The Green party has called for GDP to be replaced as a measure of national wellbeing with an index showing how much free time people have to enjoy, arguing that leisure can contribute more to overall happiness than wealth.

The policy, which followed a Green proposal for people to work a four-day week, will be unveiled on Friday by its new co-leaders, Siân Berry and Jonathan Bartley, at the party’s annual conference in Bristol.

Under the plan, the so-called Free Time Index would measure how much leisure time people have, taking into account both work hours and their commute.

It would be published once a year as part of the budget, with the aim that it should increase every 12 months.

The idea – the first policy announcement since Berry and Bartley were elected by party members in September – is intended to both improve people’s work-life balance and move society away from a culture of long work hours, given the changing expectations around employment from new technology.

Before the 2017 general election, the party called for the introduction of a formal four-day week as part of the same process, a proposal also backed last month by the TUC.

It will be a first formal outing for the new leadership duo, who replace the previous pairing of Bartley and Caroline Lucas.

Ahead of the automatic re-election process which all Green leaders and other top officials face every two years, Lucas announced she would not stand again, to focus on her work in Brighton Pavilion, where she is the party’s first MP.

The aim was also to allow a new generation of senior figures to emerge, with Lucas having long been the party’s most recognised figurehead.

In extracts from the leaders’ speech released in advance, Berry, who is a member of the London assembly and a councillor in Camden, said it was “time to shift away from the culture which sees us work harder and harder for longer and longer, often without reward or satisfaction, and to recognise that true freedom will only be found when people have more control of their time and how it is spent”.

Bartley, who leads the Green opposition on Lambeth council in London, said the aim was to allow people “the time to have a family life, relax, and pursue the things they care about”.

He said: “It should be an aim of the government to see a yearly increase in this Free Time Index, so that the quality of time which is truly our own becomes the real measure of wellbeing.”

More widely, the three-day conference will see the Greens again press their claim as a radical political force following a difficult couple of years in which their poll ratings have been squeezed by the current two-party dominance, and by the perception Jeremy Corbyn has taken some of their territory.

The Green leaders reject this, arguing that Labour remains cautious over many issues, including the environment and the idea of adapting to the new era of employment through initiatives like a universal basic income.