Palo Alto has been trying to go "carbon neutral" with its electric operation for at least five years, but it wasn't until November that the city finally determined what the term means.

The confusion over the definition of "carbon neutral" -- carbon dioxide is the main gas created by mankind that is linked to global warming -- isn't exclusive to Palo Alto. In England, the Department of Energy and Climate Change had worked with consultants for several months in early 2009 before coming up with the following definition of "carbon neutrality":

"Carbon neutral means that -- through a transparent process of calculating emissions, reducing those emissions and offsetting residual emissions -- net carbon emissions equal zero."

Palo Alto's definition, which the City Council adopted in November, isn't as easy on the ears. It reads:"A carbon-neutral electric-supply portfolio will demonstrate annual net-zero greenhouse-gas (GHG) emissions, measured at the Citygate, in accordance with The Climate Registry's Electric Power Sector protocol for GHG emissions measurement and reporting."

It's not as complex as it sounds.

Citygate, in the definition, is the main meter through which Palo Alto connects to PG&E's transmission system. Greenhouse gases are measured by multiplying the volume of energy by an "emissions factor," the percentage of gases contained in the energy.

The Climate Registry is a nonprofit operating throughout North America that sets standards for measuring and verify greenhouse-gas emissions. Its Power Sector protocol for measuring greenhouse gases is considered "the industry standard," Senior Resource Planner Monica Padilla told the council's Finance Committee in October.

Among the protocol's requirements, the city must count emissions from renewable sources in its greenhouse-gas calculations. In Palo Alto, relatively small levels of emissions come from combustion in the city's landfill gas and geothermal operation.

The protocol also allows the city to count purchases of Renewable Energy Certificates (RECs) towards its greenhouse-gas bottom line -- a method of buying "credits" to offset emissions from fossil-based energy sources. Palo Alto plans to purchase such certificates until it gets enough carbon-neutral energy sources to accommodate the city's electric load sometime around 2017.

The protocol also requires outside verification of the city's emissions statistics annually.

"Even though we report it, we have to have an outside party come and verify our numbers," Padilla said. "That lends our numbers credibility."