Famed entrepreneur Elon Musk already has quite a track record, first as a founder of PayPal, then cofounder of electric car manufacturer Tesla Motors (which acquired SolarCity), and spacecraft manufacturer and operator SpaceX. Along the way, he has amassed a fortune of US$14.8 billion, according to Forbes. But his greatest ambition, claims French magazine Le Point, is to become the energy czar of the future -- the 21st century-equivalent of what David Rockefeller was to the 20th century. \nA believer in futurologist Jeremy Rifkin’s notion of the “zero marginal cost society,” where the production cost of products moves toward zero, Musk aims to turn Tesla into the ExxonMobil of alternative energies, in a society where battery units, rather than barrels of oil, will be the basic unit of measurement.\nAt the moment, a single gram of petroleum contains a lot more potential energy than a gram of battery. A 100-kWh battery on a Tesla car weighs 600 kg, but generates the equivalent of 8.6 kg of oil. Musk hopes to reverse this weight/energy relationship the way IBM transformed computer storage. In 1956, recalls Le Point, a five-megabit storage device weighed a ton; today, a one-terabit card weighs two grams. If battery technology improved at the same rate as computer storage has improved since its early days, the Tesla car of the future could have an energy reserve of 10,000 TWh and an autonomy of 61,300 billion km — the equivalent of about 75,000 round trips to Mars.\nOf course, such numbers are still an extravagant dream, but in the field of solar energy, performance gains have been dramatic. Production costs of solar energy have been reduced by a factor of 130 in the past 40 years, allowing Tesla’s subsidiary SolarCity to produce a single watt of solar energy for US55 cents. Musk is building his 21st century energy empire one watt at a time.