The LM2941 regulator in the Linksys WRT54G, WAP54g, and BEFW11S4v4, has an absolute max input voltage of 26v. I think (not sure) that the input capacitor is also 25VDC but can't tell from the markings. I would aim for running it on about 12-15 volts, which seems like a safe region. The common 19VDC laptop power supply seems about right including CAT5 copper losses. I run mine on a gel cell (instant UPS) and slow pulse charger which runs at about 14.4VDC. I wished all such boxes were so easy to power.