Community-owned windfarms on some of Scotland's remotest islands are likely to gain from a new deal to buy their electricity at a higher price, Ed Davey, the energy and climate secretary, has said.

Davey is to introduce a guaranteed price for electricity generated by onshore windfarms in Shetland, Orkney and the Western Isles after hearing pleas from developers and islanders for help with the higher costs of building windfarms and selling power from outlying areas.

The energy secretary, announcing the measure at the Liberal Democrat party's annual conference in Glasgow on Sunday, believes the higher price will be a significant incentive to wind power developers and will particularly help island communities build their own small-scale windfarms.

Before this, the Scottish government's energy minister, Fergus Ewing, had made a competing announcement that he had given consent to the first phase of what could be Europe's largest tidal project.

Ewing said the developers MayGen had permission to build a nine-megawatt demonstration project of up to six tidal turbines in the Pentland Firth off Orkney. Eventually MayGen could install an 86mw array of tidal machines.

Davey said: "It's possible that we might see a big increase in community windfarms because of the way that the Scottish islands develop their energy resources."

He said estimates by the Department of Energy and Climate Change (DECC) suggested that this measure – the first time it has set a special renewables price for a specific part of the UK – would support the installation of 400mw of new capacity.

However, DECC officials later said the 400mw figure would cover schemes already in the planning system or already consented to, including the vast Viking project on Shetland which is due to see up to 370mw of capacity installed.

Davey said the high connection prices set by Ofgem for remote Scottish islands, and the higher construction costs, had delayed many projects in the northern and western islands, and a guaranteed price would offer a significant incentive. "There are lots of developers looking to move and waiting for these decisions to be taken," he said.

The Western Isles and Shetland in particular have been the scenes of intense disputes between residents about onshore windfarms, which some say destroy their landscapes. Opponents of windfarms insist their islands are too small to host large schemes.

Orkney has seen a boom in small-scale windpower, from privately owned turbines to several medium-sized commercial farms. It is due to meet 85% of its electricity needs from renewables during 2014.

One of the largest, at Lewis, was blocked by Scottish government ministers because of the scale of the backlash, while the Viking windfarm on Shetland is going ahead despite local opposition.

The new "strike price", or guaranteed price, expected to be £115 per megawatt hour, compared to £105 per MWh paid for all other onshore schemes, will not cover any offshore wind, wave or tidal schemes.

The significance of that decision was highlighted by Ewing's tidal array announcement: he said the waters around Orkney could generate 400mw in marine power. The minister also confirmed that two wave power companies, Aquamarine and Pelamis, would "share a slice" of a £13m wave power support fund.

The energy potential of these offshore schemes is far higher than additional onshore windfarms on the islands. Experts predict that wave, tidal and wind schemes off Scotland's islands could have an installed capacity of 11 gigawatts, equal to three coal-fired power stations the size of Drax in Yorkshire.

But developers involved in offshore projects say they face the same cost barriers selling their electricity on to the national grid as onshore windfarms, because Ofgem charges producers far more to use the national grid if they are outside southern England.

Davey said there were no plans to increase the strike price for offshore schemes off Orkney, Shetland and the Western Isles, including new windfarms.

Asked whether his decision to give the northern and western islands a guaranteed price to meet higher costs for onshore wind sets a precedent for offshore renewable schemes, Davey said that question was "hypothetical".

He said the new price could come into force in July or August 2014, after a six-week consultation by the DECC due to start this week.