Wal-Mart, the largest US retailer, says it the percentage of its customers buying energy saving fluorescent light bulbs has more than doubled over the past year, from 5 to 13 per cent, reflecting the impact of its drive to promote the sale of the bulbs across its 3,000 US stores.

The bulbs are one of five “green products” whose adoption levels Wal-Mart says it will start publicly tracking on a quarterly basis as nationwide indicators for the adoption of more environmentally friendly products.