Got a half-finished cup of tea sitting on your desk? Rather than letting it go cold, why not use its milk and sugared heat to power your desk lamp? It's all down to an ingenious device invented by the Scottish clergyman Robert Stirling in 1816. A cup-sized version of his Stirling engine is being sold by gyroscope.com for £45.99.

Starting the business only eight years ago after being forced to buy a job-lot of 12-gyroscopes rather than just the one he wanted, gyroscopes.com's owner now runs the £150,000 online business from his Gloucestershire garage. The seven sizes of model Stirling engine he now sells work by converting the thermal energy of a cup of tea into the mechanical energy of a piston or fly-wheel – attach this to a motor and you have energy for (almost) free.

Stirling engines aren't just for model enthusiasts, the New Zealand firm Whispergen has made an engine large enough to make 1kW of electricity - Powergen are importing 80,000 of these household heat and power units into the UK for launch in 2009. If you want to replace your central heating boiler with the Whispergen and simultaneously generate electricity large quantities of tea will be required, the odd cup isn't going to be enough. You'll need to brew with the world's largest teapot if you want to power your fridge and have hot radiators too.

The eight-feet high 600 gallon-sized pot standing just outside Chester, West Virginia, USA, claims to be The World's Largest Teapot. But it's a converted wooden root beer barrel and so it can't claim to be the largest useable teapot. That honour goes to the British Bramah teapot. Made in 1985 to the orders of Edward Bramah, tea entrepenur and founder of The Tea and Coffee Museum (located near London Bridge station), the Bramah pot stands 2-feet 6-inches high and holds enough for 3-months worth of my drinking needs, or 800 cups of tea.

I wonder what size of Stirling engine I could run off a Bramah? I'd better get in some more Ceylon tea from Robert Wilson then.