It's a box full of batteries, with a circuit to limit discharge to their "one tablet per day" rate.
And a warranty that's slightly shorter then the battery life.
Ha, that was my guess too! A custom-built battery with enormous internal impedance. That way it's impossible to rapidly test the total stored energy (by heavy load etc.) You could short it, and it still might take a year to run it down to zero.
Actually this has a history.
It's the Alessandro Volta perpetual motion scam of 1802! Volta himself was a "free energy" believer. He was certain that his Voltaic Piles were based on "contact electrification" and not on chemical reactions, and they would operate forever if he could just get rid of that pesky corrosion problem. Many experts of the time were confused about this, especially after experimenting with dry-electrolyte batteries, the DuLuc or Zamboni "dry piles" using thousands of layers, with dry paper as the electrolyte. Such a device could put out microwatts and drive electrostatic motors. Simple calcs show that they'd do this for several centuries before their plates finally corroded away.
See, it's the same scam: immense series impedance lets the battery keep working, but it prevents any quick measurment of net energy storage. It's perfect for "powering the controversy." It might go for centuries!
Steorn just has to word their guarantee so that, if their infinite-life battery fails after ?months?, then it's the user's fault.
The central idea here is that there's no such thing as an insulator. Materials are either electron conductors (metals,) or they're ionic conductors (electrolytes.) Glass, plastic, rubber, if they have any mobile ions at all, and you deposit a thin film of the material between plates of two differing metals, you've got a battery. It's internal resistance might be megohms or gigohms, but it's still a battery, and can (slowly) charge up a capacitor etc., as long as the capacitor dielectric has higher resistance than the "electrolyte."