Low-carbon transition is gaining momentum, but relatively little is known about the public preferences for low- and zero-carbon electricity portfolios given their environmental, health, and economic impacts. Decision science literature argues that conventional opinion surveys are limited for making strategic decisions because the elicited opinions may be distorted by misconceptions and awareness gaps that prevail in the public. We created an informed citizen panel (N=46) in Switzerland using technology factsheets, an interactive web-tool Riskmeter, and group discussions. We measured the evolution of the panel's knowledge and preferences from initial (uninformed) to informed and longer-term views four weeks after. In terms of energy transition, our elicited technology and portfolio preferences show strong support for the low-carbon electricity sector transition, especially relying on hydropower, solar power, electricity savings and efficiency, and other renewable sources. As these informed preferences are structurally different from the futures considered by many energy experts, we argue that these preferences should also inform the Swiss Energy Strategy 2050's implementation. In terms of methodologies in decision science, our factsheets, Riskmeter, and group discussions all proved effective in forming the preferences and improving knowledge, but we also intriguingly found that in a longer run the participants tended to revert back to their initial opinions. The latter finding opens up multiple new research questions on the longer-term effectiveness of informational tools and stability of informed preferences.