“Cities have been spending money on some things, and when you go and look at it, this isn’t really where the big bang is going to come, where I could spend my money more wisely,” Spiegel said during a session of Fortune’s Brainstorm E conference on Tuesday.

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Studies of different initiatives across the country have shown, for example, that the cost of replacing old-fashioned street lights with lower energy LED lights may not the best use of limited funds to curb emissions in all situations, he said, adding that light rail has also proven to be an unwise investment in some cities. Results may vary in different locations.

By contrast, the biggest savings for the cost have come from installing home and commercial automation gear, which automatically reduce electricity and fuel usage, he said.

“Those two things managing energy demand are by far and away the biggest plays,” Spiegel posited. Siemens is a leading manufacturer of smart control systems.

Cities occupy only a small portion of the planet’s landmass, but they are responsible for about 70% of all CO2 emissions—a key greenhouse gas.

Thus, many municipalities are moving to reduce their emissions via improved energy efficiency, greater use of renewable power sources, and installation of automated energy monitoring and controlling devices.