Two Swiss utilities made green power the default — 80% of customers never switched back

In a study published in Nature Human Behaviour and reported by the BBC in March 2021, ETH Zurich's Andreas Diekmann and Jennifer Gewinner examined what happened when two Swiss energy suppliers changed the default electricity offering from a mixture of fuels to renewables only — a change affecting around 234,000 private households and 9,000 businesses.

Before the switch, roughly 3% of customers chose green power; afterwards, 80–90% did. Residential consumers paid at least 3–8% more for the green tariff, businesses saw costs rise by up to 14% for energy used at night, and large companies with considerable freedom to switch suppliers mostly stayed green at around $2,300 extra per year. Even four to five years after the change, some 80% of households and more than 70% of small-business customers were still on green electricity.

The authors attribute the stickiness partly to opt-out friction — 'you have to switch to the other cheaper tariff actively... many people just don't do it,' Diekmann told the BBC — and partly to the default reading as the recommended, safe choice. The study found no evidence of increased electricity use among green-tariff customers over the six-year period, and a simulation for Germany suggested green defaults for private customers alone would save 45 million metric tonnes of CO2, about 5% of the country's emissions.

Voluntary green uptake sat at about 3%, so information and pricing alone were not the bottleneck.

Switching back to a fossil-mix tariff requires an active email or phone call that most customers never make.

The default reads as the supplier's recommendation — 'the safe choice' — which lowers the perceived risk of the dearer tariff.

Unlike organ-donation or double-sided-printing defaults, this one held up despite real extra cost, which surprised even the authors.

Defaults move behavior where price and persuasion fail: people read the default as an endorsement and rarely pay even a small effort to opt out of it.

The authors argue green defaults could cut emissions at scale, especially in fossil-reliant countries like the US, China or Germany, and say the study's stickiness estimates help suppliers calculate how many customers would stay — addressing utilities' fear of not having enough renewable supply.