England’s lights would go out without Scotland’s large and growing supply of renewable energy, according to Scotland’s energy minister.

Fergus Ewing hit back after the UK energy secretary, Ed Davey, said independence for Scotland would force up energy bills for Scottish households.

“England does require Scotland’s electricity to keep the lights on,” Ewing told the Guardian. He gave the go-ahead on Wednesday for the third largest offshore wind farm in the world, big enough to power 1m homes, as well as new financial support for floating wind turbines to exploit deep water sites.

Ewing, a Scottish National Party minister, contrasted the 20% spare electricity margin in Scotland with the 2-5% margin in the UK as a whole. “The reality is that the supplies of electricity in the UK, especially down south, are parlously tight. There have been successive warnings by Ofgem, the regulator, and it is difficult to see the response to those warnings as anything other than a serial failure to come up with any coherent strategic response,” he said. “On a security of supply basis, England will require to receive imports of Scotland’s electricity for most of the time.”

Davey said on Tuesday: “The size of the UK protects Scottish consumers from the full costs of Scottish power generation. In the UK, Scotland’s households pay less than they would in Scotland alone.”

Scotland has 10% of the UK population but a third of its renewable energy and Davey argued that citizens of an independent Scotland would therefore have to pay more towards the subsidies that support wind and marine energy. He also warned that he would look to Ireland, Iceland and continental Europe for power if Scotland became independent after the referendum on 18 September, as well as boosting renewable energy in the rest of the UK.

Ewing called the claims “political posturing” and said wind energy subsidies make up a “very small proportion” – currently 1.4% – of the costs on the average energy bill. “Over time bills would go down [in an independent Scotland] because we would have greater security of supply.”

He also said an independent Scotland would not have to pay the large subsidies agreed by Davey for a new nuclear power station in Somerset, which he estimated at £35bn. Davey, an opponent of nuclear power before entering government, countered: “The Hinkley Point C price we agreed with EDF is at a much lower rate than the strike price for offshore wind.”

Labour’s shadow energy minister in Westminster, Tom Greatrex, backed Davey. “There is no convincing explanation as to why consumers in England, Wales and Northern Ireland should pay the support costs for renewable energy technology that would be in a foreign country. People who raise the question are accused of being negative and scaremongering, but it is a legitimate question.”

The SNP leader, Alex Salmond, is committed to generating 100% of Scotland’s electricity from renewable energy by 2020. In 2013 it was 40%.

The approval today for the 1.9GW offshore wind farm off the Caithness coast is a significant step towards that, according to Lindsay Leask, senior policy manager for trade group Scottish Renewables. “We are delighted to finally see the first ever consent being granted for a large scale commercial offshore wind farm in Scotland. This is a really important step forward for the industry, which has the potential to generate massive amounts of renewable power and support significant numbers of new jobs,” he said.

While Scotland is the windiest country in Europe, England and Wales already have large offshore wind farms in place.

In his interview with the Guardian, Ewing also criticised the new “dash for gas” backed by chancellor George Osborne. “From a geo-security point of view, it is increasingly evident that the UK has failed the UK citizens in not establishing a secure policy based, as far as possible, on indigenous supply,” he said. “Being vulnerable to gas supplies from Russia in the light of recent events is not the wisest policy.”

Ewing declined to say whether revenues from future fracking and coal-bed methane production would be channeled into the sovereign wealth fund the SNP has pledged will receive oil and gas revenues in an independent Scotland. “We don’t think there is much likelihood of onshore hydraulic fracturing in the foreseeable future,” he said.

He added: “Mrs Thatcher managed to spend like some kind of unconstrained serial lottery winner and managed to work her way through, with her successors, £300bn [of oil and gas revenues]. So like Iraq, we have an oil fund of zero. Norway has a fund of £500bn.”

On onshore wind, which is as contentious in Scotland as in the rest of the UK, Ewing said a forthcoming map of “core wild areas” would stop development in some places. “It will offer a balanced measure of protection for our most precious areas, for example, no wind farms in national parks or national scenic areas.”

Wind power developers have warned the map could create large “no go” areas and the Scottish parliament’s energy committee said on Tuesday that more clarity was needed on the map, particularly on how the 1.5-mile exclusion zone around communities would be implemented.