The people had gathered to hear two Dunedin men, Jens Rekker and Mike Laba, speak about converting cars to use electric power.
About 90 people were turned away from a previous discussion by the two men after they filled a 100-seat venue at Otago Polytechnic.
Yesterday, about 200 filled Logan Park High School's auditorium.
Mr Rekker said the popularity of the talk was mainly the result of increasing petrol prices, but some there were interested because of their quest to be "zero carbon".
Others were interested in the technology.
The talk was a practical guide to backyard conversion covering the basics such as choosing a car, parts, batteries and safety.
"We wanted to let people know that it is possible, feasible, for the average person to do a conversion."
Those attending also had the opportunity to see the men's own conversions - a mini and a Ford Courier ute.
Mr Rekker said electric cars' engines were 80% efficient compared with the 15% to 25% efficiency of petrol or diesel engines.
The catch was an electric car's range - with present technology any conversion resulted in an "about town" vehicle not suitable for open-road driving.
"You have to be realistic about what you can do."
The pair have been invited to give their talk in Christchurch and Invercargill and will display their vehicles at the Otago Polytechnic's July 12 sustainability expo.
