Ensure the product meets CE and RoHS certifications to guarantee the absence of hazardous substances like lead or mercury. For the North American market, FCC compliance may be necessary for electronic components. Technically, look for amorphous silicon solar cells, which are more efficient under indoor artificial light. The button life cycle should be rated for at least 500,000 presses, and the auto-power-off function is essential to preserve the backup button cell battery (typically a LR1130 or LR44).