Largely made redundant by the prevalence of mobile phones, the green boxes are an attempt to breathe life back into a British national icon that often lingers on in a dilapidated state.
The first "solarbox" was unveiled on Wednesday outside Tottenham Court Road underground station, at the busy intersection with Oxford Street, London's main shopping thoroughfare.
The new boxes are painted green and are powered by a solar panel on the roof and more are planned in the months to come.
Inside, there are four chargers, allowing people to power up mobile phones or tablets.
"The energy is stored in a battery, so you can even charge your mobile phone during the night or when there is no sun," said Harold Craston, a graduate from the London School of Economics (LSE) university and the co-founder of "solarbox".
"We wanted to show that we could use public spaces in a positive way, and that London should try to become more green," he told AFP.
He said too many phone boxes had outlived their usefulness and "people use them as toilets when they're drunk".
"In our modern world, where hardly any Londoner is complete without a raft of personal electronic gizmos in hand, it's about time our iconic phone boxes were updated for the 21st century," said London Mayor Boris Johnson.
Designed in 1936 to commemorate the 25th anniversary of king George V's coronation, the K6 was Britain's sixth standard telephone box but the first to be installed nationwide.
When they stopped being made in 1968, there were 70,000 dotted around Britain.
While 11,000 red boxes remain in use, most of them lose money.
Under the Adopt a Kiosk scheme, decommissioned phone boxes across Britain have been fitted with life-saving defibrillators, transformed into tiny public libraries and, in one case, hosted a one-night-only pub.