Miliband sets out proposals for Labour in tough times. Press Association

The opposition leader, Ed Miliband, has set out a new leaner public spending ethos for straitened times, pointing to three areas in which he would "use the power of government in new ways" as he readies his party to fight a 2015 "austerity election".

In a speech billed as one of the more significant of his 15-month leadership, Miliband said he would "demonstrate once and for all that Labour is a party for all times, not only a party for good times".

Sketching the problem, Miliband said: "Each time New Labour won an election, it won at a time when business was prospering: when entrepreneurs could set up new firms and be confident of a return; when companies knew that there were markets for their goods, and consumers ready to spend. That growing economy meant that there were tax revenues to invest in our infrastructure, to help hardworking families and to protect the most vulnerable in our country.

"Next time we come back to power, it will be different. We will be handed a deficit. We will have to make difficult choices that all of us wish we did not have to make. So we must rethink how we achieve fairness for Britain in a time when there is less money to spend."

In his speech, he made a policy proposal intended to exemplify the new approach, proposing Labour would in future force energy companies to offer lower tariffs to the most vulnerable in society, shifting part of the job of improving living standards from the state to big business.

"How can we offer the help people need when there is very little money to spend?" he asked. "The government has already cut the winter fuel allowance so that it provides less support. That cut may be something we cannot reverse. With the economic outlook that we face, we will find it hard to do that.

"But that doesn't mean we give up. Instead, our new approach starts by looking at the root of the problem – the way our energy market is letting British consumers down. We've got six main energy companies in Britain. Competition should be good for consumers. But the way that the current market works is disadvantaging them.

"We know who the most vulnerable are, those least able to find the cheapest deal, or to be online: often the elderly. Offering different products is a good thing. But if those products end up taking advantage of older people, that is wrong.

"So I'll give the energy companies a simple rule. By all means put different products out there, and for different kinds of consumers. But we will ensure you give pensioners over 75 the lowest tariff on offer. You make it happen or we'll put it into law. There may be less money around. But for those 4 million pensioners, Labour can still deliver fairness in these tough times."

Miliband was speaking at the Oxo Tower, in London, to the London Citizens organisation, a community organisation that campaigns to improve social conditions, and which has successfully persuaded many companies to pay employees a living wage – a higher rate of minimum pay at £7.88 an hour to reflect the cost of living in London. Miliband adduced this as an example of the new mechanisms a possible future Labour government might encourage to iron out unfairness in society without the government necessarily spending money. However, Miliband refused to pledge that a Labour government would roll out a living wage across the board.

Instead of the welter of new ideas his critics crave, Miliband proposed a three-pronged strategy. "First, reforming our economy so we have long-term wealth creation with rewards fairly shared. Second, acting against vested interests that squeeze the living standards of families. And third, making choices that favour the hard-working majority."

He returned to the theme of predator and producer capitalism, which he first aired in his conference speech last year – that some British businesses make a benign economic and social contribution to the communities in which they operate, and others less so. He said: "When I talked about that in my conference speech, you might say I wasn't overwhelmed with support from other political parties. Some even said that I was anti-business." Now, he said, his ideas were being aped by the prime minister, David Cameron, and the deputy prime minister, Nick Clegg.

He said Labour's challenge was "to use the power of government in new ways. To set new rules that promote the long-term and fair wealth creation we need. To share a vision with British industry of how we pay our way in an ever more competitive world. To nurture a growing, thriving economy which creates better jobs, new companies and innovative industries.

"That means we have to end the situation where British employers want their companies to invest but can't find the finance to do so. That's why we are looking at plans for a British Investment Bank so small businesses can invest and grow. It means we have to end the situation where an industry knows it is in its long-term interest to invest in the skills of its workforce, but each firm doesn't act because it isn't in their short-term interest."

The Labour leader has been under low-level but constant pressure to say more clearly what Labour would do in 2015 rather than what it would have done differently in 2010. Since Christmas, he has been faltering, with critics within the party suggesting he has been too vague, and pointing to his weak personal polling.

For a longer time, shadow cabinet members have cast doubt on whether their economic narrative fits the public mood. The leadership will not give up their party's strident position on how it would handle the current deficit reduction programme differently – they believe the government has cut too far and fast – but it is now seeking to acknowledge that in the medium term the usual terms of what a Labour government provides have to change.

In response to journalists' questions, Miliband said: "We've got a long way to run to win the argument." Later, he said: "You don't win the race when you're one-third of the way into the race."

He also disputed that Labour had failed so far to set out what it would cut, saying Labour had long pledged it would cut the policing budget and part of the schools budget.

But he ended his speech in bullish spirits, claiming other politicians were aping his ideas: "Everyone is now joining us, talking about the squeezed middle, the next generation and responsible capitalism. But it's not enough just to talk about them. Suddenly David Cameron is falling over himself to say he too is burning with passion to take on 'crony capitalism'. Now he has accepted this is the battleground of politics, I say: 'Bring it on.'"

Before Miliband's speech, one of his closest advisers published an article warning against matching the Tories cut for cut or outflanking them on the right. Stewart Wood wrote: "For the past 50 years, Labour's approach to governing has rested on using the proceeds of growth to fund redistribution, social protection and public services. In 2012, we know that these proceeds will be in scarcer supply than in the past, and the claims of deficit and debt reduction on them when growth does return will be greater than before."

Wood argues that the deficit Labour inherited in 1997 was 3.4% of GDP, and this year it will be 8.4%. "We'll still tax and we'll still spend, and in straitened times the politics of tax and spend – making sure tax is properly progressive, making sure spending is well targeted and efficient – will become more not less important. But these choices will be tougher. The 1980s supply-side revolution from the right has run its course. What Britain needs is a supply-side revolution from the left. We will need new types of banks and stronger competition in the banking industry; corporate governance reforms to incentivise good ownership models and longer-term business strategies; ensuring that companies see the continuing upskilling of their workers as an obligation and not simply a luxury; and the courage to challenge vested interests in the economy that charge excessive prices for energy or train fares and squeeze families' living standards."