No one knows how much a renewable post-carbon grid would cost, but renewable advocates like Greenpeace, and Rocky Mountain Institute, and the proposed Google Clean Energy 2030 plan, all seem to think it will be cheap. However it is impossible for any planner to know if an all renewable grid will work, and if it will work, how much an all renewables grid would cost.

There are fundamental questions about the cost and reliability of an all or mostly renewables grid that cannot be answered without and adequate computer simulation tool, and such a tool is not available now, nor will it be for some time to come. An Argonne National Laboratory Report from the December 2008 National Power Grid Simulator Workshop, offers a realistic picture of the current status of renewables based grid planning. For example renewables planners often offer massive expansions of the electrical transmission system as a remedy for grid reliability problems created by renewables. The report states:
The national power grids are composed of many components, each with its own range of vulnerabilities to both natural and man-made events. The most apparent components are the transmission lines that stretch unprotected across open countryside. Natural threats to these lines include wind, ice storms, and fires; potential man-made threats include rifle fire, bombs, chaff, and aircraft collisions. Isolated power line outages can be quickly repaired. The disruption of large numbers of lines or those located in special locations (e.g., across rivers and wide gaps), however, can be time-consuming to resolve. . . . Renewable power genera- tion depends largely on local conditions (e.g., wind or sunlight availability). The best locations for production are often not where the greatest power demands are. Power transmission, then, could be a limiting factor in the growth of distributed, renewable energy generation.
The report also states
Tools are needed to simulate large additions of renewable and distributed power to the grids. Models must deal with the intermittency of these sources, optimize the transmission and resource mix, quantify important metrics (e.g., economics, security, and environmental impact), and assess alternative solutions.
Yet no research tool currently exists which would permit anything like adequate research on the national transmission system that would be required by a mostly renewables generating system. Despite the lack of a research tool that would determine the practicality and cost of an all renewables electrical transmission system, renewables advocates continue to assure us that electrical transmission reliability posses no problem for a future renewables based grid. Would a future renewables based grid be stable? The report states,
There is, in general, a lack of a simulation capability and associated data for national-level studies. For instance, large-scale dynamic transient stability models do not exist, though large-scale linear load flow models do.
The report also notes
Power grid models can be classified along three dimensions: operations, planning and evolution, and disruptions. . . . Most models today are limited to a single axis of this taxonomy. The three dimensions described above, though, are not independent. Issues raised along one dimension (e.g., an evolution toward more renewable energy generation) affect issues along the other axes (e.g., changing the consequences of a wide-area weather event).
It is clear then that plans for a renewables dominated grid are highly premature in the absence of a tool or tools that can accurately asses the reliability and cost of such a grid, and compare that assessment with an assessment of other options, for example the modeling of a nuclear dominated grid. Some assessments suggest that the cost of a nuclear electrical generation system, often said to be high, might be actually far lower than the cost of a renewables dominating electrical system.