The benchmarkstudy, by the Natural Resources Defense Council and the Electric Power Research Institute (EPRI), found that plug-in hybrids are cleaner than regular cars in each of nine scenarios.
The charity wants to use this large-scale study to create a benchmark for a "well-being index" - so that the impact of future changes in policy can be measured.
But an unofficial study using the IMF's methods calculated a benchmark of about 2.9% of GDP over the medium term, which suggests China's surplus is about where it should be.