Imagine the island of Manhattan covered with 150 feet of crude oil – almost enough to drown the Statue of Liberty – or 1,000 football stadiums filled to the brim with black gold.

That's a cubic mile of oil, or the amount of oil alone the world now consumes in a year, Ripudaman Malhotra told an audience Monday at Greentech Media's Greentech Innovations: End-to-End Electricity conference in New York.

And Malhotra, associate director of SRI International's Chemical Science and Technology Laboratory in Menlo Park, Calif., wants people to think of all the world's energy usage in terms of cubic miles of oil, or CMOs, because "That exercise will bring us face to face with the enormity of the challenge we are facing" in moving to a renewable energy future.

Including electricity generation, which takes up about 40 percent of the world's energy usage, and all other forms of energy, the world uses the equivalent of three cubic miles of oil per year, he said.

But renewable energy makes up only a tiny portion of that measure of world energy supply, he said – about two-tenths of a cubic mile including large hydropower projects, and a mere 0.005 cubic miles of oil for all the solar and wind power now in place today.

It will be very difficult for renewable sources to grow to meet the world's projected demand growth to somewhere between six to nine cubic miles of oil by 2050, he said. (see IEA Paints Dire Picture of Energy Supply and Demand).

While the sun provides an equivalent of 22,000 cubic miles of oil in energy to the earth, capturing that energy will take new technologies deployed on a scale that is hard to fathom, he said.

"If we are at .005 CMO [for solar and wind power] and want to get to 5 CMO, that's a thousand-fold increase" over the next 40 years, Malhotra said by way of example. And current technologies are not effective enough to make up for "a big part of that gap, so I'm looking to see some new things," he said.

The first problem is the sheer scale of the expected growth in energy demand, he said. Second is the disadvantages that solar and wind power have in terms of intermittency – not producing power consistently throughout the day and year – which will require new forms of energystorageto capture the power for use when it's needed most, he said.

In terms of cubic miles of oil, Malhotra laid out some daunting figures for how much solar and wind would be needed to meet demand.

Getting just one CMO-equivalent of photovoltaic solar power in the next 50 years would require 4.2 billion 2-kilowatt solar rooftop systems, or 250,000 installed every single day over that timeframe, he said.

Looking at concentrated solar-thermal power, which produces power using the sun's heat at lower cost than photovoltaic solar systems, Malhotra said it would take 7,700 solar-thermal parks of 900-megawatts capacity – or three built per week over the next 50 years – to add up to one cubic mile of oil equivalent.

As for wind power, getting to that one CMO-equivalent within 50 years would require putting up 1,200 1.6-megawatt wind turbines every week over that time, he said.

Using solar-thermal and wind power effectively will also require a massive investment in transmission to bring the power from the remote areas where it's best produced to the cities that need it most, he said. (See NREL Hunts for Solar-Thermal Hot Spots and National Grid: Dream or Reality?)

Biofuels – which have the advantage of being the only form of renewable energy that can be easily stored – offer other problems, such as the energy that's required to make them and the potential environmental and greenhouse-gas emission costs of clearing land or cutting down forests to make room for planting the crops to make them, he said.

Creating one CMO-equivalent of biodiesel, for example, would require an 85-fold increase in the current amount of land now dedicated to growing soybeans around the world, he said.

As for nuclear power, which Malhotra said must be a part of the world's energy future, getting to the equivalent of one cubic mile of oil would require the building of 2,500 nuclear plants, or one every week for the next 50 years.

And fossil fuels, which now account for the vast majority of the world's energy supplies, will have to increase in use as well, even as the world tackles how to keep their use from increasing greenhouse-gas emissions and thus the threat of global warming, he said (see Can You Spare $45T to Curb Global Warming?).

Given these massive challenges, "The operative conjunction is ‘and,' because it will take nuclear and solar and wind and whatever" to meet the world's energy needs, he said.

And the solution won't just lie in increasing energy supplies, Malhotra said. Efficiency – doing more with less energy – and conservation, or foregoing the use of energy, will play an equally important role, he said.

Efficiency has "historically never reduced net consumption," since people tend to increase their energy use as efficiency improvements make it cheaper, he said. Still, doing things like replacing one billion incandescent light bulbs with compact fluorescent lamps, or CFLs, could save the world an equivalent of one cubic mile of oil per year, he said.

Making more energy-efficient buildings, which now account for about half the world's use of energy, could also have a big impact – if such improvements in lighting, heating and air conditioning systems, as well as more efficient building materials, can meet the price demands of developing countries like China and India, he said.

Bringing a host of efficiencies to the United States' roughly $500-billion-per-year building renovation market could yield a savings of about one-tenth of a CMO, he said.

Bringing the same innovations to India and China, where several trillion dollars of buildings are built and renovated each year, would raise that energy savings to as much as two CMOs, he said. But right now "we really need major innovations to produce cost-effective solutions" for those markets, he said.

"Conservation, or avoiding [energy use], is tougher," he said. But, given the enormous challenges in increasing energy supplies and improving energy efficiency, it will have to be part of the equation, he said.

Conservation and efficiency together could reduce the world's energy demand by three cubic miles of oil equivalent, he said.

"The question is, what about the rest? Those solutions will have to come form the supply side now – and as we've seen, we have absolutely nothing on the supply side to meet this right now."