I found this 48 Watt LED for $16 on Amazon and tried it for indoor testing. I only got about 180 mW out of my solar panel, about a factor of ten too weak, but still, good enough for some hard-and software development. Meanwhile, I saw a youtube video by mjlorton at https://youtu.be/bIzeLG-pMkE where he points out the potential of growth lights for sun simulation. For example, there is a MarsHydro 300 Watt light for $99 http://amzn.com/B00XC3LBI2. These lights might be closer to the actual sun frequency spectrum, but I'll wait and see what his tests reveal.