sq. cm

The panels the team have been testing are about 6 sq cm in size and capable of producing just 0.25 volts, meaning about 20 are needed to power a light bulb, or charge a cellphone.

Dr Peter Watson, chief medical officer at New Zealand's National Burns Unit, said an estimated 1.2m sq cm of replacement skin would be needed for the patients.

In the UK and US, microwaves must not leak more than 5mW of radiation per sq cm at a distance of 5cm from the microwave’s surface.

Just under 10 sq cm, it might be easy to miss: a small brass stone, embedded directly underfoot, in the cobblestones of the street.

SPF ratings are based on the assumption that a 2mg blob will cover 1 sq cm of skin.