The highly conservative International Energy Agency predicts the cost of solar energy will fall to around 4c/kWh in coming decades as the sun becomes the dominant source of power generation across the world.

As we reported yesterday, the IEA now expects solar to become the biggest single source of energy by 2050 and has now doubled its forecast capacity for solar PV.

Rooftop solar, it says, will now account for one half of the world’s solar PV installations, because as a distributed energy source the technology is “unbeatable”.

On costs, it says all solar technologies will fall dramatically in coming decades, with solar PV falling to as low as 4c/kWh, utility-scale solar to around the same level, and solar thermal with storage will fall to as low as 6.4c/kWh.

As this graph below shows, the minimum price tends to occur in regions with great sunshine, and it also assumes a low capital cost of around 8%.

Indeed, IEA executive director Maria van der Hoeven said capital costs were a key element in bringing down the cost of solar technologies. To do this, she said, it was critical to have stable and long-term policies.

This graph below illustrates how financing costs have a critical bearing on the overall cost of solar – any technology for that matter, but particularly one where the bulk of the cost is in the upfront capital outlay for installation.

Van der Hoeven said policy makers needed to make clear, credible and consistent signals, which can lower deployment risks to investors and inspire confidence.

“Where there is a record of policy incoherence, confusing signals or stop-and-go policy cycles, investors end up paying more for their investment, consumers pays more for their energy, and some projects that are needed simply will not go ahead.” The implications for Australia, where large-scale investment has ground to a halt because of policy uncertainty, could not be any clearer.

The forecasts from the IEA are not the most dramatic that can be found, but they are significant because the IEA is essentially a conservative organisation that was created in the 1970s to defend developed countries’ access to fossil fuels

It has a history of underestimating the impact of new technologies such as solar, as we pointed out in this article – even though it has doubled its forecast for solar PV deployment in just the last few years. Other agencies, such as IRENA, have a much more bullish forecast for solar.

The IEA insists that its figures are not forecasts, but what the world should be aiming for. The deployment of solar could be much higher – assuming costs come down faster than thought. The IEA’s base model still relies heavily on “baseload” generation.

If the IEA figures are right, solar PV’s share of global electricity will reach 16 per cent by 2050, a significant increase from the 11 per cent goal in the 2010 roadmap.

This will require 4,600 GW of installed PV capacity by 2050 – more than half of it in China and India. This requires the installation rate to nearly quadruple to 124GW year. It notes solar PV will be a highly effective abatement tool, avoiding up to 4 gigatonnes (Gt) of carbon dioxide (CO2) annually on its figures.

The IEA says variability is an issue, but it can be overcome with interconnections, demand-side response, flexible generation, and storage. The overall cost will require an increased investment of $44 trillion – but this will generate savings of $115 trillion in fuel costs.

The IEA says the dramatic cost falls in solar PV means that the sector is around five years ahead of where it thought it would be.

Solar thermal with storage, on the other hand, was lagging because its “dispatchability” was not being fully valued.

That would change, however, because solar thermal with storage would play a critical role in the energy systems of the future and would be a match for solar PV because of its ability to store energy for use at night or times of peak demand. In some countries, solar thermal deployment is expected to be greater than solar PV. In others, the opposite will be true.

Paulo Frankl, the IEA’s solar expert, says half the large PV deployment considered in this roadmap would take place on buildings or nearby (such as over parking lots). More than half of this would be on commercial buildings rather than residential.

The forecasts rest, in part, on the concept of grid parity – when the cost of distributed PV generation is equal or below the per-kWh component of retail electricity prices – and on self-consumption. “At the utility level, solar PV has many competitors. At the distributed level, solar PV has a competitive advantage and is unbeatable.”