Eneloop and other brands of low-self-discharge NiMH cells will work just fine in solar lights. Traditionally most solar lights use NiCd cells, but that's only for cost reason and not because NiCd is more suitable for such applications. NiMH is actually better because it offer 2-3x higher capacity than NiCd. As the demand for battery capacity increases, even solar lights manufacturers are starting to offer NiMH replacement batteries, such as: Moonrays 97143 AA NiMh Pre-charged 1500mAh Rechargeable Batteries for Solar Lights, 4-Pack
The Sanyo eneloop AA cells offer higher capacity that Moonrays NiMH (2000mAh vs. 1500mAh) and availavle at lower price, so it is a much better deal.