The REN21 Renewable Energy Policy Network formed as a result of the Bonn Renewables 2004 conference in June 2004. The network recognized the need for a summary of the current status of renewables worldwide and commissioned the Worldwatch Institute to produce the report. The report was intended to be factual. Thus it does not contain analysis, recommendations, or conclusions.

An extensive research and review process over several months followed, led by Eric Martinot and involving more than 100 researchers and contributors (who are acknowledged in the report). Some of these researchers had specific tasks, such as estimating the number of jobs worldwide from renewables or determining the market capitalization of the leading global companies. Others compiled and reported on market and policy developments in specific regions or countries. Contributors provided substantive reviews, supplemental data and sources, and interviews for expert opinion.

The report drew from over 250 published reference sources, plus a variety of electronic newsletters, unpublished submissions, personal communications, and web sites.

There is generally no single source of information for any fact globally, as most existing sources report only on developed (OECD) countries or on regional or national levels, such as Europe or the United States. Thus, global aggregates were built from the bottom up, adding or aggregating individual country information for most indicators and statistics. Developing countries in particular required country-by-country sources, as very little material exists that covers developing countries as a group.

All of the information, graphs and tables in the report are built from multiple sources, often involving triangulation of conflicting or partial information. However, some key sources exist for certain topics. Installed solar PV data comes primarily from Paul Maycock and PV News. Solar hot water data comes primarily from the IEA Solar Heating and Cooling Programme, in a series of reports led by Werner Weiss, along with reports by the European Solar Thermal Industry Federation and Li Hua of SenterNovem for China. Wind power capacity data come from BTM Consult and the Global Wind Energy Council (GWEC), although the two sources disagree slightly with each other. A key source of material for installed capacity statistics for OECD countries comes from the International Energy Agency's Renewables Information 2004 and Electricity Information 2004 reports.

Other key sources include the U.S. Energy Information Administration; the World Energy Assessment produced jointly by the United Nations Development Programme (UNDP), the United Nations Department of Economic and Social Affairs (UNDESA), and the World Energy Council (WEC); several UN agencies; the World Bank; the EurObserv'ER information series contained in the bulletin Systemes Solaires; and many industry associations.

Key sources of information for policies include the International Energy Agency's on-line databases (IEA countries and JREC countries, which includes some developing countries). For the United States, the DSIRE database of state-level policies is the best source. See the renewable energy policies page of this website for links to some of the main policy references.

The global investment numbers (Figure 10) reflect a database of installed capacity by technology and by year, since 1995, drawn from the sources mentined above and in the report's notes. These installed capacity figures are multiplied by assumed average capacity costs (i.e.,in $/kW or $/m2). Capacity costs are taken globally, with the exception of small hydro and solar hot water in China, to which special cost estimates are applied. The report notes provide details of the specific capacity costs used. Some costs, such as biomass power generation, vary very widely, and a global average becomes problematic. Thus the investment numbers are approximate, probably to within plus or minus 10-15%. But the time progression, using a consistent set of assumptions, should portray the growth of investment over time quite well.

Data for developing countries is often some years older than data for developed countries, and thus extrapolations to 2004 must be made from older data, based on assumed and historical growth rates. This is one of the reasons that capacity data (kW) instead of energy data (kWh) are reported, as capacity expansion is easier to extrapolate than energy production. (Another reason is that capacity expansion is more closely tied to annual investment; in general, the debate about reporting kW vs. kWh is significant and should be treated more explicitly in the future.)