that would cost £555. (not that there's anything relevant about a kWh other than it costs about 13 pence from your local electricity supplier)

I should have added that only the first kWh would cost that, you could then recharge them but I'll leave someone else to work out how much it would drop to per kWh if you got 500 charge cycles out of the cells!