4 ms·

sunlight. It has the best price / benefit ratio (0) and an extremely long operational lifetime (billions of years).

• newsbinator 5y ago
  There's no usable sunlight (on skin) for 4 ~ 8 months a year in big chunks of the populated world.
• postsantum 5y ago
  Great! Unfortunately, where I live, this item is out of stock for the next few months