Great Britain’s electricity system has recorded its greenest ever summer after growing numbers of wind and solar farms cut the need for gas power plants to fresh lows.

Analysis of energy generation data, commissioned by the Guardian, revealed that Britain’s reliance on fossil fuels fell in August to less than one-fifth of all electricity, or 4 terawatt hours (TWh), its lowest ever level for a one-month period.

This allowed the carbon intensity of the power grid to plummet to the lowest level recorded for a single month, at 144g of CO₂ per kilowatt-hour in August, 40% lower than in the same month last year, according to the analysis.

The data, analysed by the energy thinktank Ember, showed that the record lows were sustained even when averaged over the summer months from June to August, meaning the grid experienced its greenest summer ever.

The Labour government aims to run the UK’s power grid on virtually zero carbon electricity by 2030 thanks to a surge in new wind and solar farms. Its flagship auction this week for renewable energy subsidies awarded contracts for 131 new projects, or enough new clean electricity projects to power 11m UK homes. However, it secured just half the offshore wind capacity needed every year for the rest of the decade if the government is to hit its net zero target.

Frankie Mayo, an analyst at Ember, said: “Having the lowest monthly fossil fuel share on record shows that homegrown wind and solar can reduce reliance on imports. This is a great starting point on the path to clean power by 2030 for the new government.

“But gas in the mix still threatens energy bills. Clean power is delivering cheap, low-cost power – ramping up deployment at scale can’t come soon enough.”

The analysis confirmed forecasts, revealed by the Guardian earlier this month, that Great Britain was on track for a record summer for solar power. Electricity generation from the sun reached a monthly average of 1.86TWh over the calendar summer, up by almost 20% from the same period last year.

Windfarms also generated a record summer high of 7.04TWh in August, Ember’s analysis shows, more than 46% higher than in the same month last year. The average monthly wind output between June and August climbed to 5.6TWh, up 40% from last summer.

Emma Pinchbeck, the chief executive of the industry body Energy UK, said: “It wasn’t that long ago that coal was providing 40% of our electricity and the prospect of running the grid on predominantly low-carbon power would have been dismissed by many as impossible.

“The regularity with which new records like this are set shows the pace at which cleaner homegrown sources are providing an ever-increasing share of our power.”

Luke Clark, a director at Renewable UK, the green power trade association, said: “These record-breaking figures show that we’re making great progress, but to achieve the new government’s target of decarbonising our electricity system by 2030, we’ll need to increase the rate at which we build new wind and solar farms by securing even higher volumes of new capacity in each annual auction for contracts.”

Ed Miliband, the energy secretary, said: “While these figures are to be welcomed, we have a mission to make Britain a clean energy superpower, with solar and wind power at the heart of our plans.

“Just this week we achieved a record-setting round of renewables projects, with enough power for 11 million homes – essential to give energy security to families across the country. And we’ve set up Great British Energy, which will unlock billions of private investment, delivering new energy projects, tens of thousands of high-quality jobs and protecting bill payers permanently.”