German utility got ten times more green power signups by flipping the default

German electricity customers could choose a green-energy tariff sourced from renewable generation at a small premium — about 3 cents more per kilowatt-hour, roughly $15 a year for a typical household — over the standard grey-power plan. Under the conventional opt-in structure, where customers had to actively select the green plan to get it, uptake stayed low: economists Felix Ebeling (University of Cologne) and Sebastian Lotz (Stanford) found only about 7% of an opt-in group chose it.

Ebeling and Lotz ran a field experiment with roughly 42,000 German households, randomly assigning customers to either the standard opt-in green tariff or a version where the green tariff was the pre-selected default and customers had to actively opt out to get cheaper grey power instead. Nothing about the price, the underlying electricity, or the marketing pitch changed between the two groups — only which option required action to select.

Flipped the default: made green the passive option, so staying took no action and switching away did, converting indifference into adoption.

Most customers stay with the option that requires no action, so the default becomes the outcome.

Holding price and pitch identical isolates the default's effect, proving it's the mechanism.

Opt-out converts latent indifference into apparent preference without persuading anyone.

The tenfold jump shows defaults can outperform marketing for costlier preferred options.

When persuasion only converts a few, flip the default: make the desired option the passive choice and require action to decline.

About 69-70% of customers in the opt-out group stayed on the green default, compared to roughly 7% who actively chose it under opt-in — a roughly tenfold increase in green-tariff adoption from the identical price and product, published in Nature Climate Change in 2015.